10673282 Ansys Tips and Ansys Tricks

ANSYS® Tips

ANSYS Tips and ANSYS Tricks

Peter Budgell

Burlington, Ontario, Canada

© 1998, 1999 by Peter C. Budgell -- You are welcome to print and photocopy these pages.

These tips and comments are intended for user education purposes only. They are to be used at your own risk. Thecontents are based on my experience with ANSYS 5.3 -- more recent versions may change things. The contents donot attempt to discuss all the concepts of the finite element method that are required to obtain successful solutions. Itis your responsibility to determine if you have sufficient knowlege and understanding of finite element theory to apply thesoftware appropriately. I have attempted to give accurate information, but cannot accept liability for any consequencesor damages which may result from errors in this discussion. Accordingly, I disclaim any liability for any damagesincluding, but not limited to, injury to person or property, lost profit, data recovery charges, attorney's fees, or any othercosts or expenses.

As one writer put it, This information is free, and may be well worth the price.

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FEA Modeling Issues Page

The ANSYS manuals explain many things and give some examples, but they do not give many �ps to the user.

Here is a collec�on of things I have noted or learned. (Use at your own risk...) Necessity is the mother of inven�on,

and I learned virtually everything here as a result of need, or as a result of trial and lots of error. I'm also thankful

to my local ANSYS distributor for many helpful conversa�ons. The comments in these pages are based on my

experience with ANSYS 5.0 through ANSYS 5.3. I hope these �ps will shorten your learning curve. An analyst

frequently does not have a mentor for guidance, so considerable effort can be needed to deduce how to

accomplish some tasks. ANSYS users need to spend a generous amount of �me reading the manuals and training

materials, and returning to read them again as the user's knowledge of the program increases. Don't use

anything here verba�m... understand why it works, and whether my comments are in error or inappropriate for

your situa�on, before employing any of these sugges�ons.

The teaching of FEA at the academic level is intended to educate the mind, teach how FEA methods are derived

from first principles, and to develop students who can invent and code new elements, test their behavior, write

research or industrial quality so2ware, and apply it to difficult academic or research problems. Some professors

feel strongly that the purpose of an undergrad course in FEA is further educa�on in how applied math,

engineering, con�nuum mechanics, energy methods, and analysis of structures come together, building on the

Strength of Materials courses already taken ‐‐ I have no argument with that. A user with a comprehension of

what underlies FEA work will know when to apply and how to evaluate FEA work, have more crea�vity, learn

quickly, problem solve be5er, be more innova�ve, and make fewer serious modeling errors. The professors do

not feel that the course is intended to concentrate on modeling details or learning the interface to a commercial

FEA program. (Students, on the other hand, want to graduate having used an FEA package to do something

significant. Assignments and projects with ANSYS/ED are a good way to get there.) I've heard the opinion

expressed that with FEA technology maturing, there is less research grant money for FEA work in universi�es, and

the supply of advanced FEA graduate students may be shrinking. The teaching of commercial FEA program use is

principally focused on training people to use the interface to and commands of the par�cular so2ware package,

and how to perform basic analysis types. Some instructors pepper their presenta�ons with �ps, but the

a5endees may be drowning from informa�on overload. Li5le is available to lead the user through the techniques

that can be used in modeling complex structures, and around the traps that exist, except help from good vendor

support people, co‐workers, or other users, and substan�al reading, thought, trying examples, and tes�ng

techniques on the part of the analyst. I hope that these pages will provide some helpful details.

CONTENTS:

ANSYS Tips and ANSYS Tricks h5p://www3.sympa�co.ca/peter_budgell/ANSYS_�ps.html

1 of 54 1/17/2009 11:41 AM

Tip 1: Use Annotations

Tip 2: Making Room for Annotations

Tip 3: Using Parameters in Annotations

Tip 4: Use Small Annotations

Tip 5: Mathematical Functions Available

Tip 6: Start 16-Bit Applications before Starting ANSYS under Windows NT

Tip 7: Running ANSYS at Low Priority under Windows NT 4.0

Tip 8: Operating on (Scaling) Loads

Tip 9: Ramping Loads Down to Zero

Tip 10: Starting ANSYS Graphs at t=0

Tip 11: Pressure on Lines

Tip 12: Ramping Some Loads, Not Others

Tip 13: Force and Pressure on Flat Plates or Flat Shells

Tip 14: Linear and Nonlinear Buckling

Tip 15: Nonlinear Analysis and the Arc-Length Method

Tip 16: Animating Results from a Nonlinear or Other Analysis

Tip 17: Getting the Mass or Weight of a Model

Tip 18: Using Fnc Calls from Macros

Tip 19: Use ENSYM and ENORM to Turn Over Shell Elements

Tip 20: Shell Types to Try

Tip 21: Moving a Model from ANSYS Mechanical to ANSYS Linear/Plus

Tip 22: Deleting Nodes with Nodal Coupling

Tip 23: Convergence with Shell Finite Element Models in Nonlinear Analysis under ANSYS

Tip 24: Working with Load Step Files in ANSYS

Tip 25: Plotting Shell Stress -- Surface, Mid-Plane Stress, Load Paths, ESYS and RSYS

Tip 26: Nodal Coupling (CP) versus Rigid Region (CERIG)

Tip 27: Vibration Modes with Pre-stress

Tip 28: Creating New Elements by Copying or Reflecting Existing Structure

Tip 29: Adding to a Model Comprised of Elements and Nodes Only

Tip 30: Zero Mass Beam Elements Form Rigid Region

Tip 31: Turn off Symbols When Changing a Model after Solution

Tip 32: Are the "Free-Free" Vibration Modes Relevant?

Tip 33: Selecting a CAD or FEA System -- Cover Yourself

Tip 34: Creating Lines Perpendicular to, or at Angle to Existing Lines

Tip 35: Use the /UI command in Your ANSYS Toolbar to Bring up GUI Dialog Boxes

Tip 36: Reaction Force, Nodal Force, and Load Paths

Tip 37: Inputting Temperatures with BF, BFE, and TUNIF in Structural Analysis

Tip 38: ANSYS Toolbar Use

Tip 39: ANSYS Piping Element Lengths

Tip 40: Graphical Output from ANSYS

Tip 41: Check Nodal Loads at Bolts, Rivets, Spot Welds and Links

Tip 42: Use QUERY to Check Results with Picking

Tip 43: Loads on Geometric Entities Overwrite Loads on Nodes and Elements -- Easy Error to Make

Tip 44: Use Components for Load Input, and for Results Review

Tip 45: Simple Substructuring Examples -- Bottom Up and Top Down

Tip 46: Plot Applied Temperatures

Tip 47: Skipping Over Statements in an Input File

Tip 48: Static Analysis Followed by Transient Analysis

Tip 49: File Compression for Model Storage

Tip 50: Organizing Large FEA Models

Tip 51: Selecting Nodes in a Stress or Strain Range

Tip 52: Selecting Nodes that are Subjected to Nodal Coupling

Tip 53: /NOPR and /GOPR Speed Up Input Files and Macros

Tip 54: Using Commands IMMED and /UIS and /SHOW,OFF

Tip 55: What's the Bauschinger Effect? Comments on Material Yield

Tip 56: Thought Experiments

Tip 57: Control of Meshing

Tip 58: Four View Plot

Tip 59: Quick Review of Mode Shapes

Tip 60: Using ANSYS Help

Tip 61: The FEA Job Hunt

Tip 62: *VPUT and DESOL

Tip 63: How to Divide One Element Table Column by Another

Tip 64: Element Tables (ETABLE) and Arrays -- An Example

Tip 65: Error Estimation, PowerGraphics, and ERNORM

Tip 66: Concatenate and Mesh Last

Tip 67: ANSYS Output of Data to Files for Use by Other Programs

Tip 68: Writing Array Columns to Output or to Files

Tip 69: Synthesizing Parameter Names and Manipulating Jobnames and Long Strings in APDL


2 of 54 1/17/2009 11:41 AM

Tip 70: Solid Elements 95 and 92 -- Efficiency and Interconnection

Tip 1: Use Annota�ons:

Only a one‐line �tle is possible on the ANSYS screen or plot. Considerably more informa�on can be included in

annota�ons on the screen. The annota�ons are kept through all plots un�l they are deleted with the command:

/ANNOT,DELE or via picking with the graphical user interface (GUI).

At the top of the Annota�on dialog box, there is a list box from which the user can choose Text, Lines, etc., on

down to Controls. These selec�ons bring up different menus. The Controls selec�on offers a SNAP seEng that

makes it much easier to get the text aligned nicely. (Hint: ANSYS, Inc. should put this SNAP selec�on up front

under Text, or even on every menu.) Ac�vate the Snap seEng, then go back to Text to enter the annota�ons.

Tip 2: Making Room for Annota�ons:

The /PLOPTS command controls what goes into the legend at the right (by default) side of the ANSYS screen and

plot. If you turn off LEG2 (the rela�vely useless "view" informa�on), you will get extra room at the bo5om of the

legend. This area can be used for annota�ons if the number of contour levels in stress plots is not too great (the

default is fine).

Tip 3: Using Parameters in Annota�ons:

Just as in a �tle created with the command /TITLE, ANSYS permits the use of a parameter in an annota�on, as

discussed in the Commands Manual descrip�on of the /TLABLE command. When typing the annota�on using the

GUI, include the parameter in percent signs like this: %pname% where pname is the parameter name. The

parameter can contain either numbers or text. The value of the parameter will be plo5ed in the annota�on

string. The ANSYS func�on NINT can be used to round a number the nearest integer, some�mes improving the

appearance of the annota�on for large numbers in which the frac�onal part is irrelevant (e.g. NINT(123.456789)

= 123 ). For this, the parametric expression should be enclosed in percent signs. Annota�ons are usually created

in the GUI, but can be entered with code like that shown below. Entering a single annota�on line containing

Result = %pname% generates log file contents such as:

! The following commands place an annotation on the screen.! For information only. Use at your own risk.! In this example, "pname" is a parameter with a numerical value such as 123.456789/ANUM ,0, 1, 1.2303, -.74699 /TSPEC, 15, .600, 1, 0, 0/TLAB, 1.010, -.747,Result = %pname%

The last line in the above example contains the string that the user types manually. The other data set up the

string posi�oning on the screen, and the proper�es of the characters. To apply the NINT func�on to the

parameter, manually enter Result = %NINT(pname)% as the annota�on:

! For information only. Use at your own risk.! Type the annotation in one line, so the log file contains:/ANUM ,0, 1, 1.2303, -.74699 /TSPEC, 15, .600, 1, 0, 0/TLAB, 1.010, -.747,Result = %NINT(pname)%

The beauty of doing this is that if the value of the parameter pname should change, then when the next plot

command is executed, the annota�on will automa�cally update to reflect the new value! Try it: a2er crea�ng an

annota�on on the screen that includes a parameter, change the parameter's value, then do a /REPLOT. Running

a macro could get informa�on that goes into the parameter that a /REPLOT will automa�cally put it on the

screen. This makes it possible to automa�cally include far more informa�on than can go into the �tle, and to do

it for a series of automa�cally generated plots or graphs.

Tip 4: Use Small Annota�ons:

The default character size seEng for an annota�on is 1. The size of an annota�on can be decreased using the

GUI. A size of 0.6 is quite readable and permits far more informa�on to be packed into a plot. Note that there is a

limit to the number of characters possible on an annota�on line – this is character size independent.


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Tip 5: Mathema�cal Func�ons Available:

Under the Help lis�ng for the *SET command there appears lists of mathema�cal func�ons available in ANSYS.

Another list is in the ANSYS User's Guide on APDL, Chapter 14 of the Modeling and Meshing Guide. The

commands are usable anywhere. They include:

ABS(X) Absolute value

ACOS(X) ArcCosine

ASIN(X) ArcSin

ATAN(X) ArcTangent

ATAN2(X,Y) ArcTangent of (Y/X) with the sign of each component considered (see a

FORTRAN manual if you don't know what this means.)

COS(X) Cosine

COSH(X) Hyperbolic cosine

EXP(X) Exponen�al

GDIS(X,Y) Random sample of Gaussian distribu�ons where X is the mean, and Y is the

standard devia�on. Might be used in a Monte Carlo Simula�on to explore

the distribu�on of outputs based on randomized loadings and material

proper�es. For an explana�on, see a good modern engineering design

textbook.

LOG(X) Natural log (to base e)

LOG10(X) Log (to base 10)

MOD(X,Y) Modulus (X/Y), it returns the remainder of X/Y. If Y=0, returns zero (0)

NINT(X) Nearest integer (nice for outputs of stresses to /TITLE or annota�ons (see Tip

3 above))

RAND(X,Y) Random number, where X is the lower bound, and Y is the upper bound.

(Useful for Monte Carlo Simula�on, etc.)

SIGN(X,Y) Absolute value of X with sign of Y. Y=0 results in posi�ve sign.

SIN(X) Sine

SINH(X) Hyperbolic sine

SQRT(X) Square root.

TAN(X) Tangent

TANH(X) Hyperbolic Tangent

Note:

The func�on form of the *GET commands can also be used to get informa�on from the model ‐‐ see the

APDL guide men�oned above for a lis�ng of available func�ons. The APDL guide also gives func�ons to

retrieve the values of parameters, both numerical and character. The *VFUN command has a list of

func�ons that act on an array entry. The Commands manual lists func�ons that act on Element Tables in

the sec�on "POST1 Command for Element Table". Crea�vely used, the array and ETABLE algebra


4 of 54 1/17/2009 11:41 AM

SeEng Process Priority in NT

commands can be surprisingly powerful.

Tip 6: Start 16‐Bit Applica�ons before Star�ng ANSYS under Windows NT:

It has been my experience that some large commercial 16‐bit applica�ons will not start properly when ANSYS is

already running. If you start them before launching ANSYS, there will be no problem. If you intend to work with

those 16 bit applica�ons in the foreground while the ANSYS SOLVE is running in the background, this will be a

useful �p. I have seen other applica�ons start up very slowly (e.g. Internet Explorer) or wait un�l ANSYS was done

before proceeding (setup.exe for many Windows install programs).

Tip 7: Running ANSYS at Low Priority under Windows NT 4.0:

Under Windows NT 4.0 the priority

level of individual processes can be

user‐adjusted. To do this, bring up

the Task Manager (right click on the

Windows NT taskbar), and click the

tab for "Processes". Right click on the

process �tled "ANSYS.EXE", and "Set

Priority >" comes up. Set the priority

to "Low" to help make foreground

applica�ons run more smoothly

while ANSYS is running SOLVE in the

background. This may help more if

you have a large RAM in the

computer.

When ANSYS has completed the

SOLVE process, return the priority to

"Normal" so that ANSYS is not

slowed down when you start doing

plots through the GUI.

Tip 8: Opera�ng on (Scaling) Loads:

You can operate on loads on nodes and elements in order to scale them up or down. Unfortunately, scaling loads

on geometric en��es (keypoints, lines, areas and volumes) seems not to be available. If any load on your

structure has been applied to a geometric en�ty, rather than directly to elements or nodes, that load will be

transferred to the elements and nodes at solu�on �me. The transfer will overwrite any scaling of loads that you

have applied. (Guess how I figured this out!)

So what can you do about this? Method 1: Transfer the loading from geometric en��es to the elements and

nodes, then write a load step file. This records loading on elements and nodes. Delete the loading on geometric

en��es, then read the load step file that was just wri5en. Now the loading can be scaled up or down freely.

Method 2: For a faster method, see the "LSCLEAR,SOLID" command, which will not require wri�ng a load step

file. Method 3: Transfer the loading from geometric en��es to the elements and nodes, then delete the

rela�onship between geometry and the FEA mesh with the MODMSH,DETA command. Method 4: Transfer

loading from geometric en��es to the elements and nodes, then un‐select the geometric en��es, before

execu�ng SOLVE. The element and node loading can be scaled a2er it has been transferred from geometric

en��es. An un‐selected geometric en�ty will not transfer its loading to elements or nodes when SOLVE is

executed. Warnings: Method 3 ruins the rela�onship between geometry and the mesh. Save the model under an

appropriate file name before execu�ng MODMSH,DETA. Method 4 is fine, as long as you do not forget and

re‐select the geometric en��es ‐‐ ALLSEL will do this.

Scaling displacements (nodal constraint values) is also possible. One thing that has not worked for me is an

a5empt to reduce applied displacements to zero by using 0.0 as the scaling factor. What did work for me was to

use "_TINY" as a value, which mul�plied displacements by a factor of roughly 10^(‐31) and reduced loads to


5 of 54 1/17/2009 11:41 AM

virtually zero. A5empts to use 0.0 as the factor resulted in NO change to the applied displacements.

Tip 9: Ramping Loads Down to Zero:

If you are ramping force, pressure, and accelera�on loads up and down as part of an analysis, you may want to

return loads to zero. I do this when I want to inspect permanent deforma�on that results from plas�c yielding. If

you delete the applied load, the loading will drop immediately to zero, even if you have load ramping turned on.

The thing to do is to set the loading to virtually zero or the scaling factor to virtually zero, not delete the load. It is

important to appreciate that to ANSYS, reducing a load to nearly zero is not the same thing as dele+ng it (zeroing it),

for the purposes of ramping loads. The �me substep sizes to use will depend on your model.

SeEng displacements to zero or near zero is, of course, very different from dele�ng constraints.

Tip 10: Star�ng ANSYS Graphs at t=0

Graphs start at the first data point, which means that if you do a �me‐history trace, you don't get a t=0 data

point. If you leave �me as 0.0 on the TIME command, you get the default 1.0 in your output. The only way to get

a graph from zero that I have found is to do a first load step with "t" extremely small, in comparison to other

�mes in the analysis, e.g. t=0.0000001. The load at this �me must be appropriate so that the response ramps up

correctly. (If your intent was to ramp up from zero load, just leave the loads as zero.) The next load step con�nues

as usual.

Tip 11: Pressure on Lines:

Applying pressure on a line results in loads being applied to the nodes associated with that line. The loads on the

nodes that the FEA program applies will be appropriate given the formula�on of the elements. If you want to

apply a total force to the line, you can use a *GET command to find the length of the line, then divide the force

by the length and use the result as the pressure.

Note that pressure on a line acts in the plane of the area that is a5ached to the line. If two areas are a5ached at

90 degrees or another angle, two loads are set up, ac�ng in each of the area plane direc�ons. You can use select

logic on the areas to get some interes�ng effects as to the direc�on in which the applied forces act, but only if

both areas are meshed, and the elements are selected. If you un‐select one of the areas, pressure on the line will

only be exerted in the direc�on of the area that is selected. The select logic must s�ll be in place when you

SOLVE, or else your carefully cra2ed load case can be overwri5en. As above, transferring loads from geometric

en��es to nodes and elements, wri�ng them as a load step, dele�ng all the loads on geometric en��es, and

reading in the load step will protect your load case, and make scaling the loads possible. Alterna�vely, consider

the "LSCLEAR,SOLID" command.

NOTE: Pressures on surfaces follow the deformed shape during a Large Displacement (geometrically nonlinear)

analysis. Forces on nodes maintain their orienta�on in space, even under Large Displacement. This difference

will govern how loads should be applied in some models.

Tip 12: Ramping Some Loads, Not Others:

To hold some loads constant and ramp up or down others, run a first load step with all the loads at their star�ng

values, ramping from zero only if appropriate.

If you want, use an extremely small value on the TIME command, e.g. 0.0000001, and run this as a first load step.

Then set up a second load step, with ramping ac�vated. Change those loads to be ramped from their star�ng

values to new values. Hold the other loads constant. The TIME command can be used with a new value, such as

1.0.

An example is the applica�on of a gravity load before other loads are to be ramped up from zero. In some cases,

this could give a more realis�c assessment of nonlinear buckling caused by applied forces other than gravity

loading. (You will want to check the codes that regulate your design work before deciding on this. Codes that I

have seen were generally started before FEA was widely available, and do not address this concept. Find out

what is considered good prac�ce in your industry.) Applying gravity first can give much be5er convergence when

assessing the effect of thermal expansion moving structures across fric�on contact elements, where the normal


6 of 54 1/17/2009 11:41 AM

load on the contact elements is caused by gravity.

I suspect that this is not possible with the Arc‐Length method. I have not experimented with it, but do not see

how controlled ramping of only some loads could be implemented under Arc‐Length control of applied loading ‐‐

any opinions?

Tip 13: Force and Pressure on Flat Plates or Flat Shells

There is a rule of thumb, that if the out‐of‐plane deflec�on of a flat plate or shell is greater than half the

thickness, then membrane forces start to become significant in resis�ng the applied load. In ANSYS, this calls for

ac�va�ng a Large Displacement solu�on (a.k.a. geometric nonlinearity). Ignoring this can result in your design

missing out on inherent strength, OR in grossly inadequate underdesign. Know what you are doing.

Tip 14: Linear and Nonlinear Buckling:

Linear eigenvalue (classical Euler) buckling is a "quick" check on a structure, but the ANSYS manuals go to

considerable pains to point out that in many situa�ons, a Large Displacement solu�on (geometric nonlinearity)

needs to be run also as a check on the buckling adequacy of a design. As with linear buckling, nonlinear buckling

may need to be assessed with respect to a number of load cases. In some structures, a diagonal tension field is

developed in a web, and elas�c buckling failure does not develop at the first eigenvalues predicted. In other

structures, buckling failure may occur before the first eigenvalue, and only nonlinear analysis will predict this.

Linear eigenvalue buckling has to assume that gap and contact elements are either closed and ac�ve, or open

and inac�ve. Nonlinear analysis will follow the effects of these elements as they go in and out of contact, when

the loading is applied.

A2er any Large Displacement nonlinear elas�c buckling analysis (if it doesn't diverge), see whether the elas�c

stress limits have been exceeded (this includes the surfaces of shell elements, and be careful that nodal

averaging does not hide anything). If significantly overstressed, the structure may not be adequate.

Combined bending and axial compression in a beam is a classic place where inadequacy in strength can be

predicted in FEA only by Large Displacement nonlinear analysis (i.e. a linear analysis says it is OK, but a nonlinear

analysis shows it is NOT). For some structures undergoing elas�c Large Displacement analysis without contact

and gap elements, the user may want to consider a Southwell plot.

If elas�c stress limits are exceeded in the Large Displacement model, it may be desirable to do a combined Large

Displacement and Plas�c Deforma�on model. If the structure is overloaded, it may begin to collapse (perhaps

only locally), and the Arc‐Length method may be needed for convergence control. A need to strengthen the

structure may be predicted or iden�fied. The material proper�es to use are applica�on domain and industry

specific ‐‐ start by talking to your co‐workers, supervisor, and suppliers.

Tip 15: Nonlinear Analysis and the

Arc‐Length Method:

The basic way to do nonlinear analysis

in ANSYS is to use NR itera�on and

many default seEngs. At �mes,

convergence will become a problem;

I've encountered this with shell

structures under compressive stresses.

The arc‐length method can some�mes

cope be5er with nonlinear solu�ons,

because of its ability to follow force‐

deflec�on curves that rise and fall. Be

prepared for long run �mes if your

model is large.


7 of 54 1/17/2009 11:41 AM

My experience with the arc‐length

method is that in its default seEngs

for step size mul�pliers, it does not give sa�sfactory results when compressing some shell‐based models. What

may work is to set a number of �me substeps, such as 10, so that each substep is 1/10 of the load step. Set the

Arc‐Length maximum mul�plier MAXARC to 1.0 so that no substeps larger than 1/10 of the load step are taken.

Set the Arc‐Length minimum mul�plier MINARC to 0.1, so that the smallest load substep is 1/100 of the full load

step. I found this to help considerably. You may want to user a larger or smaller MINARC seEng, but my

experience to date suggests that one should not get greedy with MAXARC. Obviously, you may want to play with

the number of �me substeps.

The solu�on may s�ll diverge but it is likely that you will get more informa�on than without arc‐length analysis.

You will want to set a termina�on condi�on for the analysis if buckling is expected to result.

I find it desirable to save the

results at every �me substep

when doing this type of analysis

(it helps to have a large hard

drive) in order to review the

process. When you review the

results of a single load case run

under Arc‐length control, the

TIME value on the ANSYS plots

shows the decimal frac�on of the

full load being applied to the

model. As you move forward

through the plots, if the

load/displacement curve for the

structure is falling, the decimal

frac�on will fall, even though

some displacements are visibly

geEng larger.

As men�oned above, something

I have not tried is to get the

Arc‐length solu�on control to

ramp some loads and not others,

by having run a preliminary load

step. Is this even possible? If not, then the user may face the prospect of gravity being ramped up and down, in

addi�on to other applied loads, and the physical realism of the model may be affected.

Tip 16: Anima�ng Results from a Nonlinear or Other Analysis:

It can be helpful to watch the increasing stress levels that result as a nonlinear analysis loading is ramped up. To

create an anima�on, first run your analysis with loads ramped up, and a number of substeps. Have all substeps

wri5en to the results file. Do a stress plot of interest to set the type of stress plot to be animated by the macro

that will be run. Make the ANSYS Graphics window as small as you want the anima�on window to appear (most

screens will have lower resolu�on than a CAD worksta�on), keeping the aspect ra�o correct. Smaller graphics

windows result in smaller anima�on files, if size ma5ers. Anima�on files under Windows NT (AVI files) from

ANSYS o2en compress very well for storage purposes. Use the PlotCtrls menu selec�on on the U�lity Menu, and

choose Animate to get a sub‐menu of choices. Choose "Dynamic Results" to create an anima�on of your saved

load substep results with the �me shown in the legend. This seems to work only for the last load step (read the

ANSYS macro). The resul�ng AVI file can be viewed with the media player, distributed, put on a web site, and so

on. The media player can be stepped manually for slow viewing. It makes it easier to watch the changing stress

pa5ern or deforma�on as nonlinear effects take over the model.

In anima�ng a changing stress or other contour plot, you may wish to specify the contour levels before

genera�ng the anima�on file. View the load step or substep with the worst results as part of deciding where to

set the contour levels.


8 of 54 1/17/2009 11:41 AM

I have not found that any of the ANSYS supplied anima�on macros do the one simplest thing I want. Usually I

want to animate every substep of every load step stored in the results file. The following simple macro does this

for me under Windows NT. There is virtually no error checking in this macro. Note that this simple macro does

not update element table data at each frame. Consequently it will not work properly for plots of element table

data. If stresses, strains, or other data with amplitude informa�on are to be plo5ed, the user may want to fix the

contour map levels ahead of �me. The user will want to set the displacement amplitude scaling with /DSCALE in

advance‐‐automa�c scaling will not be sa�sfactory. In general, it may not be sa�sfactory to have /ZOOM,OFF

ac�ve, since the view will change if plots of significant deflec�on are included in the anima�on. Manually seEng

a view may yield a be5er anima�on. Modify this macro as you wish.

This macro must be called from within /POST1. The file that contains the results must have already been

selected, and a prototype plot command executed so that calling /REPLOT will generate the type of plot the user

wants:

! --------------------------------------------------------------------! MY_ANIM.MAC A quick-and-dirty animation of all of the substeps! --------------------------------------------------------------------! For information only. Use at your own risk.! User must indicate how many frames are to be animated! This macro starts with the first substep in the results file! by using the SET,FIRST command internally! User implicitly indicates how many times to use the SET,NEXT command.! The number of frames needed must exist in the RST file, else errors.! NOTE: This does NOT work for plots of data in an element table.! Plotting element table results would require a macro in which! the element table results are updated at each substep.!! Virtually NO Error Checking Is Performed ! ! ! ! !!! What will be plotted is based on /REPLOT therefore, on the last user plot executed! before this macro is called.! Scaling, etc. are all based on the last user plot. Only the SET value is updated.!! Call with:!! my_anim, time_delay_for_frame, number_of_frames_including_first!ar11=arg1*if,arg1,eq,0,then ar11=0.1*endif*if,arg2,ne,0, then /NOPR /gsav,xxx,gsav,,temp /seg,delete /seg,multi,,ar11 set,first /replot *do,_iii,1,arg2-1,1 set,next /replot *enddo /seg,off anim,1,1,ar11 /gres,xxx,gsav /gopr*endif

An alterna�ve to this macro could step through all substeps on the RST file by using a *GET command of the type

*GET,NTOTAL,ACTIVE,0,SOLU,NCMSS to check the number of substeps as the SET,NEXT command is issued. The

parameter NTOTAL will be re‐set to 1 when the anima�on is complete, and the *IF and *EXIT commands can

check this and break out of a do loop ‐‐ see Tip 59 below for the example of automa�cally ploEng all mode

shapes. The user would then not need to specify the number of substeps to plot, improving the automa�on, and

leEng the solver use variable substep sizes without the user having to check on the number of substeps that

resulted.

Tip 17: Ge;ng the Mass or Weight of a Model:


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A reader has been helpful by poin�ng point out that mass (or weight, depending on your units) of keypoints,

lines, areas, or volumes in a model can be retrieved, when a5ributes have been assigned to these en��es, by

using commands available in /PREP7. Using the graphical user interface, enter into "PreprocessorOperateCalc

Geometric Items" to see the choices: "Of Keypoints, Of Lines, Of Areas, Of Volumes, Of Geometry". These items

execute the "sum" commands: "KSUM, LSUM, ASUM, VSUM, GSUM" respec�vely. If no a5ributes have been

assigned to the geometric en��es, unit densi�es are assumed in repor�ng mass and center of gravity

informa�on. A2er the execu�on of these commands, the *GET command can be used to assign to a variable the

implied volume of an area (based on the thickness associated with its a5ributes) or the volume of a "volume".

The volume of a series of areas or "volumes" can also be retrieved with the *GET command a2er a "sum"

command is used. The *VGET command can also be used, where appropriate, in retrieving informa�on made

available a2er one of the "sum" commands is executed.

For some unstated reason, ANSYS will not directly give the total weight or mass of a model (retrieved from the

mass matrices of the elements), except to print it to output during the solu�on of a problem. The user can run a

par�al solve in order to get this weight or mass printed reasonably quickly. In Imperial units, it may be desirable

to convert between pounds mass and pounds weight. There is no *GET command that directly returns the weight

of selected elements. However, the volume of an element can be returned, and the volume of a set of elements

can be put into an element table, and summed.

You can get the weight of many models into a parameter by: step through all material types, selec�ng elements

for each material type. Get the volume of those elements, and mul�ply by the density of that material type. Sum

the masses or weights of all the material types. This will not include added mass and mass elements at nodes

(check this carefully against the output mass in the solve module) or other things that I may not have thought of.

Of course, you can get the weight (assuming you gave densi�es in the material defini�ons) by removing all loads

(don't let thermal expansion, nodal rigid region, nodal coupling, various gap and contact elements, or loads on

constrained nodes trip you up ‐‐ use the minimal constraints needed to stabilize all bodies in 3‐D), applying 1 g

ver�cal, having constraints on ver�cal mo�on, running SOLVE in a linear analysis, and finding the ver�cal

reac�on force. In such a run, a combina�on of the FSUM (select ver�cally restrained nodes only, with all a5ached

elements) and *GET commands in /POST1 might help you to get the weight into a variable. However, a par�al

solve will give the answer more quickly (but not put it into a variable). Depending on your system of units,

remember, you may want to convert between weight and mass.

I base my comment, about the inability of ANSYS to directly return the weight of the model with *GET, on

comments in the manuals on Op�miza�on. The op�miza�on examples work to reduce model volume, not

weight.

Tip 18: Using Fnc Calls from Macros:

Before using macros for the first �me, read about the *USE command in the ANSYS Help manual, in addi�on to

other relevant parts of the ANSYS manuals. The *USE command help discusses the macro calling parameters and

their local scope. Note a slight difference in calling parameters AR19 and AR20 when the *USE form of a macro

call is used, versus the "unknown command" form.

There are �mes when calls from macros directly to the Func�on form of an ANSYS command will be the only way

to get the func�on called with picking. It may be desirable to sent the user a message that explains why the

picking has been requested. The func�on must be called with the exact use of upper case and lower case

characters. An example: Fnc_ENSYM will work, whereas fnc_ENSYM will not, because the capital F is missing.

Tip 19: Use ENSYM and ENORM to Turn Over Shell Elements:

ANSYS has two commands, ENSYM and ENORM, for re‐orien�ng shell elements so that a set of shell elements can

all have their "top" surface face the same way. This makes applica�on of pressure, contact elements, and review

of results more feasible. This orienta�on should be done before running SOLVE; the results are not re‐oriented in

the database when these commands are applied, nor in the results file, so if the elements are re‐oriented a2er

SOLVE, the stress results will no longer apply to the correct shell surfaces and a meaningless mess will result.

These commands work with shell elements that are a5ached to areas, as well as with independent shell

elements. Note: If you clear the elements a5ached to an area, then re‐mesh, the new elements will have the

same orienta�on as the area. (Hint: ANSYS ought to do this re‐orienta�on for Areas, making it easier to pressurize


10 of 54 1/17/2009 11:41 AM

the interiors of containers defined with shell elements.)

See HELP,ENSYM for informa�on on what this command will do. ENSYM can be used to "flip over" a shell

element so that the opposite side (Top or Bo5om) is showing. To do this would require reversing the node order

in the database so that Face 1 (Bo5om) and Face2 (Top) get switched.

For more powerful capabili�es in re‐orien�ng shell elements, see HELP,ENORM. This command will search

outward from a chosen element that the user considers "correct", re‐orien�ng a connected set of shell elements

so that they face the "same way" (this takes some interpreta�on), even working around corners. It searches

elements from the selected set of elements, un�l it hits the edge of the model, or un�l two or more elements are

a5ached to one element edge. The user should experiment with this command in order to understand exactly

what it does, and inspect the model thoroughly a2er ENORM is applied, to verify that the results are as desired.

The correct use of ENORM can make the applica�on of pressure or contact elements to a complex model

substan�ally easier.

It would be very helpful if ANSYS had a special command that would plot shell elements with the sides colored

according to whether they were FACE1 or FACE2 of the element. This command could be extended to color the

(up to) six sides of solid elements, according to their face number. A similar command for ploEng areas would

help, too. It could even be done for beams displayed with /ESHAPE showing the outer envelope. At present, with

ANSYS 5.3 running on Windows NT, I get different colors for Face 1 and Face 2 of shell elements when

PowerGraphics is ON, and "No Numbering" plus "Colors" or "Colors and Numbers" has been chosen under

PlotCtrls,Numbering. I have not seen this documented. This does not happen for areas, or for solid elements.

Tip 20: Shell Types to Try:

I have used Shell 63 (for Elas�c), Shell 43 (for Plas�c), Shell 93 (8‐Node, for Elas�c & Plas�c), Shell 143 (for Plas�c),

and Shell 181 (for Plas�c). The Revision 5.4 for ANSYS will include a bug fix for a Shell 181 problem. Shell 143 is no

longer supported, but is s�ll embedded (hidden) in Revision 5.3 of ANSYS for compa�bility reasons.

I have recently found Shell 93 to be useful in modeling some curved structures, because of its ability to follow

curved surfaces. (Shell 63 elements are flat, and can make a mess of a general curved surface under free

meshing.) Shell 93 gave me good convergence for both elas�c and plas�c Large Displacement (nonlinear

geometric) analysis. It does not like to follow too large an angle of curvature with one element, so the number of

elements on an area fillet can be large. Set the angle subtended by Shell 93 elements during meshing to a value

that is small enough to avoid warning messages. Watch out for aspect ra�o warnings. (Lack of warnings is not a

complete guarantee of acceptable element shapes.) If the structure has pressurized flat surfaces, Shell 93 o2en

converges be5er when stress s�ffening is ac�vated for Large Displacement analysis. Stress s�ffening for Shell 93 is

ac�vated at the solu�on phase of the analysis, whereas Shell 63 is (apparently) only stress‐s�ffened by seEng

one of the KEYOPT values. (I have obtained different Large Displacement convergences with Shell 63 with no

stress‐s�ffening set, with the KEYOPT stress‐s�ffening set, with stress‐s�ffening set in /SOLU, and with stress‐

s�ffening set in both places.) Like Shell 63, Shell 93 also has the virtue of being supported by the Linear/Plus

version of ANSYS for Large Displacement elas�c analysis, so models can be moved back and forth.

When forcing mapped meshing of curve‐sided Shell 93 elements on a plane area by concatena�ng perimeter

lines, I have occasionally had mid‐side nodes created, in the interior of the area, such that there was too much

element curvature distor�on in the plane of the element. One fix is to have the elements created with the sides

straight, which is tolerable if the elements are flat, and if it does not cause trouble on the perimeter of the plane

area being meshed. "Trouble" here means poor representa�on of curved boundaries‐‐other elements on these

boundaries may need to curve to follow curved surfaces, or it may be desired to have a curved fit to an outside

edge. If flat element sides cause trouble on the perimeter, then start by meshing areas on the other side of the

perimeter with elements that have curved sides‐‐these elements could even be triangular. Next, mesh the area of

interest with the elements sides set straight, then clear the surrounding areas, if the surrounding areas are not

intended to be meshed, or need be5er element shape control. This will leave the plane area of interest meshed

with elements that have straight edges in the interior, and curved edges on the perimeter. This is illustrated by

the following images of an inten�onally extreme example. In the first image, a line plot of element edges shows

extreme distor�on in the plane. An intended hole is meshed with triangles. All these elements are Shell 93,

having mid‐side nodes.


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In the second image, meshing with mid‐side nodes posi�oned on straight lines is being chosen.

In the third image, the consequence of meshing the part with straight‐sided elements is shown. The elements at

the hole have a curved side, because the hole is already meshed with curved‐sided elements.

In the fourth image, the elements bordering the hole are shown, a2er the hole has been cleared of elements. The

element curvature at the hole is visible. The interior of the plane area is meshed with straight‐sided elements.

The same problem and a similar fix can be encountered with mid‐side noded SOLID95 brick elements that have

20 nodes. The surface areas of a volume can be meshed with 8‐node SHELL93 elements with curvature, then the

volume meshed with SOLID95 elements with the sides straight, then the shell elements on the areas removed

with the ACLEAR command. This will leave the volume meshed with SOLID95 elements that are curved on the


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surface areas, but with straight sides in the interior. There are rare occasions when this will eliminate element

distor�on warning messages.

Tip 21: Moving a Model from ANSYS Mechanical to ANSYS Linear/Plus:

Because versions of ANSYS sell for different prices, a company may own one version to be used for nonlinear

models, and several licenses for linear work, or just for model crea�on and results review. Occasionally, a model

will be moved "down" from a fuller version of ANSYS to the Linear/Plus version.

A user can run into difficulty moving a model from ANSYS Mechanical (or ANSYS Structural, etc.) to the less

expensive ANSYS Linear/Plus. The Linear/Plus version limits the number of nodes allowed. Unfortunately, it

implements this control by not allowing node numbers that exceed a limi�ng value. This means that compression

of node numbers (and element numbers) may be required in order to get larger models to be accepted by ANSYS

Linear/Plus. Otherwise, the program quits without an opportunity to compress the numbering (more recent

ANSYS versions may be more tolerant, but the numbering will have to be compressed at some point).

When the node and element numbers are compressed, coordina�on of loading with the numbering expressed in

load step files is lost. The way around this that I have used is to read in the original database, read in a load step

file, compress the numbering, and write the load step file. The process, reading in the original database, must be

repeated for every load case (a macro could be wri5en to automate this.) Finally, the original database is read in,

numbering is compressed, and the new database is wri5en.

Unsupported element types cannot be used in ANSYS Linear/Plus; neither can too large a wavefront (can the PCG

solver get around this?). The unsupported elements need to be deleted or changed before moving the model

(e.g. change SHELL181 to SHELL63). Then, if the number of en��es does not exceed ANSYS Linear/Plus limita�ons,

the database can be moved to the other program.

The next problem in moving models to ANSYS Linear/Plus, is that nonlinear material models must be deleted in

ANSYS Mechanical (Structural, etc...) before moving the database to ANSYS Linear/Plus. This is because the

ANSYS Linear/Plus program will complain that the material nonlinearity is included, but not accept the

commands to delete it (Hint: ANSYS should add this delete func�on to Linear/Plus.) Of course, I found all this out

the hard way.

On rare occasions, a model from a more recent version of ANSYS may be moved back to an earlier ANSYS version.

If IGES is not sa�sfactory, a user could use CDWRITE to write out the element and node model and other model

data to a file (the DB op�on), then manually clean up the file so that the earlier version of ANSYS could accept it.


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This includes modifying commands for element crea�on, a2er deducing what format is needed. Wri�ng the

element data with EWRITE then cuEng and pas�ng with the CDWRITE file may be easier ‐‐ I haven't tried it. A

user‐wri5en program can expedite cleanup for a large model.

Tip 22: Dele�ng Nodes that have Nodal Coupling:

When dele�ng a set of nodes for which some were members of coupled node sets, delete the coupling equa�ons

BEFORE dele�ng the nodes. Otherwise, unwanted coupling equa�ons may be ac�ve if you create more nodes.

The coupling equa�ons are not automa�cally deleted when the nodes are deleted‐‐is this a bug? Select the

nodes to be deleted, then delete node coupling equa�ons for which any nodes are selected, then delete the

nodes. (You will have had to first delete the elements.) Clearing solid model en��es is the same as dele�ng the

elements and nodes simultaneously.

I find it very helpful to turn on the symbols for nodal coupling when checking for proper use of these details.

Tip 23: Convergence with Shell FEA Models in Nonlinear Analysis under ANSYS:

First, remember that there are three basic kinds of nonlinearity: (1) Large Displacement (geometrically nonlinear)

analysis, and (2) Plas�c Material proper�es are the obvious types. In addi�on, nonlinear solu�ons occur (3) when

nonlinear elements such as gap elements, hook elements, and surface contact elements are used. Because of (3)

it is clearer to refer to a "linear" analysis as "small displacement elas�c", since "linear" may be perceived as

meaning that there are no nonlinear elements present. A nonlinear analysis will take longer, usually

considerably longer, than a linear analysis. For a large finite element model, it helps to have a computer with an

extremely fast CPU, large RAM, large hard drive, and fast hard drive data transfer (high‐speed SCSI may help on

PC's) for nonlinear analysis.

In ANSYS, the Shell 63 element will do Large Displacement, but is NOT capable of material nonlinearity

(plas�city). Shell 43, Shell 143, and Shell 181 are capable of both Large Displacement and material nonlinearity.

These four elements are 4‐node quad elements. ANSYS also has an 8‐node shell element, Shell 93. The Shell 93

element is capable of both Large Displacement and material nonlinearity. Shell 93 has the advantage that it can

follow a curved surface. There are also shell elements for composite materials and for P‐element solu�ons. I will

restrict my comments to the basic shell elements: 63, 43, 143, 181, and 93.

The elements should have acceptable aspect ra�os, not be ridiculously large or small, not be pathologically

deformed, and not generate warnings about being warped. If warped quad elements are unavoidable during

meshing, it may be desirable to use either small triangles, or the Shell 93 element. Note that within the ANSYS

manuals, high order elements are not considered to be ideal for nonlinear work. However, I seem to have had

some success with the Shell 93 element (can't say if the results were ideally accurate). You can evaluate the

model quickly by doing a par�al solve (Par�al Solu in the GUI), only genera�ng the element matrices, and geEng

warnings (if any) and other informa�on in the ANSYS Output window.

If a Large Displacement solu�on is chosen, some solu�ons are improved by seEng Stress S�ffening before

running the solve process. Stress s�ffening for elements 63, 43, 143, and 181 can (apparently) only be set with

one of the KEYOPT values (Keyopt(2)) for the element (see Op�ons when using Add/Edit/Delete to add element

types with the GUI). Some beam elements are like this, too. It apparently (I find the manuals difficult to interpret

on this) can NOT be set within the Solve module, even though the GUI has a selec�on box for Stress S�ffening.

However, I seem to have had convergence differences with Shell 63 with stress‐s�ffening set and not set in the

solve module. For Shell 93, stress s�ffening IS set within the Solve module, by choosing it under Analysis Op�ons

in the GUI (SSTIF). The use of stress s�ffening for convergence improvement is contraindicated by some

condi�ons such as the substan�al use of nodal coupling or nodal constraint equa�ons... see the ANSYS manuals

on this. Note that SSTIF is NOT the same thing as the command PSTRES.

A second thing that helps many nonlinear solu�ons (both Large Displacement and plas�c) to converge when

substeps are being used is to ac�vate the Predictor (PRED) in the Solve module. (This may be more of a hindrance

than a help when gap and other nonlinear elements will be changing status frequently.)

There are other seEngs that can be tried when a5empts at convergence are not working. I usually s�ck to leEng

the program decide how to use Newton‐Raphson itera�on and adap�ve descent in the Solve module. Under the

Nonlinear seEngs of the GUI, the user can modify the Convergence Criteria. I o2en use only convergence on


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forces (not moments) when analyzing shells if I am not inpuEng any moments directly. I usually reduce the

number of Equilibrium Itera�ons to 15 when doing shell models, preferring to use smaller substeps instead.

However, in a model with gap or contact elements it may be desirable to have a much larger number of

Equilibrium Itera�ons. I rarely try Line Search.

Making a good choice of �me substep sizes is cri�cally important in geEng models to converge. If shell models of

flat plates subjected to pressure or perpendicular forces are included in the analysis, the shell will at first act as a

flat plate in bending. Once the shell has curved, by movement as small as half its thickness, the shell will start to

carry the applied load with membrane forces. In a model of this type, star�ng with very small substeps (e.g. 1/100

of the full load) may be needed to achieve convergence. I would start with a very small first substep, but allow

the largest substep to be as large a frac�on as 1/4 of the applied load. If there are no perpendicular loads but the

loading is causing Large Displacement, or if buckling is to be considered, it is likely that small �mesteps will be

needed toward the end of the force applica�on ramp. Where there is no pressure or perpendicular force on flat

shells, I would start with a substep such as 1/10 or 1/4 of the applied load, but allow a minimum substep as small

as 1/100 of the full load. If these approaches will not work, it is likely that convergence control commands in

addi�on to �me substep size will need considera�on.

If the structure is buckling or undergoing plas�c failure, or "simply will not converge" it may help to use the

Arc‐Length method. As I have noted elsewhere, I don't use the default Arc‐Length seEngs. I usually start with a

number of substeps (NSUBST), and don't let the Arc‐Length solver increase the size of a step beyond my

maximum substep size. I let the Arc‐Length solver use a minimum step size that is 1/10 or 1/100 of my substep

size. I let the Arc‐Length solver use a maximum step size mul�plier of one. The Arc‐Length method can follow a

rising and falling force‐displacement rela�onship. I find PlotCtrls/Animate/Dynamic_Results to be useful in

reviewing the behavior during an Arc‐Length analysis, and other nonlinear analyses. I prefer to save the results at

every substep when doing this (Output Ctrls). When using Arc‐Length analysis, it is usually desirable to set a

criterion to stop an analysis (NCNV). I usually use maximum displacement as the criterion for shell work.

Remember to ramp up your loads, permit automa�c �me stepping, and in the NSUBST command, allow the

program to bisec�on by seEng the maximum number of substeps greater than the minimum number of

substeps.

If you are having trouble with convergence, save the results at intermediate substeps so you can review the

stress and displacements. If you are doing combined Large Displacement and plas�c deforma�on, and having

trouble with convergence, consider a study in which you do (1) an elas�c small displacement analysis as a check

on element shape, loading, and constraints, (2) a Large Displacement elas�c solu�on, and possibly (3) a plas�c

small displacement solu�on. If these work without significant warning messages, you should be making some

progress. If gap or contact elements are being used, consider (4) so2ening the normal and tangen�al s�ffness

values in a preliminary analysis (KN and KS). You can also (5) try relaxing the convergence criteria on force and/or

moment error. If desperate, a coarsely meshed model may improve speed enough for you to study what helps

get an answer. These preliminary studies may help you to find what seEngs help you to get convergence or

discover modeling problems before you do more �me‐consuming accurate analysis. If you are trying a new

technique, consider tes�ng it on a toy‐sized problem, before applying it to a large industrial‐sized problem that

runs for hours or days, in order to learn the peculiari�es and pi[alls of a par�cular �me‐consuming method.

If gap or contact elements are the only nonlineari�es in a model, consider substructuring the linear regions of the

model. This can result in a tremendous increase in solu�on speed. If only a sub‐region of a model will behave in a

nonlinear manner, it may reduce solu�on �me to substructure the region that can be regarded as ac�ng in a

linear manner. This speedup effect or may not occur with large displacement modeling, when the substructure

itself will be undergoing large displacement ‐‐ I have done only limited tes�ng of this technique. See below for a

brief discussion and for simple examples of substructuring.

Tip 24: Working with Load Step Files in ANSYS:

Load step files can be used to automate the applica�on of a number of different load cases on a structure. A load

step file contains loads on elements and nodes. It does NOT contain loads on geometric en��es. Consequently, a

load step file can be generated a2er all loads from geometric en��es have been transferred to a model. A2er all

loading on geometric en��es has been deleted, the load step file can be read back in, recovering all applied

loads. Alterna�vely, consider the "LSCLEAR,SOLID" command. These loads can then be scaled.


15 of 54 1/17/2009 11:41 AM

The user needs to be careful when manipula�ng load step files. The load step files may contain the KUSE

instruc�on telling ANSYS to re‐use the TRI file if the constraints have not changed. If the user deletes a load step

file, changes the order of their execu�on, or manually modifies their contents, invalid analysis might result.

If the model is re‐numbered a2er load step files are generated, the node and element numbers in the load step

file will no longer be synchronized with the model, and will be invalid. A way around this is men�oned elsewhere

in these notes (See Tip 21).

The reader should take note of the ANSYS user guides comments on the LSCLEAR command. This deletes all loads

and resets all load step op�ons to their defaults. This can "clean up" the load step data before using LSREAD to

read a load step file for modifica�on. What this implies is that the load step execu�on process does NOT execute

an LSCLEAR command when a load step file is read in. If it did, then ANSYS would have to implement substan�al

checking to see whether a TRI file was safe to re‐use, under the frontal solver (TRI file re‐use saves considerable

�me). Load step implementa�on can cause havoc when the user employs load step files in a manner for which

the method was not designed. It may help to read the contents of the LSSOLVE.MAC macro in predic�ng what

will happen, and to see what LSSOLVE does to avoid trouble. The LSSOLVE.MAC macro at ANSYS 5.3 includes

some undocumented commands including DMARK, FMARD, SMARK, BMARK, and a *GET command that

retrieves the error number in the /SOLU process. It also uses an "LSCLEAR,SOLID" command that removes loads

on geometric en��es before reading in load step files. It selects all DOF labels, sets xCUM labels to "replace", and

does a few other things. I do not consider the manuals to pursue this topic adequately ‐‐ a user ought to read the

macro.

The ANSYS manual comments on the LSREAD command. The command does NOT clear ALL current loads on the

model when it reads in a new load step file (it does clear some... read the manual).

When using load step files: If loads on nodes and elements are set with BF and BFE commands (for example

applying temperatures for a thermal deforma�on stress analysis), then if you set up a subsequent load step, if

these temperatures are to be returned to ambient it may be necessary to use the BF and BFE command to set the

nodes and elements to the reference temperature (by default 0) rather than just dele�ng the loads using BFDELE

and BFEDELE and using BFUNIF to input the uniform temperature. It may help to use commands such as

"nsel,s,bf,temp,‐999,99999" and "esel,s,bfe,temp,‐999,99999" to select all of the nodes or elements to which

temperatures have been applied, if you are going to change them. Be very careful with the BFE command. If you

set the value of the temperature at, for example, four loca�ons on an element with BFE, and in a later load step

set the value at only two loca�ons within an element, the temperature at the other two loca�ons will s�ll be

"hanging around" at the previous value. It is very easy to make this mistake when running a series of load step

files. (Another thing I found out the hard way, in a model where both piping crea�on commands and beam

elements were used.)

If the user is dele�ng displacement constraints using DDELE, and then wri�ng an addi�onal load step file, the old

constraint may s�ll be present when the series of load step files is read in under LSREAD; check for this in your

results. Be careful with this. It may compromise the use of load step files, or require some interven�on like

wri�ng an input file that calls load step files in using LSREAD, implemen�ng fix‐up commands as needed ‐‐ be

careful that a TRI file is not re‐used because a load step file contains "KUSE,1" when your changes to constraints

mean that a new TRI file should be generated. Statements in the LSSOLVE.MAC macro can provide guidance on

using LSREAD effec�vely. You may need to look inside the load step files with a text editor. Be warned that

changing the contents of load step files with a text editor can be tricky because of unintended side‐effects.

In general the user will have to be careful that the "residue" from the loads and displacements from one load

step do not appear inappropriately in later load steps. This is true when genera�ng the load step files in the first

place, and may apply when reading in load step files with LSREAD. As noted, LSSOLVE.MAC uses cleanup

statements.

The user will have to be careful to change loads between load steps in a manner consistent with geEng smooth

ramping of loads and displacements, for those cases when this is desired, either for transient analysis, or for good

nonlinear analysis convergence, or when intermediate results are desired at in‐between loads.

Before reading in load step files to solve with LSREAD, ensure that loads on geometric en��es and elements and

nodes have been deleted, unless you are keeping them inten�onally (as noted, loads on geometric en��es


16 of 54 1/17/2009 11:41 AM

overwrite loads on elements and nodes). As noted, LSSOLVE.MAC in ANSYS 5.3 contains the command

"LSCLEAR,SOLID" to remove the solid model loads on the model before proceeding.

If Large Displacement analysis is going to be used in analyses run by load step files, the NLGEOM flag must be set

in the first load step file. There will be no NLGEOM command generated in subsequent load step files. Because

ANSYS does not permit the kind of analysis to be changed when applying a series of load steps, error messages

will be result if the user changes the value of NLGEOM in the middle of a set of load step files.

Tip 25: Plo;ng Shell Stresses ‐‐ Surface, Mid‐Plane Stress, Load Paths, ESYS and RSYS:

In the ANSYS database, shell stresses

(and strains) for the basic shell

elements (63, 43, 143, 181, and 93)

are reported at the top and bo5om

surfaces of the shell element. The

user can has four op�ons in ANSYS

5.3 for ploEng shell stresses (and

strains). Three of them are selected

with the commands: "SHELL,TOP" ,

"SHELL,MID" or "SHELL,BOT". These will cause ploEng of shell stresses (and strains) to be based on the values at

the top surface, mid‐plane, or bo5om surface of each shell element. This is a bit misleading. The mid‐plane stress

is based on the average of the stresses at the top and bo5om (this may not be correct, at least for some elements,

considering Sec+on 2.3.4 of the Theory manual, which refers to stress on the mid‐plane of a shell element separately

from the top and bo2om, and forms the force per linear unit from a weighted average of top surface, mid‐plane, and

bo2om surface stress ‐‐ what's going on here?). What cons�tutes the top and bo5om of a shell element depends

on the element's orienta�on when it was defined (see elsewhere in these pages). It is possible to have adjacent

elements, one with a "top" surface poin�ng upward, and its neighbour with the "top" surface poin�ng

downward. In complex structures it happens all the �me. If nodal averaged plots are done, for example with

"PLNSOL,S,EQV", when either top surface or bo5om surface ploEng is chosen, then with such adjacent

elements, the plo5ed top surface and bo5om surface results will get blended, causing a misleading mess to be

displayed. (See Tip 19 for commands that can re‐orient shell elements.)

More insight into the flow of stress in a model can be gained by ploEng the stress vectors, using the "PLVECT,S"

command. With shells, these vectors will be plo5ed for the mid‐plane principle stress components. At �mes you

will want to use vector graphics with no hidden surface removal, to give the best view of these vectors. If there is

local compression, the vectors point inward. These vectors can give insight into load paths in a structure.

Where there are intersec�ons of planes of shell elements, e.g. corners or "Tee" intersec�ons, or where elements

of differing thickness meet, the averaging of node stresses can render local stress plot informa�on meaningless at

the intersec�on. This is true of both surface and mid‐plane stress plots. This is one way in which excessive

stresses will be uninten�onally missed.

Any �me that nodal averaged ploEng is done, it is possible for the averaging to "wash out" local stresses that

may be important, yet it is common to do nodal averaged plots because of their much cleaner appearance (I do

them myself). The fourth op�on in ploEng shell stresses is to switch on the ANSYS Powergraphics feature. This

causes shell results to be displayed, even averaged, for the visible surface. Op�ons ac�vated with the AVRES and

/EFACET commands can refine the way the results are plo5ed under Powergraphics (look them up).

Powergraphics has the op�ons to discon�nue the averaging of stress contours where there are certain

discon�nui�es in the material or geometry in the model. I'm going into this detail, because a high stress that is

washed out by nodal averaging could be a stress that causes serious fa�gue or other damage, such as cracking, or

a weld being torn apart.

The only shortcoming is that Powergraphics will not work with mid‐plane stress. The user has few op�ons here.

Some�mes it is important to select only regions of a model when doing nodal averaged mid‐plane stress plots

(using "SHELL,MID", without Powergraphics) so that the averaging does not wash anything out. A mid‐plane

stress plot without Powergraphics can be done for element stresses, using a command like "PLESOL,S,EQV". This

will look messy, but at least it doesn't hide an extreme stress. An alterna�ve I used is discussed elsewhere in

these pages: I wrote a macro to get the mid‐plane averaged stress (all components) at every node of every


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element (a given node has different results with reference to each of the elements to which it is a5ached, so a

given node will be looked up as many �mes as the number of elements to which it is a5ached), and transfer it to

the top and bo5om surfaces, so that Powergraphics would plot mid‐plane averaged stress neatly, with

discon�nui�es. CAUTION: This ruins the results database. The macro is extremely slow to run. The method

(under Powergraphics) does, however, give far be5er looking plots than using the "PLESOL,S,EQV" command to

plot mid‐plane element stresses without nodal averaging (without Powergraphics).

LOAD PATHS: The macro I men�on above could be modified to mul�ply the mid‐plane averaged stress

components by the local shell element thickness at each node. The resul�ng values would yield a contour plot of

force per linear inch (or other dimensional unit) "averaged" at the mid‐plane of the shell ‐‐ this could help to

make load paths visible in a complex shell structure. "PLVECT,S" plots that would now show arrows

corresponding to the load‐per‐unit‐length on the mid‐plane and show the principal direc�ons in which it points,

helping to illustrate the load paths. This macro would also ruin the database for any other use. Before ploEng

"load‐per‐unit‐length" data, the user needs to decide how to orient the results data coordinate systems with

RSYS for informa�on such as Sx or Sy that contains direc�on informa�on (stress and strain with EQV does not

contain direc�on informa�on).

Note: The Output Data sec�on on Shell63, Shell43, and Shell93 includes In‐plane element X, Y, and XY forces

called TX, TY, and TXY. Consequently, shell "force per unit length" data can be obtained directly in an Element

Table very quickly, though with a resolu�on of one value per element. (For Shell63, 43, and 93, use SMISC seEng

1, 2, or 3 when genera�ng the element table data.) The Theory Manual uses the term In‐plane forces per unit

length while the elements manual refers to just forces as above ‐‐ a simple test I ran shows the data to be force

per unit length. The elements manual ought to clarify this. The Element Table data can be contour plo5ed, but

there are no principal stress style vector plots of table data. (Clarifica�on: PLVECT can plot vector arrows based

on 3 ETABLE columns, but not the double‐headed arrows for an ETABLE as in a principal stress vector plot.) The

Elements manual shows the TX, TY, and TXY values not being available under "Miscellaneous Element Output" at

every node, only at the centroid. The Elements manual does not explicitly show that S,EQV or S,INT stress

informa�on can be extracted at the mid‐plane. Their value is extracted with the component name method. Brief

experimenta�on shows that if the command "SHELL,MID" is followed by "ETABLE,SEQVMID,S,EQV" that the

column called SEQVMID will contain an average SEQV value for the mid‐plane. If "SHELL,TOP" or "SHELL,BOT" is

called, the ETABLE value of SEQV will change if the update command "ETABLE,REFL" is executed. Warning: When

ploEng ETABLE shell element element table data with PLETAB the plot informa�on legend will read TOP, MID, or

BOT according to the current seEng of the SHELL command. This bit of informa�on DOES NOT reflect the SHELL

surface seEng condi�ons in effect when the ETABLE data was stored, and could be misleading. For this reason,

the label used for the column should indicate the shell layer seEng in use when the element table data was

loaded, as with "SEQVMID" above. Doing an element table update with ETABLE,REFL will re‐fill columns with

results data. A change of the SHELL layer seEng can change stress results that are loaded in an update.

Consequently, loading shell element data must be handled very carefully in order that the layer choice is

controlled. Element table data from the CALC module (adding columns etc.) is NOT updated and has to be


18 of 54 1/17/2009 11:41 AM

explicitly re‐calculated.

NOTE Also: The direc�on of the element table load‐per‐unit‐length TX, TY, and TXY is as taken from the element

in Element Coordinates. Unlike SX or SY, the values of TX, TY, and TXY appear to be insensi�ve to the RSYS seEng.

The Element Coordinate System will vary orienta�on from element to element, par�cularly under free meshing,

and affects the usefulness of TX, TY, and TXY data. The element table data can be processed by the user to yield a

new table column containing the "load‐per‐unit‐length intensity" in the sense of a Mohr's circle, giving rapid if

somewhat coarse plots of load path informa�on along the shell mid‐plane. The plots will usually be more

informa�ve without nodal averaging. Sec�on 2.3.4 of the ANSYS Theory manual discusses Forces and Moments

per unit length on shell elements ‐‐ the sugges�on is that internally, at least for some shell elements, the

mid‐plane stress is NOT simply the average of the top and bo5om stresses. The way around the problem of

element coordinate systems being arbitrarily oriented is to define local coordinate systems before meshing areas

(or otherwise genera�ng shell elements) and use ESYS to get all shell elements oriented with the local coordinate

systems. ESYS assigned to elements can be modified a2er the fact but before SOLVE, by using the EMODIF

command in /PREP7. It may be desirable to have a local coordinate system aligned with each flat area to be

meshed with shell elements so that all shell element coordinate systems can be aligned in the plane of the area

‐‐ a �me consuming process unless a macro is used. Curved surfaces would be difficult.

The problem of orien�ng coordinate systems in the ploEng of results is illustrated by the images below. The first

shows 3 elements that were created during free meshing. The elements are plo5ed using vector graphics, with

the element coordinate systems shown. Each element has its coordinate system oriented differently. The image

below it lists the elements and their node numbers. Look at the sequence of node numbers for the three

elements to see why the element coordinate systems point in such different direc�ons.

The next two images show a plot of TX done from an element table. The element table was filled by the TX values

for the elements (this is the load‐per‐unit‐length in the element coordinate system X direc�on). The values differ

so much from element to element because of the difference in the element coordinate systems. The plot

consequently tells us too li5le. The following element plot of Sx shows the stress in the X direc�on. The results

are shown in the global coordinate system.


19 of 54 1/17/2009 11:41 AM

The final images in this sec�on show a group of Shell63 elements that have had their element coordinate

systems aligned with local coordinate systems at the �me of the crea�on of the elements, by the use of the ESYS

command. This will permit element table results TX, TY, and TXY to be aligned in a known manner. This also

permits Sx, Sy, and Sxy to be aligned in the plane of the elements crea�on if RSYS,SOLU is ac�ve when ploEng

stress results. Knowlege of the alignment of the loads and stresses can make plots more useful in understanding

load paths, reduce the total number of plots required in model assessment, and help facilitate an evalua�on of

loading on welds. The first plot with vector graphics shows the elements with their element coordinate systems.

Note that they are aligned. There are two local coordinate systems at work in this example ‐‐ they are numbered

11 and 12 and their symbols are plo5ed. Elements have been created aligned with number 11 in one plane, and

aligned with number 12 in the other plane of elements. A line pressure has been applied in the global ‐Y

direc�on. The second plot with raster graphics is of Sx at the shell mid‐plane. Because RSYS,SOLU was ac�ve

when the Sx plot was generated, there are Sx values shown in all elements. If RSYS,0 were ac�ve when the Sx plot

was done, the plane of elements that is perpendicular to the global X axis would show zero stress in the X

direc�on in this example.


20 of 54 1/17/2009 11:41 AM

There is an alterna�ve to using ESYS and RSYS,SOLU to align element coordinate systems for the purposes of


21 of 54 1/17/2009 11:41 AM

stress plots like Sx, Sy, and Sxy. During postprocessing in /POST1, a local coordinate system can be aligned with

the plane of shell elements of interest, and RSYS set to that local coordinate system, before ploEng Sx, Sy, or Sxy.

However, this would do nothing for TX, TY, and TXY which depend on the element coordinate system and are

generated in an Element Table.

I leave the topic of whether to plot surface or mid‐plane shell stresses to the reader to determine. Too much is

industry or applica�on domain specific. Hint: Check mid‐plane plus both shell surface stresses. Surface stresses

and strains can cause local bending, cracking, breaking of protec�ve coa�ngs, fa�gue, and imply possible

overload or prying of welds and fasteners, and can highlight other troubles.

Tip 26: Nodal Coupling (CP) versus Rigid Region (CERIG):

I have seen analysts mistakenly use nodal coupling where rigid region constraint equa�ons should have been

employed. (The nodes concerned were not at the same loca�on in space.) Rigid region constraint locks together a

selected set of nodes so that they translate AND rotate in space as if they were locked together by an infinitely

s�ff structure. Nodal coupling locks together selected degrees of freedom (transla�on and/or rota�on)

individually, so that the same degree of freedom value will result for the nodes in the coupled set. Nodal

coupling will not combine the rota�ons and transla�ons that are necessary to imply rota�on as a rigid body in

space.

Note that rigid region constraint may not be appropriate for Large Displacement, when the displacement

rota�ons are significant (sin(theta) differing from theta, etc.). This is because ANSYS uses a linear approxima�on

to the rigid body rota�on matrix. A rigid region grouping can be implied by tying nodes together with extremely

s�ff beam elements (zero‐mass beam elements a few orders of magnitude s�ffer than the structure to which they

are a5ached.) The beam elements should have the advantage that they work under Large Displacement. The

beam elements should not be too s�ff, or ill‐condi�oned matrices could result. If the beams are of very widely

varying lengths, then some may be too s�ff, others too flexible ‐‐ remember that flexibility is propor�onal to

length cubed.

I ran a model in which about one thousand beam elements were used to posi�on gap elements. These beam

elements would ideally have been infinitely s�ff. I needed elements, instead of nodal coupling or constraint

equa�ons, because of thermal expansion considera�ons. The beam elements were widely varying in length. This

created solver trouble, un�l I wrote a macro that assigned each beam element a unique REAL value, which set

values for each BEAM4's Ixx, Iyy, Izz, and Area as a func�on of the element's length. I found it sufficient to set

their s�ffness a couple of orders of magnitude s�ffer that contact s�ffness for the gap elements.

Turning on the symbols for nodal coupling and for nodal constraint equa�ons is very helpful in reviewing the

correctness of a model.

Tip 27: Vibra�on Modes with Pre‐stress:

Calcula�on of natural frequencies and modes of vibra�on CAN be done with pre‐stressing of the structure under

ANSYS. There is a "PRESTRESS" flag to set under modal analysis. This is available in the dialog box for Modal

Analysis Op�ons. First, do a sta�c analysis with the prestress flag set. Exit Solu�on (click Finish or enter "/fini").

Re‐enter Solu�on, and do a modal analysis with the prestress flag set again. This does not seem to work when

the stress run is done with Large Displacement ac�vated.

I leave the ques�on of how a performer plays music with a hand saw and a violin bow as an "exercise for the

reader" :‐)

Tip 28: Crea�ng New Elements by Copying or Reflec�ng Exis�ng Structure:

In order to create new elements by reflec�ng or copying exis�ng elements, there are a few things to do. First,

select the elements to be copied and get their nodes with NSLE. Copy or reflect the nodes, no�ng the nodal

number offset that will be used ‐‐ write it down. Copy or reflect the elements, using the nodal offset number that

you wrote down. ANSYS should default to a nodal offset number equal to your highest numbered node. If you

make it smaller, you run the risk of changing the loca�on of nodes that already exist, resul�ng in a lovely mess. If

you are running something like ANSYS/ED you may want to compress your node numbers first, for if a node

number results that exceeds the ANSYS/ED limit, the program will terminate immediately (the more recent


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ANSYS revisions may give a non‐fatal warning message and quit some �me later if you don't clean up). You could

compress the node numbers, and then make the offset number equal one plus the difference between the

maximum node number of the whole model and the lowest node number of those nodes to be copied or

reflected. You can find these node numbers with *GET commands. (Remember that compressing node or

element numbers will destroy synchroniza�on with Load Step files.)

The same nodal offset number will need to be used if nodal coupling is to be copied as well. In order to copy

nodal coupling, use "Generate Coupled DOF Sets with same DOF" for which you will need the same nodal offset

number. Do a replot to see the newly created nodal coupling. Cau�on: Be sure that if nodes were deleted earlier,

that nodal coupling equa�ons that in the past included those deleted nodes were also deleted. If you forget, you

may get a pre5y mess.

Remember that if there are nodes on the plane of reflec�on, new nodes will overlay them. Merge commands

may be wanted for the nodes on the reflec�on plane. Now the tricky part: elements lying in the reflec�on plane

(shell elements will do this) get generated with the node order reversed, because of the mirror imaging. They Will

Not Merge with the element from which they were reflected. They may have to be deleted, depending on what

you are trying to accomplish. Alterna�vely, do not select elements that lie in the plane of reflec�on when

reflec�ng the structure. You s�ll need to reflect the nodes on the plane of reflec�on, in order to reflect the

elements that will join them to the remainder of the reflected structure, so the nodal merge will s�ll be needed.

Tip 29: Adding to a Model Comprised of Elements and Nodes Only:

It may happen that a model that consists of nodes and elements only has to have a sec�on replaced, or requires

the addi�on of more structure. The way to a5ach new geometry onto exis�ng nodes and elements is to: (1) Place

keypoints on the nodes onto which new geometry is to be built (i.e. gra2ed). (2) Join these keypoints with lines.

(3) Set mesh density along these lines to only one element. (4) Build new geometry outward from these keypoints

and lines. This gets messy if you are building solids. (5) Mesh the new geometry. (6) Select the nodes (new and

old) along the interface between the old nodes and the nodes of the new geometry. (7) Merge ONLY these nodes

along the interface using the NUMMRG,NODE command. Alterna�vely (much more work unless a macro is

wri5en or the CPINTF command is used correctly), fully couple the PAIRS of nodes with the CP command. In the

event of elements with mid‐side nodes, lines will have to be created curved so that a single line spans three

keypoints placed on the three nodes along the edge of an element. It is probably advisable to connect elements

with mid‐side nodes to other elements with mid‐side nodes.

This a5aches the new geometry and mesh to the old elements and nodes. Be sure to double check that the

merging has been done correctly and according to your inten�ons ‐‐ I have found this to be a surprisingly

error‐prone opera�on.

Tip 30: Zero Mass Beam Elements Form Rigid Region:

An analyst could use very s�ff beam elements (a few orders of magnitude s�ffer than the surrounding structure)

in order imply a rigid region grouping of nodes, which works under Large Displacement (a CERIG group does not

work with large displacement). This is an old FEA trick ‐‐ it is not perfect. A separate material should be created

for these beams, and be given zero mass (set the material density to zero) so that no gravita�onal or other iner�al

load acts on the material. A thermal expansion coefficient should be input if appropriate ‐‐ it would usually be

iden�cal to the coefficient value for the structure that it approximates.

I wrote a macro to create a rigid region using beam elements. It is called a2er the set of nodes to be connected is

selected. The lowest numbered of the set of nodes is a5ached to each of the other nodes in the set by a beam

element. The beam element to use has to be set up in advance, and the appropriate MAT, REAL, and TYPE set by

the user. A macro like this is very fast to run. Cau�on: Such a macro would become complex if it checked for

duplicate nodes at the first node loca�on (ANSYS can't use zero length beams), and checked for widely varying

beam lengths. This is not a guaranteed method.

Tip 31: Turn off Symbols When Changing a Model aDer Solu�on:

If you have run SOLVE, the results database will be full of data. If you then change a model, and create anything

that plots a symbol, all symbols become ac�ve, and plots become extremely slow. Turn off symbols with

/PBC,ALL,,0 to speed things up. I put this command in the Toolbox for convenience. I have found that ploEng can


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become slow with very large models when loads have been applied, and even when applied and deleted.

Presumably ANSYS is checking to see if any symbols should be shown. The ploEng speeded up considerably

when symbols were turned off with "/PBC,ALL,,0" even though there were, in fact, no symbols to be plo5ed.

Tip 32: Are the "Free‐Free" Vibra�on Modes Relevant?:

Simple supports on a structure may be appropriate for sta�c analysis and gravity loading, since the structure will

"sink" un�l the simple support reacts enough to withstand the applied load. If a modal vibra�on is excited, small

amplitude vibra�ons may result in very li5le response from the support, and vibra�on similar to a structure that

is free in space may result (this is obviously very problem dependent). If so, it may be desirable to run a modal

vibra�on analysis with no constraints. More than six modes must be requested, since the first 6 represent the

free transla�on and rota�on, and give Zero eigenvalues. A be5er approach would be to characterize the

flexibility of the constraint points. With some structures, you may get a few surprises, as torsional and other

vibra�on modes appear.

Tip 33: Selec�ng a CAD or FEA System ‐‐ Cover Yourself

It is common to evaluate a few CAD or FEA packages when trying to make the right choice for a purchase. Watch

out for this stunt: (I've seen it done, and been threatened with it once (I laughed at her).) A losing vendor writes

a le5er to your boss, or even to the head of your company, claiming that the engineers are incompetent (stupid,

uninformed, can't spell, and so on) and making a huge mistake. If the boss is not an engineer and cannot

understand the issues, this could get awkward. (Certain Dilbert cartoons come to mind.) Warn your boss(es) in

advance that a few vendors pull this move and that you and your group will evaluate the products in a thorough

manner. Write down some criteria and your assessments. Also, be careful that you cannot be accused of leaking

informa�on unfairly from one vendor to another ‐‐ date your correspondence carefully, and work through your

purchasing department if that is appropriate at your firm. Some sales‐types are very greedy for their

commissions, and petulant when they lose. (Names will not be men�oned, to protect the guilty. If you've been

around the block a couple of �mes, perhaps you can make a few guesses.)

(The ANSYS vendor I've dealt with has been very professional.)

Tip 34: Crea�ng Lines Perpendicular to, or at Angle to Exis�ng Lines

When crea�ng structures in the /PREP7 por�on of ANSYS, I find that the commands

that create lines that are perpendicular to exis�ng lines, or at an angle to exis�ng

lines, are extremely useful. Look at the commands LANG, LTAN, L2ANG, as well as the

others. These commands break lines where new lines intersect, even though the

original lines are already a5ached to areas. Since I o2en model shells that are to act

as if they are welded together, I need the lines to be shared where areas contact

each other. These commands give the connec�vity I need.

The command that meets another line at an angle may do be5er if it is entered manually, with the first guess of

the contact point set at 0.0, 0.5, or 1.0, depending on your inten�on. This o2en succeeds when the interface

command fails.

Tip 35: Use the /UI command in Your ANSYS Toolbar to Bring up GUI Dialog Boxes

Take a look at the /UI command in ANSYS. You can use it in your Toolbar to ac�vate certain GUI dialog boxes with

one‐click simplicity, instead of finding your way though the menu system. I some�mes get odd results from the

Hard Copy command when I do this ‐‐ I have no idea why.

Tip 36: Reac�on Force, Nodal Force, and Load Paths

I worked on a model subject to aerodynamic pressure and gravity load. We needed to know the load that the

structure would apply to its founda�ons. Prin�ng the Reac�on Force would give this value, however the +/‐ sign

is in the direc�on of the force that the constrained node (or nodes) applies TO the structure. If nodes are selected

with the three commands NSEL,S,D,UX $ NSEL,A,D,UY $ NSEL,A,D,UZ the Nodal Force at the constrained nodes


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can be printed. This is the force with which the nodes press on the supports. (NOTE: You may need to include

nodes where there are constraints on rota�on, depending on what you are modeling.)

WARNING: A number of things can go wrong with this approach.

If you ask for nodal forces without limi�ng the node selec�on to nodes where there are constraints, you

will get nodal forces wherever forces and pressures have been applied to your structure. (For the curious,

prin�ng nodal forces when only pressure has been applied to shell and high order elements will illustrate

that FEA so2ware inputs a complex set of forces and moments because of how the elements are derived

from first principles. What is being printed is the force with which the nodes react to the forces input from

outside ‐‐ if a moment is input, a nodal "force" moment is output in reac�on.)

1.

If an input force has been applied to a constrained node, the nodal force and the reac�on force

magnitudes will differ. When I tested this, the reac�on force that ANSYS listed was modified by the

presence of a force applied directly to a constrained node, whereas the nodal force (that is based only on

element deforma�on) was not affected.

2.

IMPORTANT: All the elements to which the selected node is a5ached must be selected in order to get the

total force with which the node pushes on the outside world (use ESLN a2er selec�ng the nodes). The

genera�on of Nodal Force (and Reac�on Force, if I remember correctly) is determined from the

deforma�on and s�ffness of a5ached elements. If elements a5ached to a node of interest are not selected,

then the contribu�on of those elements to the force at the node is not included and will be missing.

3.

Cau�on: If you have used a rigid region with the node of interest, the lack of element deforma�on means

that you will NOT get the Nodal Force or Reac�on correctly ‐‐ you may need to work from the set of nodes

where the rigid region a5aches to the flexible part of the structure. I'm not sure what kind of effects nodal

coupling would have.

4.

There are various other uses to which you can put Nodal Force. You can plot the Nodal Force vectors along with

your model (see the /PBC command), a2er SOLVE, giving visual cues during your review. You can use NODAL

FORCE to find out about the load being carried in certain Load Paths:

Determine where to posi�on a "cut" in the model. Locate it where you want to determine the force carried

across the cut. The "cut" should follow a path along the edge of a set of adjoining elements. Select all the

elements on ONE side of the "cut".

Select the nodes on the "cut" side of those elements.

Prin�ng the Nodal Force (forces only) will tell you about the forces that your selected elements apply to

those nodes. The sum that is printed tells you the total force carried across the "cut" in the X, Y, Z

direc�ons, based on the selected elements.

Cau�on: GeEng the moment across the cut is not so easy, because moment is determined with respect to

an axis. You would have to do extra work to pursue moment across a cut, determining your "neutral" axis,

and using other commands. See, for example, ANSYS manuals informa�on on the SPOINT command.

Note my earlier comment Tip 25 on making load paths visible in shell models. For further informa�on, read the

ANSYS manuals on the FSUM and NFORCE commands.

Tip 37: Inpu;ng Temperatures with BF, BFE, and TUNIF in Structural Analysis

As discussed by ANSYS in Chapter 2.6 of the Elements Manual, Body Loads (temperatures for structural analysis

that cause thermal strains and affect temperature dependent material proper�es) may be input in a nodal

format or an element format. "Either the nodal or the element loading format may be used for an element, with the

element format taking precedence. Body Loads are designated in the "Input Summary" of each element." This

means that if both BFE and BF are applied to an element and its nodes, and the inputs differ, the BFE seEng will

govern. If temperature is input on a nodal basis, the temperature input at a node will influence all the a5ached

elements. If temperature is input on an element basis, the temperature(s) input will influence only the element

to which it was applied. The commands TUNIF and BFUNIF can be used to set all nodes to one default

temperature that differs from the reference temperature. Then, BF or BFE commands can to used on specific

regions of the model to put in other temperatures.

If you use piping commands to create pipe elements, and have applied temperatures, ANSYS will apply the

temperatures on an element basis (to check this, generate a Load Step file and inspect its contents, or use the


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BFLIST and BFELIST commands). For the user applying temperatures directly, it can be a li5le simpler to apply

temperatures on a nodal basis with BF, since the nodes can be selected by loca�on. InpuEng temperatures on

an element basis with BFE permits control of things such as temperature differences between the inside and

outside of pipe elements, or between the top and bo5om of beam elements. The element lis�ng in the Elements

Manual should be consulted before applying temperatures with BFE. As I discussed elsewhere, if you change

temperatures that were previously set with BFE, the temperatures have to be changed at all the loca�ons within

each element to which temperatures were applied. Otherwise, the old temperatures will s�ll be there. You may

want to clean up with a BFEDELE or other cleanup command before star�ng. The BFDELE and BFEDELE

commands only act on selected nodes and elements ‐‐ if you want to remove all temperature applica�on, select

the full model first. Be wary of what happens when you use Load Step files.

Tip 38: ANSYS Toolbar Use

The ANSYS Toolbar can be very helpful in giving "one click" access to frequently used commands. Toolbar bu5ons

can also call macros, or the func�on form of commands, for example Fnc_Pl_Symbols to bring up the dialog box

for seEng symbols. If you want to get fancy, a toolbar bu5on could be used to ac�vate an alterna�ve toolbar.

In the toolbar shown here, a variety of bu5ons have been enabled. Some of the cap�ons

are a li5le cryp�c; this is because the cap�ons are limited to only 8 characters. The

command that gets executed cannot include the $ sign. Consequently, only one

command can be executed, however, a macro can be called in order to perform a

complex set of instruc�ons. The toolbar edi�ng is brought up from the menu item

"MenuCtrls". In the example toolbar shown here, the bu5ons are not in a highly logical

order. In order to modify the bu5on sequence, save the toolbar (I suggest the

unimagina�ve file name "toolbar") and re‐order the lines in that file with a text editor.

Keep the eventual sizing of your toolbar in mind. The example here is sized for six rows

deep, and seven columns wide. Use the "Save Menu Layout" menu selec�on to save the

layout of all of your ANSYS windows including the toolbar shape. (This seEng is

destroyed if you modify the "GUI configura�on" under your ANSYS Interac�ve startup

dialog box.) When you are happy with the layout of your toolbar, you can append the

toolbar file's contents to the end of the "Start.ans" file located in the ANSYS "DOCU"

subdirectory.

Tip 39: ANSYS Piping Elements

The use of ANSYS piping elements, Pipe16 and Pipe18, can simplify the work required to create models of piping

systems that will sa�sfy certain code requirements. Piping commands can be used in /PREP7 to directly create

models of piping. In using piping crea�on commands, a user works out the intersec�on points of the runs of

piping as if there are sharp angle bends. Each run of pipe is entered as dx, dy, dz, crea�ng Pipe16 elements, and

then a radius of curvature at the previous intersec�on can be applied, crea�ng Pipe18 elements. The Pipe18

elements are taken out of the two Pipe16 elements that met at the last corner intersec�on. If these two Pipe16

elements are too small to encompass the Pipe18 bend elements, difficul�es will result. If the user is defining

U‐bends, it is easy to have zero‐length Pipe16 elements generated. My approach to this is to inspect the model

for zero‐length Pipe16 elements, and delete them, a2er I make sure that all pipe nodes are merged. I use a macro

to inspect the model and do the dele�ons. Checks are included in the macro, because Pipe18 elements always

return a zero length. I have also seen users do a U‐bend with a small extra space so that a very small Pipe16

element will remain between the two 90 degree bends that make up the 180 degree U‐bend, avoiding a

zero‐length element problem.

To list or plot useful stress informa�on from the piping model usually requires puEng selected results data into


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element data tables, and the use of appropriate PLLS commands. Fortunately, ANSYS includes many output

possibili�es for the two piping element types, so typical piping code requirements can be met. See the element

manual for these elements for informa�on on the available output data.

For obvious reasons, I leave proper use of design codes within piping analysis as an "exercise for the reader." :‐)

Piping crea�on in ANSYS includes the possibility of added mass due to fluid in the pipes, and from insula�on

added to the piping. The insula�on addi�on is simple ‐‐ the user can input thickness and density. This lets the

added mass presence of heat exchanger fins be easily faked by inpuEng the product of fin_thickness x

fins_per_inch x fin_material_density as the "insula�on" density, and fin height as the "insula�on" thickness.

(Subs�tute the appropriate dimension for fins_per_inch, etc. for your system of units.)

The deflec�on behavior of pipe elements is based on ANSYS beam elements. If accurate vibra�on behavior is to

be modeled, at least several pipe elements will be needed between supports. If accurate gravity‐induced

deflec�ons and stress are wanted, be5er results will come from the use of a consistent mass matrix, if element

density between supports is low.

Developing an understanding of the func�on of the ANSYS element crea�on commands (BRANCH, RUN, BEND,

and so on) will require crea�ng some elements with material and dimensional informa�on, then reviewing what

element TYPE and REAL data has been created in the model database. Model review is enhanced by ploEng the

elements with the /ESHAPE op�on ac�ve.

Where piping is connected with sliding supports to the outside world, the use of gap elements may be needed if

sliding fric�on is to be included in the model. ANSYS does not differen�ate between sta�c fric�on and sliding

fric�on coefficients, so a reasonable and conserva�ve value for coefficient of fric�on (as well as contact s�ffness

of the gap element) will have to be determined by the analyst. If there are thermal expansions in the piping,

stresses predicted by the model will usually be reduced if the gaps in the support structure are included in the

model (depending on the nature of the structure) rather than having "�ght" fits at the sliding connec�ons.

Tip 40: Graphical Output from ANSYS

If you start up ANSYS under Windows NT with "win32" selected for graphics, the stress plots will be shaded. If you

select "win32c" for the graphics, the stress plots will not be shaded, and will usually look be5er when plo5ed to

paper, especially when plo5ed from ANSYS with HardCopy to ink jet printers. They can be selected with the

commands /SHOW,WIN32 and /SHOW,WIN32C when using the GUI.

PloEng to the screen window with Z‐buffering as the hidden surface control can give very sa�sfactory and o2en

quicker results. Hard copies of these Z‐buffer plots, however, will look "pixelated", being limited to a coarse

resolu�on. Be5er looking hard copies to paper will usually result if the screen is set to "Precise Hidden" or even

to Centroidal hidden surface control. This is usually true of plots sent to a file, for subsequent processing with the

ANSYS DISPLAY program.

Plots can be redirected to files by using the /SHOW command. This permits the DISPLAY program to do various

things with the results, including the genera�on of anima�ons. Under Windows NT, an anima�on can be

generated as an AVI file.

I occasionally find it helpful to generate an anima�on file based on a single stress plot of a load step, in which I

spin the model about the screen X or Y axis. You can use the /ANGLE command and the /REPLOT command to

accomplish this. A simple macro does /REPLOT calls with the model set at a series of angles from 0 to 360 degrees.

You can even execute this command on one line using the "$" symbol to separate the commands. The command

"*DO,III,0,355,5$/ANGLE,ALL,III,YS,0$/REPLOT$*ENDDO" will achieve this for you. The scaling of the display

should NOT be set with /ZOOM,OFF or else the image will "move in and out" in order to fill the screen as the view

is rotated ‐‐ set the zoom level manually with picking; you may want to move out so that the model fits in all

views. You may need to experiment. Node plots without symbols are a quick way to assess the behavior while

tes�ng. If the plots have been re‐directed to a file when this command is executed, the plots in the file can be

animated by the ANSYS DISPLAY program.

AT the ANSYS 5.3 level, and presumably above, you can do a /SHOW,VRML plot to get a 3‐D VRML file produced

of a 3‐D model plot. This could be a stress contour plot of a 3‐D model. With the right op�ons ac�vated for a


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good VRML viewer plugged into a Web browser, the stresses on the 3‐D model can be reviewed at any viewing

angle with the posi�oning control a VRML viewer. This ought to be par�cularly interes�ng on a computer with a

fast 3‐D graphics accelerator.

There are u�li�es that can convert a Postscript output file from the ANSYS DISPLAY program into a bitmap image

file. A free conversion program is Ghostscript, once you figure out how to use it. The user should get a front end

for the Ghostscript program, for ease of use.

Under Windows NT, the Alt/PrintScreen key combina�on will copy a window to the Clipboard. This can be used

to capture an ANSYS graphics window for pas�ng into a word processor document, or into an image processing

program for conversion to a GIF or other bitmap file. GIF files can be used in WEB pages to show the results of

ANSYS work. I recommend GIF over JPEG files for images from ANSYS, because GIF files precisely reproduce 256,

16, and 2 color images (you have to reduce the colors to 256 or fewer levels in the image processing program, or

accept the default color reduc�on used when the GIF file is generated.) Before capturing the Graphics window of

ANSYS, set its size to your sa�sfac�on. Bitmap image size changes in an image processing program are not

sa�sfactory with this type of graphical output. If you want to get really fancy, generate a GIF file that contains an

anima�on of a ANSYS model. (Animated GIF files can be generated from individual images with so2ware that

you can find on the Web or purchase.)

Re‐sizing of the ANSYS graphics window under Windows NT is painful if a model has been plo5ed, because

ANSYS wants to keep re‐ploEng the image as the window edge or corner is dragged. This problem goes away if

you set the Windows NT Display Proper�es to NOT show window contents while dragging. I keep my PC

permanently set this way for this reason.

Tip 41: Check Nodal Loads at Bolts, Rivets, Spot Welds and Links

Wherever connec�on by bolts, rivets, or spot welds has been represented by various simplifica�ons or

representa�ons in an ANSYS model, the load on those connec�ons should be checked, and compared with

allowables. Spot weld review may require assessment of moments (especially about an axis perpendicular to the

sheets that are spot welded together), as well as assessment of forces. One way to do this is to select the

appropriate node(s) at the connec�on, select elements on "one side" of the node(s), and check nodal loads. The

connec�on devices should not be overloaded. The hole in which a bolt or rivet is placed must not be overloaded

or too near an outside edge of a sheet or plate, either. Addi�onally, building codes usually forbid or substan�ally

limit "prying" loads on bolted and riveted connec�ons. If the FEA model has good detail, including gap or contact

elements, a high prying load can be demonstrated in some models (never assume your FEA model will

automa�cally show you all trouble spots).

Similarly, links or "spars" that are loaded should be checked for stress, and be checked for buckling. Since a link

will be represented by one element that is pin connected at the ends, and only cross sec�on area is entered,

ANSYS will not generate buckling informa�on about the link, not even in a Large Displacement analysis. The user

must do some work to compare compressive load with cri�cal buckling load (use a good margin of safety). A user

could write a macro to step through all link elements, iden�fying the compressive stress and force, and

calcula�ng buckling informa�on. The ANSYS Link10 element supports a tension‐only and a compression‐only

capability. Where it is not known in advance whether all links will remain in tension, and the links are slender,

this element could be used to imply that no link can support compression for what may be a "worst case"

evalua�on of some models. An example would be the stays that support the mast on a sailboat, with

pre‐tensioning implied with ini�al strain. (If the stays are woven rope or steel cable, geEng a representa�ve

crossec�on for the link elements will require some extra work.)

Spot weld representa�on in large structures is usually an inexact science in FEA modeling. Spot welds will be

found, for example, in many automobile body structures. Plug welds are a stronger alterna�ve, applicable to

thicker steel sheets and plates. The crudest and quickest representa�on of spot welds is to merge coincident

nodes from the two joined layers where nodes have been inten�onally created coincident at the spot weld.

Alterna�vely, the nodes can be fully coupled with the CP command if they are coincident. They can be joined as

a rigid region with CERIG if the nodes are close but not touching as when shell elements are kept at the

mid‐plane posi�on of two sheets that are spot welded together. (Remember that CERIG is valid only in small

displacement analysis ‐‐ coupling with zero‐mass s�ff beam elements could be subs�tuted if large displacements

were needed.) The shell nodes can be joined with a beam element that has proper�es that reflect the diameter


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of the spot weld. The roughest approxima�on will merge or couple just one node pair. If nodal coupling is used,

rota�ons should be coupled as well as transla�ons, for spot weld representa�on. NOTE: With shell elements,

read the "drilling mode" comments in the ANSYS Elements Manual ‐‐ it may be necessary to set a KEYOPT value

to transmit rota�on and torque about an axis perpendicular to the shell elements when a spot weld is crudely

represented by single node pair coupling, merging, CERIG, or beam elements. Contact elements between the

joined shells or materials may want considera�on. Exactly what to do for spot weld representa�on is very

problem, industry, and material dependent. These very approximate techniques tell us li5le or nothing about

stress, fa�gue, or fracture possibili�es near or at the weld. More elaborate modeling (more nodes and elements,

and special element types) of each spot weld could give more informa�on about local stresses, when local

stresses ma5er. Studying the "crack" that is hidden between the sheet metal layers in a spot weld is an

"advanced topic" ‐‐ discuss this with an expert or consultant. I doubt that you would find many spot welds used

with aluminum, not only because of the difficulty of welding aluminum, but also because of the fa�gue

considera�ons ‐‐ consider how commonly aircra2 use rivets and modern adhesives.

There is a document on spot weld fa�gue and FEA on the MSC/Nastran website. (There is a variety of other good

reading at the site, too.) Take a look at the paper in PDF format by Heyes and Fermer, which, although it is

MSC/Nastran related, is interes�ng and includes the following references:

Rupp, A., Störzel, K. and Grubisic, V. "Computer Aided Dimensioning of Spot-Welded Automotive Structures". SAE Technical Paper 950711, 1995.Smith, R. A. and Cooper, J. F. "Theoretical predictions of the fatigue life of shear spot welds." Fatigue of Welded Structures, Ed. S. J. Maddox, pp. 287 - 293, TheWelding Institute, 1988.British Standards Institution. Code of Practice for Fatigue Design and Assessment of Steel Structures. BS 7608, 1993.Radaj, D. "Local Fatigue Strength Characteristic Values for Spot Welded Joints." Engineering Fracture Mechanics, Vol. 37, No. 1, pp. 245 - 250, 1990.Sheppard, S. D. and Strange, M. E. "Fatigue Life Estimation in Resistance Spot Welds: Initiation and Early Growth Phase." Fatigue and Fracture of EngineeringMaterials and Structures, Vol. 15, No. 6, pp. 531 - 549, 1992.Sheppard, S. D. "Estimation of Fatigue Propagation Life in Resistance Spot Welds." ASTM STP 1211, Advances in Fatigue Life Prediction Techniques, M. R.Mitchell and R. W. Landgraf, Eds., pp. 169 - 185, ASTM Philadelphia, 1993.Heyes, P., Dakin, J. and StJohn, C. "The Assessment and Use of Linear Static FE Stress Analyses for Durability Calculations." SAE Technical Paper 951101,1995.

I have never worked in aerospace, but I recently had a look inside a old helicopter that was on public display. In

addi�on to rivets, what was either a caulking or an adhesive appeared to have been used between some ribs and

the outer shell. This may prevent corrosion in the gap, and help reduce vibra�on and freEng or galling. If it is

purely a so2 caulking, it might be ignored in FEA, but if it func�ons as an adhesive, the load on the rivets is

probably reduced. Presumably the manufacturer has standards for this type of design.

Tip 42: Use QUERY to Check Results with Picking

In /POST1 the "Query Results" capability applied to nodes makes it easy to check on results (stresses, strains,

deflec�ons, etc.) by picking nodes. To see an image of this in ac�on Click to See Image and use your browser's

Back bu5on to return. For the shell element illustrated, the result will be reported for the Top, Middle, or

Bo5om, according to how the SHELL command was issued (the usual rules as to what cons�tutes the Top and

Bo5om of a shell element apply). It will do this even if PowerGraphics is ac�ve for the plot on the screen. Note

that the nodal stresses are based on averages if more than one element that is connected to a node is selected.

You can inspect the consequence of element selec�on on nodal stress easily with this feature. The element query

returns only data on energy and error es�ma�on.

Tip 43: Loads on Geometric En��es Overwrite Loads on Nodes and Elements ‐‐ Easy Error to Make

My "dumb move of the week" was to retrieve an old model of a beam with redundant supports, change the load

on a node, and re‐run the model. I then updated the element table results, and used PLLS to plot the result, as

shown below. This is a plot of top surface bending stress, with gravity loading included. Both applied point loads

and reac�ons are shown as colored arrows. Stress colors have been gray scaled for prin�ng to a black and white

laser printer. Upon inspec�on by a co‐worker, he no�ced that the results were the same as the results the last

�me the model was run, under different loading, a month before. What was wrong?

The model database file had been saved with the original loading and results. The original model had the load

applied to the keypoints. I changed the load on a node. When I ran SOLVE, the load on the keypoint OVERWROTE

the load on the node, and I got the old result. When I listed the applied forces with FLIST before running SOLVE, I

saw my modified loads. When I listed the applied forces with FLIST a:er running SOLVE, I got the OLD loads on

the nodes. The same principle applies to loads on lines, areas, and volumes. Presumably, it happens with applied

displacements, also. Since loads on geometric en��es cannot be scaled, there may be li5le reason to keep the

loads on geometric en��es a2er these loads have been transferred to nodes and elements, EXCEPT when


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meshing may be changed in the future. The use of components is an alterna�ve way to select parts of the model

for loading.

Sugges�on: The user should add a warning annota�on sta�ng that loading is on geometric en��es, before

archiving a model. Should ANSYS add a warning message about SOLVE transferring loads from geometric en��es,

which requires user acknowledgment?

A poten�ally dangerous mistake ‐‐ watch for it!

Tip 44: Use Components for Load Input, and for Results Review

A user‐wri5en input file could be used to apply loads to components that the user has defined. An even more

convenient use for components is for reviewing stresses due to a load. The components can be called up and

stresses plo5ed without the need to do manual selec�on over and over for each load case. I wrote a macro that

automa�cally steps through all components, ploEng the stresses for each component from a couple of

viewpoints, for each load case. When the plots were diverted to a plot file, the file could be used in ANSYS

DISPLAY to plot stresses for all components for all load cases. Statements in the macro would put the component

name and weight (based on volume only) in an annota�on; the �tle already contained the load case name.

Tip 45: Simple Substructuring Examples‐‐ BoIom Up and Top Down

ANSYS/ED is capable of only a small number of Master Degrees of Freedom (50 the last �me I looked), so any use

of substructuring in ANSYS/ED will have to be done with a very small number of nodes for master degree of

freedom use. A 2‐D element such as PLANE42 may be best for many substructure experiments with ANSYS/ED. In

Large Displacement substructuring, rota�onal degrees of freedom are needed at the nodes, and ANSYS/ED will

only handle very small numbers of nodes ‐‐ 2‐D beams may be best for learning experiments with Large

Displacement. The problem with using beams elements for learning is that review of stresses is more complex;

element tables must be used to hold and display beam stress informa�on. For an alterna�ve, consider SHELL63

elements with very few MDOF nodes (8 nodes x 6 DOF/node = 48 DOF), in Large Displacement substructuring

studies.

Substructuring has become more rare in FEA work, because of the capacity of modern computers for large

models. There are s�ll �mes when it is desirable, such as when gap elements or contact elements are employed

in large models, or when extremely large models are in use. The user will have to employ some insight to select

substructures in a way that minimizes the resul�ng number of degrees of freedom and wavefront size.

Substructuring is a rela�vely tricky procedure, par�cularly with mul�ple substeps or mul�ple substructures. For

serious use, the ANSYS manuals on substructuring should be purchased and studied in detail.

The reader is reminded that the elements inside a substructure are treated as linear. Any nonlinear elements

grouped inside the substructure will be treated as if they were in their ini�al condi�on, without material

nonlinearity. The two simple examples below do not address use of mul�ple substructures, mul�ple load cases,

g‐loading, vibra�ons, and other complica�ons. Nonlinearity (Large Displacement) is men�oned only briefly. If

you do not turn to expert help for substructure work, I recommend substan�al tes�ng of any techniques on small

models before doing any real work.

Warning: Read the ANSYS Elements Manual sec�on on MATRIX50 the superelement. Note its warning that if

gravity is applied during the "gen" pass when the superelement is created, and gravity is applied during the "use"

pass, it will be applied TWICE to the superelement substructure DOUBLING the gravity load on the superelement


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region of the model. For this reason, gravity load would have to be introduced "carefully". Unfortunately, a

detailed desc�p�on of this careful applica�on is not included in the base ANSYS manuals. In the "Top Down"

example below, I set "ACEL" for the model to ZERO in all three global coordinate direc�ons during the "gen" part

that generates the superelement. If the user has applied gravity to the model file that is read in, it will be applied

during the "use" part of the analysis, and so only applied once to the superelement. This may affect the accuracy

of the solu+on ‐‐ I have not yet done comparison runs to test this. The example does not address centrifugal loading

or other complica�ons. Unfortunately, linear accelera�on loading (e.g. gravity loading) is more accurately

represented when applied as a load vector. This presents a problem when there are elements with mass that are

not included in substructures. I have not yet determined whether gravity could be applied to a superelement in

the GEN pass, without having a superelement mass matrix generated ‐‐ if it could be done, then the "accurate"

applica�on of gravity loading to the superelement could be accomplished without coun�ng gravity load twice.

Using rotated superelements introduces another set of problems with the direc�on in which loads are applied ‐‐

read the manuals.

Note that non‐zero applied DOF displacements are not to be applied by a load vector, so MDOF should be

applied to nodes where non‐zero DOF values are to be applied during the analysis. Loads and constraints created

in the GEN pass (i.e. in a load vector) cannot be changed in the "USE" pass, except by uniform scaling. Brief

tes�ng I did suggests that load ramping DOES work for load vectors ‐‐ the user should check this independently.

The excep�on to uniform scaling is with respect to angular mo�on ‐‐ read the ANSYS tutorial and user's guide

manuals on substructuring.

The substructuring examples given in Chapter 4 of the ANSYS Advanced Analysis Techniques manual leaves out

the rou�ne steps ‐‐ leave out too many, in my opinion. The user should purchase an ANSYS manual and tutorial

manual on substructuring before doing serious work. (ANSYS 5.5 has added some helpful comments to its

Advanced User's Guide on Substructuring.) The command EXPSOL has to be added in the expansion pass of the

bo5om‐up example in order to get any results in the expansion results file. The SFE command is needed only if

loads were applied to the superelement ‐‐ if SFE is used, it has to point to the element number of the

superelement that was read in with the SE command as well as the appropriate load step number. A *GET

command could find the element number of the superelement right a2er the SE command. The commands

manual does not explain this adequately in ANSYS 5.3. The following examples are fairly brief. In the bo5om up

example, the coupling command CPINTF is used to join the superelement with the non‐superelement por�on of

the model. The example shows the stresses in the superelement a2er the expansion pass completes. The results

of the use pass are saved in the file "use.db" for later review by the user.

Bo5om Up Substructuring Example:

! Substructuring demonstration *************************************! For information only. Use at your own risk.fini ! finish whatever was active previously/clear ! clear the database/title,Substructure Technique Test

/filname,gen ! filename for the generation pass/prep7 !et,1,shell63 ! element type 1 set to SHELL63r,1,.05 ! shell is 0.05 thickmp,ex,1,30000000 ! set value of Eblc4,-.5,0.5,1.0,-1.0 ! create a rectangular arealesize,all, , ,3,1,1 ! 3 elements per line -- user can change thisamesh,1 ! mesh the rectanglefini/solu antype,subst ! substructure analysisseopt,gen ! generation passlsel,s,line,,2 ! line at right sidensll,s,1 ! select all nodes on linem,all,all ! make these nodes Master Degrees of Freedomlsel,s,line,,4 ! line at left sidensll,s,1 ! select all nodes on lined,all,all ! constrain nodes against all motionallselsave ! save this part of model as gen.db for expansion pass ! the save need not follow "solve"solve ! generates the gen.sub filefini


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/clear,nostart /title,Shell elements are attached to a superelement/filname,use ! filename for the use pass/prep7 et,1,50 ! element type 1 set to superelement MATRIX50 type,1 ! set type 1se,gen ! read in the superelement matrix from generation pass ! after reading superelement, create remainder of model:et,2,shell63 ! element type 2 set to SHELL63r,2,.05 ! shell is 0.05 thickmp,ex,2,30000000 ! set value of Eblc4,.5,.5,1.0,-1.0 ! create a new rectangular arealesize,all, , ,3,1,1 ! 3 elements per line -- user can change thisaatt,2,2,2 ! assign mat=2, real=2, type=2 to the unmeshed areaamesh,1 ! mesh the area -- note superelement node numbers are not usedcpintf,all ! automatically couple coincident nodes at interfaceeplofini/soluksel,s,kp,,3 ! keypoint at upper right cornernslk ! select node at this keypointf,all,fy,-1 ! put a load on node at upper right cornernsel,all ! select all nodes! SFE,1,1,SELV, ,1 ! no load applied in generation pass, this statement not needed solve ! results go in the file use.rstsave ! save use.db to review results in the non-superelementsfini/post1/pbc,f,,1 ! show applied force symbols/pbc,cp,,1 ! show nodal coupling symbolsplnsol,s,eqv ! plot the stresses in the non-superlementsfini

/clear,nostart /filname,gen ! filename for the expansion passresume ! brings up gen.db saved above/solu expass,on ! activate expansion passseexp,gen,use ! options for the substructure expansion passexpsol,1,1 ! THIS IS NEEDED ! (read about NUMEXP also) **************! OUTRES,ALL,ALL ! not required for one load step solutionsolve fini/POST1 /title,Stress in the Substructure/pbc,mast,,1 ! show master degrees of freedom symbols/pbc,u,,1 ! show displacement constraints/pbc,rot,,1 ! show rotation constraintsplnsol,s,eqv ! look at the stress in the superelement

In the above example, the user can change the mesh density. The numbers and posi�ons of nodes along the

common interface between the superelement and the normal por�on of the model have to be the same for

CPINTF to successfully connect the two parts of the model.

The model is created with the "bo5om‐up" approach. In the "use" part of this example, the superelement is read

in with SE before the remainder of the model is created. If the remainder of the model was created before the

superelement was read in, then the user would have to add statements to control the node numbering, so that

none of the master nodes coming in with the superelement would replicate the node numbers of the exis�ng

elements. If the superelement has master nodes that have the same node numbers as the exis�ng model, the

model nodes will be redefined, and a mess will result. Check the manual, and look at the SETRAN command to

act on the superelement, or at the NUMOFF command to act on the exis�ng model, to prevent node replica�on

problems. The PARSAV and PARRES commands can be used to put model parameter informa�on into a coded file,

and retrieve it a2er the /CLEAR command has been issued. The maximum node number can be put into a

parameter by *GET and put into a file with PARSAV during the genera�on pass. It can be retrieved during the use

pass by PARRES, and used to guide the offset of node numbers in either the already generated superelement

with SETRAN, or in the remainder of the model with NUMOFF.

Top Down Substructuring Example


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The following example is NOT a subs�tute for a detailed understanding of ANSYS substructuring. It is for

demonstra�on purposes only. Get the ANSYS Substructuring Tutorial guide and the Substructuring Guide for

serious work.

The top down substructuring technique makes it possible to take an exis�ng model, and have a por�on of it

changed into a substructure. This can boost efficiency in a number of ways, such as dealing with contact surfaces

and gap elements, and handling very large models that have already been generated. In the example presented

below, a model database is read in from a user‐prepared file named "model.db". This model in "model.db" must

have had a por�on of the elements grouped into a component called "super" using the command

CM,SUPER,ELEM. This component will be rendered into a substructure. The intended substructure should, in

general, consist of linear elements. The model must have had constraints and loads applied. The SFE command

used in this example expects loads to exist inside the superelement, but should work without them. Some nodes

can have been declared by the user to be master degrees of freedom. In order to create master degrees of

freedom through the GUI, the analysis type has to be Substructure. In order to use the example below, the

analysis type will have to be changed back to the type desired a2er crea�ng extra master degrees of freedom ‐‐

usually to sta�c analysis. Note that for dynamic analysis, master degrees of freedom are needed throughout the

substructure ‐‐ they are not created by the example below. In the example presented, master degrees of freedom

are automa�cally generated for the nodes on the interface between the component "super" and the remainder

of the model. (There is no check for redundancy with user‐declared master degrees of freedom.) The full model is

used ‐‐ the analysis is not limited to the selected set of elements in the file "model.db" when it is loaded. The

example will automa�cally perform the substructure genera�on and the subsequent analysis, and will plot

results to plot files. I have added a plot of the results for the full model, with results files for both the substructure

and the non‐substructure being read. There is no error checking in the example. This example has had limited

tes�ng‐‐let me know about errors.

When dealing with gap elements and/or contact surfaces, the usual procedure would be to select all the linear

elements in the model (not the gap or the contact elements), and in this example, call them the component

"super" for substructuring. Because the substructure matrix is usually much smaller than the full model matrix,

the itera�ons required for convergence with gap and contact elements will usually run far faster than itera�ons

involving the full model, once the substructure matrix is generated. This makes otherwise infeasible modeling

into a possibility.

In dealing with extremely large models, where the objec�ve is simply to deal with the size, not nonlinear

elements like gap elements, there may li5le advantage in turning the en�re large model into a substructure ‐‐ it

could take as long to generate the superelement as to solve the model for one load case. It would be more

common to turn por�ons of the model into one or more substructures. The connec�ng regions between the

substructures would be chosen to involve as small a number of nodes as possible, to minimize substructure

matrix size, and model wavefront size.

Although a MATRIX50 substructure superelement can undergo Large Displacement, it will act internally as a

linear elas�c structure. Methods to use MATRIX50 in nonlinear applica�ons should be thoroughly tested by the

user before applica�on, including plots of displacement and stress to look for compa�bility in results among the

substructure regions and the remainder of the model, and checks that reac�on forces equal the total applied

forces. I have encountered difficul�es combining Large Displacement with Substructuring ‐‐ see the image below.

To use the following example: (1) Create a model, and (2) select a por�on of the elements to become the

substructure. Give this selec�on set of elements the component name "super" with the command

"CM,SUPER,ELEM". (3) The model should have loads and constraints applied, and the analysis type defined. The

analysis type must be acceptable for substructure use. (4) Save the model with the database name "model.db".

(5) Call the rou�ne below with the /INPUT command. Graphical results for the last load substep in the results files

will be plo5ed to disk files. If run interac�vely, the user will have to click the "OK" bu5on a few �mes, and a plot

to the screen should result when done. Expect warning messages related to par�al element selec�on, and to

reading from results files. NOTE: This example sets gravity load to ZERO in the "gen" por�on of the analysis;

otherwise, gravity would be DOUBLED on the superelement if the user's model includes gravity ‐‐ see the

Elements Manual for MATRIX50. (If gravity is applied, be sure a density was applied to the materials in the model.

If there is no gravity, the example can have the "seopt" command changed to NOT generate the mass matrix for

the superelement.)


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The loading on MDOF nodes would also be DOUBLED if it was used in the superelement load vector, and used in

the "USE" pass. For this reason, correc�ons to this rou�ne have been added (Nov.2, Nov.4 1998). A complica�on

for substructuring: Only a master node from a coupled node set or a constraint equa�on node group can be used

as an MDOF for substructuring. This complica�on is NOT addressed in the present example. The example is for

stress analysis. It does not address centrifugal loading. To address other types of analyis, start with a look at the

ANSYS Advanced User's Guide, and look at the table of loads applicable in a substructure analysis.

Automa�ng a substructure analysis is somewhat tricky ‐‐ this file will NOT be applicable to all types of analyses. I

have been tes�ng it with simple stress examples. The "POSTPROCESS" pass seems to work in loading stresses

from the two different sources for viewing, "USE.RST" and "GEN.RST", although I haven't seen this documented. I

tried the SUBSET command, but was geEng warning messages about the nodal force and other results not

necessarily being correct. I haven't thoroughly inves�gated this. Have a close look at how the substeps are

output with OUTRES and selected for expansion:

! "Top-Down" Substructuring Example -- In Development!! - For information only. Use at your own risk.! - There is no error checking in this example.! - Warning messages will be generated.! - ANSYS/ED supports very few master degrees of freedom.!! WARNING: Gravity would be applied TWICE to the superelement if ACEL! were not zeroed in the "gen" pass. For models that do not! include inertial loads, change the "seopt" command to generate! STIFFNESS only. See the Elements manual for MATRIX50.! This example NOT designed for other inertial loads.!! WARNING: In the "use" pass, nodal loads on superelement MDOF nodes! are deleted so loads on MDOF nodes are not counted TWICE.! FDELE and DDELE are used.!! The model to be processed is in the file "model.db". The user must! have identified the region to be substructured as the component "super"! with the command "CM,SUPER,ELEM" and saved the model as "model.db".! Anything nonlinear in the component "super" will be treated as linear.! Analysis type is defined by the file "model.db" -- must be acceptable type.! The "USE" pass has OUTRES set to write ALL substeps to the RST file.! The "EXPAND" pass has a *DO loop that expands solutions at ALL substeps.! The model must have had its loading and constraints applied.! This example is for one load case only. Some Master Degrees of Freedom! can have been applied by the user -- needed for dynamic analysis.! Master Degrees of Freedom Nodes will be generated between the substructure! and the remainder of the model. No check for redundancy is performed.! Unless this file is run BATCH, the user will have to click the "OK" button! whenever the CLEAR command is executed, and if error messages appear.!

fini ! finish whatever was active previously/clear ! clear the database

/COM,############ GEN ############/COM,############ GEN ############/COM,############ GEN ############

/show,part1,grp ! file for storing plotsresume,model,db ! read the model to be processed ! - all loads and constraints must already be applied ! - the SFE command is employed in the "use" pass to ! apply loads to the substructure ! - only one substructure generated in this example/filname,gen ! filename for the generation pass/prep7allsel*get,nmx,node,,num,max ! get the highest node number*get,nmn,node,,num,min ! get the lowest node numbercmsel,s,super ! select the elements identified as the component "super"nsle ! select nodes of these elementsesel,invert ! select the elements that are not part of "super"nsle,r ! reselect nodes connecting "super" to remainder of modelm,all,all ! make these nodes Master Degrees of Freedom (MDOF)cmsel,s,super ! select the "super" elements again


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nsle ! select their associated nodesnsel,r,m,,nmn,nmx ! reselect all of these nodes that are MDOF (don't want nodes ! outside the "super" that the user called MDOF)fdele,all,all ! delete loads on these MDOF nodes for "gen"ddele,all,all ! delete displacement loads on these MDOF nodes for "gen"nsle ! select nodes of the component "super"/pbc,mast,,1/pbc,f,,1/pbc,m,,1/pbc,u,,1/pbc,rot,,1/title,Elements of the to-be-substructureeplo ! plot the elements of the to-be-substructurefini/solu antype,subst ! substructure analysisseopt,gen,2 ! generation pass -- generate STIFFNESS and MASS matrices ! - if no inertial load, change setting to STIFFNESS onlysave ! save this part of model as "gen.db" for expansion pass ! - SAVE need not follow the command "solve" ! - component "super" and its nodes currently selectedacel,0,0,0 ! set gravity to Zero AFTER "save" but BEFORE "solve"solve ! generates the "gen.sub" filefini

/COM,############ USE ############/COM,############ USE ############/COM,############ USE ############

/clear,nostart /show,part2,grpresume,model,db ! bring the model in again. restores "acel" if any. ! "model.db" has to define the analysis type -- it should not ! be a substructure generation/filname,use ! filename for the use pass/prep7 allsel*get,nmn,node,,num,min*get,nmx,node,,num,maxcmsel,s,super ! select the portion intended for the substructureesel,invert ! select the remainder of the modelnsle ! select nodes of the remainder of the modelnsel,a,m,,nmn,nmx ! add MDOF nodes for visibility (not needed for solve)*get,ntp,etyp,,num,max ! get max element type number in the model in parameter ntpet,ntp+1,50 ! new element type ntp+1 set to superelement MATRIX50 type,ntp+1 ! set type ntp+1 before reading "creating" superelement with SEse,gen ! read in the superelement matrix from generation pass ! - master D.O.F. nodes already are at the interface ! - no need to couple coincident interface nodes this example ! - new element number assigned should be above maximum*get,snm,elem,,num,max ! get the element number of the superelement just loaded ! - needed for SFE loading the superelement below ! - extra work needed if more than one superelement/pbc,all,,0/pbc,f,,1/pbc,m,,1/pbc,mast,,1/title,Remainder of model attached to substructureeplo ! plot the elements in the non-substructure plus "outline" view ! of the substructurefini/solu ! "model.db" analysis type for substructure is neededSFE,snm,1,SELV, ,1 ! load applied in generation pass was in "model.db" ! - apply load to to superelement number "snm" found above ! - extra work needed if more than one superelementoutres,all,all ! save results for the all substeps of load step ! - change here and "EXPAND" below if desired to changesolve ! results go in the file "use.rst"save ! save "use.db" to optionally review non-substructure resultsfini ! "use.db" and "use.rst" now contain non-substructure results/post1set,last ! plot results at the end of the load step/title,Stress in the non-substructure elementsplnsol,s,eqv ! show nodal stress in the non-substructure*get,lastlstp,active,,set,lstp ! get the last load step number


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*get,lastsbst,active,,set,sbst ! get the last substep numberparsav,scalar,parameterstore,parm ! store them in file for retrieval belowfini

/COM,############ EXPAND ############/COM,############ EXPAND ############/COM,############ EXPAND ############

/clear,nostart /show,part3,grp/filname,gen ! filename for the expansion passresume ! brings up "gen.db" saved above, "super" is selectedparres,new,parameterstore,parm ! retrieve data on last load step/substep ! parres must follow resume statement/solu expass,on ! activate expansion passseexp,gen,use ! options for the substructure expansion pass*do,iii,1,lastsbst expsol,lastlstp,iii,,yes ! expand result at last load step/substep ! - (read about NUMEXP also) outres,all,all ! all data written solve*enddofini ! "gen.rst" now contains substructure results, last step/POST1 /title,Stress in the Substructureplnsol,s,eqv ! show nodal stress in the substructuresave,stresses_in_super,dbfini

/COM,############ POSTPROCESS ############/COM,############ POSTPROCESS ############/COM,############ POSTPROCESS ############ ! ! WARNING: The following is my own invention; use at your own risk. ! Warning messages will be generated by ANSYS.resume,model,db/show,part4,grp/post1cmsel,s,supernslefile,gen,rstset,lastesel,invert ! Select the elements NOT in substructure component "super"nsle ! Select the nodes of these elementsfile,use,rst ! Point to file "use.rst" that contains the rest of the resultsset,last ! Read in load step data for selected elements, last substepesel,all ! Select all elementsnsle ! Select the nodes of the elements/pbc,all,,0/pbc,f,,1/pbc,m,,1/pbc,mast,,1/title,Stress in the Full Structureplnsol,s,eqv ! Show nodal stress for the full model. ! Because of averaging, PLNSOL stresses on the interface of the ! substructure and non-substructure regions cannot exactly ! match values for these locations plotted separately, above. ! Element stress and displacement should exactly match in ! a small displacement linear analysis.save,stress_allelem,db ! Save the model with all stresses on elements/show,term ! Back to screen -- only works if used interactivelyplnsol,s,eqv ! Show the stress results for all elements if interactive ANSYS

The "top down" example saves the results of the "use" pass and the "expansion" pass in database files. These

can be loaded to inspect results in the non‐substructure and in the substructure parts of the model, respec�vely.

If the file is run interac�vely, the user will have to click the "OK" bu5on each �me the /CLEAR command executes,

and for a variety of warning messages that can appear. It may be preferred to run the file under Batch control,

and to later review the results in the plot files, and in the resul�ng database files. Remember to check for error

and warning messages. Because of the complexity of substructure analysis, the user should run checks on

balance of forces, and do other typical checking of results.

Large Displacement Nonlinearity and Substructure: The ANSYS 5.5 Advanced User's Guide, Chapter 5 gives more


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help on large rota�on (large displacement, geometrically nonlinear) substructured analysis than at the 5.3 level.

Note the comment that constraints should be applied in the "use" pass, not in the "gen" pass, for large rota�on

analysis.

If the file "model.db", used in the above example, has had Large Displacement ac�vated with "NLGEOM,ON"

then a nonlinear solu�on will be sought. Convergence criteria, ramping of loading, substeps, and other nonlinear

controls may be desired. Because the substructure will act linearly internally, convergence may not be as easy as

the user would wish. When the run does converge, the results will not be an exact match for the result without

substructuring. The output plots should be examined to see if they read "Substep 999999", indica�ng failure to

converge. If you test the above example with Large Displacement, use a Large Displacement model that

converges easily without a substructure approach. An a5empt has been made in the above example to cope with

a model that develops the Large Displacement solu�on in a load step containing a set of substeps. This is the

reason for statements that record the last loadstep and substep numbers. However, the example does NOT

reserve applica�on of all DOF constraints for the "USE" pass, as recommended in the ANSYS 5.5 guide, so it will

NOT be appropriate for models with constraints applied to non‐MDOF nodes in the substructure region. The user

can get around this by manually assigning MDOF to all the nodes to which constraints are applied in the

component "super", in "model.db".

The master degrees of freedom for the superelement must have rota�onal degrees of freedom for Large

Displacement work. The user can try assigning MASS21 elements to the master degree of freedom nodes if the

elements in the model do not have rota�onal degrees of freedom. The MASS21 elements can have a REAL value

that contains zero values for the masses and mass moments of iner�a. This will introduce the requisite rota�onal

degrees of freedom.

When using elements like SHELL63, which have rota�onal degrees of freedom, I have encountered a rather odd

result: The Large Displacement solu�on for the elements in the superelement (stored in "gen.rst" in the example)

is for the displacement of the substructure nodes with respect to a coordinate system embedded in the

superelement, not with respect to the global axes. This is not the case for small displacement solu�ons, which

appear displaced correctly. Since the superelement can undergo large rota�on, the displacement that is reported

and plo5ed for the nodes inside the superelement will be far smaller than the displacement reported and

plo5ed for the remainder of the model, in a Large Displacement solu�on. This is because the coordinate system

embedded in the superelement moves with the superelement. In limited tes�ng, the SEQV stress plots appear to

be OK, if the load step that is to be expanded is iden�fied properly. I have not inves�gated what happens to

stress components in the Global and Element Coordinate Systems. Rota�onal transorma�on of the stress and

strain tensors could be very complex. See the image below for a result combining SHELL63 elements,

substructuring, and Large Displacement.

A possible visual displacement fix (for the displacement plot problem of 6 DOF elements in Large Displacement

substructures) is to transform the displaced posi�on coordinates of the non‐MDOF nodes in the superelement on

the basis of the rota�ons and transla�ons of the origin of the superelement in Global Coordinates. The origin of

the superelement will be the MDOF node that reports no displacements or rota�ons inside the superelement (in

superelement coordinates); it appears to be the MDOF node with the lowest node number. Applying a

transforma�on properly will require deducing or looking up the order of the sequence of rota�ons that ANSYS

uses in Large Displacement work, or that ANSYS uses to report node rota�ons. A reading of the Theory Manual

suggests that ANSYS internally uses quaternions for large displacement rota�ons in space. This would be for the

usual reason that quaternions do not have a singularity in any orienta�on, in contrast to Euler angles. It appears

that the rota�ons reported at a node represent 3‐D components of a single rota�on vector, rather than Euler or

other angles, so the transforma�on will need to be based on rota�on about a vector that starts at a known point

in space (the origin of the superelement), plus transla�ons. I will be working on this as my next project for this

web page. The reported rota�on may be complicated by rotated nodal coordinate systems (NROTAT) or

superelements that the user has employed... this will require checking. Reader feedback would be appreciated. If

I get anywhere with this, I will limit myself to displacements only. Transforming stress tensors would be a bit

much!


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The above plot was generated using the above sample /INPUT file on a Large Displacement model of a can�lever

beam created with SHELL63 elements. A similar displacement discon�nuity results with BEAM4 elements in a

similar applica�on, as shown in the images below:

Tip 46: Plot Applied Temperatures

In a thermal stress analysis, temperatures will be applied as a "load". Temperatures can be applied to nodes with

the BF command, to elements with the BFE command, or implied using other commands. (Check the BFE

command and the element type in the ANSYS documenta�on for details on using BFE.) A colored element plot of

applied temperatures can be generated by using the commands /PBF,TEMP,,1 and EPLO, which Shows body force

loads as contours on displays, per the ANSYS Commands manual.

When using beam, link, and pipe elements, if the element thickness is shown with the /ESHAPE command before

execu�ng EPLO, temperatures can be made visible with contour coloring for these line elements. It may be

desired to exaggerate their displayed thickness with /ESHAPE in order to make the temperature informa�on

more visible.

Tip 47: Skipping Over Statements in an ANSYS Input File

ANSYS commands can be developed in a file that is executed with the /INPUT command. This can permit very

flexible and sophis�cated use of the program. Here is a well known programming trick that can be used to

temporarily skip over part of an ANSYS input file. Set a parameter ("SKIP" in this example) to a value that tells an

*IF statement to jump over a sec�on of code that you want to skip. This is much quicker than commen�ng out a

block of code, or cuEng and pas�ng as an input file is developed and modified. *IF statements that use this


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parameter could be located in a number of posi�ons in the input file ‐‐ this permits changing the value of one

parameter at the beginning of the input file to cause skipping of input code in a variety of loca�ons.

! Input code to ANSYS...! ...SKIP=1 ! Set to 1 to skip, 0 to run the code inside the *IF...*ENDIF commands*IF,SKIP,EQ,0,THEN ! ANSYS commands that are optionally executed...*ENDIF! ... more code follows

Since there is no compila�on of the input file, the "skip" technique uses li5le �me in choosing to execute or

bypass the blocked off commands (ANSYS s�ll has to read the blocked out code in order to check off the number

of *IF and *ENDIF commands).

Tip 48: Sta�c Analysis Followed by Transient Analysis

Transient analysis by ANSYS can model transient vibra�ons, or the dynamics of a flexible mechanism in mo�on,

in addi�on to more complex effects. Ini�al condi�ons can be applied, followed by transient analysis. One type of

ini�al condi�on is a zero velocity ini�al posi�on with stored energy. The stored energy can be poten�al energy of

posi�on, elas�c energy, or both. Another ini�al condi�on is an ini�al velocity. A model can have both ini�al

velocity and stored energy. A sta�c analysis may be desired to develop the stored elas�c energy, before star�ng a

transient analysis. Remember that for transient analysis, the mass of the model must be input in the appropriate

mass units, not as weight.

The following ANSYS input file illustrates the execu�on of a linear elas�c sta�c analysis that sets an ini�al

condi�on, followed by a transient analysis. The model is of a can�levered beam that has a force applied to the

free end in a sta�c analysis. The transient vibra�on that results when the force on the free end is removed is

obtained. No gravity is used. No damping has been applied, and ANSYS defaults for the numerical integra�on are

implicit. This is a linear elas�c solu�on, so the numerical integra�on should be stable, given the ANSYS algorithm

used. The �me substep size for the transient analysis should be smaller than 1/20 of the period of the first few

modes of vibra�on. (The user could tweak the ANSYS numerical integra�on parameters so that very high

frequency response modes are numerically damped. Stability in Large Displacement nonlinear transient analysis

is probably not guaranteed, although damping and small �me substep size should help.) The use of a consistent

mass matrix (default) should in general yield more accurate results than a reduced mass matrix if the element

density is coarse, however the use of a reduced mass matrix may shorten the solu�on �me in large models. The

movement of the �p of the beam is plo5ed ‐‐ it is not a perfect sinusoid because the ini�al deflected shape of the

beam is not an exact match to a mode of vibra�on.

! Transient vibration, cantilever beam, "plucking" the tip.! For illustration purposes only. Use at your own risk.fini/clear ! Start fresh/title,Transient Vibration of Cantilever Beam/PREP7 ET,1,BEAM3 ! 2-D model of beamR,1,1,1,1 ! beam crossection propertiesMP,EX, 1, 30000000 ! Young's modulus, BIN unitsMP,DENS,1, 7.34E-04 ! beam mass density, BIN unitsK, , 0.0 ! keypointsK, , 10. ! 10" longL, 1, 2 ! lineLESIZE,ALL,,,8,1,1 ! 8 element divisions LMESH, 1 ! mesh with beam elementsFINISH

/SOLU ANTYPE,4 ! Select transient analysis F,2,FY,-50000 ! apply down force on RHS node (unrealistically high)d,1,ux ! constrain first node at LHS, in X directiond,1,uy ! in Y directiond,1,rotz ! and constrain rotation

time,0.0005 ! small time increment, static OUTRES,ALL,ALL ! save all substep results timint,off,all ! no time integration -- treat as Steady State nsubst,2 ! two substeps to imply zero initial velocity for transient


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kbc,1 ! step change load solve ! find the static deformed shape

TIME,.002 ! time at end of transient (pre-determined to show oscillation) NSUBST,100 ! time steps small enough to show vibrationKBC,1 ! step change load fdele,all,all ! delete force -- show vibration after force is released OUTRES,ALL,ALL, ! save all substep results timint,on,all ! activate transient analysis solve ! find the transient vibration of the beamfini

/post1 /dscale,1 ! automatic scaling, to easily view final resultpldisp,1 ! show final deformed shapeFINISH

/POST26 NSOL,2,2,U,Y,UY ! results variable for plotting /title,Transient Vibration of Cantilever Beam: Motion of TipPLVAR,2 ! graph oscillation of the tip of the beam FINISH

NOTE: The use of the TIMINT command controls ac�va�on of the sta�c and transient por�ons of the solu�on.

The sta�c solu�on is obtained at two �me substeps so that an ini�al velocity of zero is implied. An anima�on of

the transient solu�on can be generated for the full beam in /POST1, showing the transient vibra�on in ac�on. For

an anima�on, the user will have to set a sa�sfactory displacement scaling value with the command /DSCALE, not

use automa�c scaling. In the anima�on of the Large Displacement mo�ons of a mechanism, a /DSCALE seEng of

1.0 will generally be wanted, so that angles of rota�on look correct. A zoom seEng other than /ZOOM,OFF will

usually yield a be5er anima�on.

Tip 49: File Compression for Model Storage

If no restart is to be executed on an ANSYS model, it will o2en be sufficient to save only the model database file

(*.DB) and the results file (*.RST) when archiving an ANSYS model. If the model was generated from command

input files, these will require storage. If only one load step was wri5en to the results file, the results file may not

require archival if the results are also contained in the database file. Load case, graphics output, and other files

may be wanted for archival. The database, graphics, and results files can be extremely large. They o2en compress

well using data compression programs such as the UNIX compress and gzip u�li�es (gzip is more powerful than

compress). On Windows computers, gzip is also available for NT (it handles long file names), in addi�on to the

shareware ZIP u�li�es, though you will need to dig on the Internet to find gzip for Windows NT ‐‐ have a look at

GZIP on the web and look for instruc�ons and the version for your computer (test before use). In FEA work, I find

the advantage of the gzip u�lity to be that the compressed file name is simply the original file name with .gz

appended, and the uncompressed file is removed. The data storage requirement may be reduced by roughly 25%

to 80%, both on the hard drive, and on tape or removable disk. The data compression is significantly more

effec�ve, though much slower, than with the disk compression scheme that can be used by Windows NT 4.0,

which also does not keep the files compressed when they are sent over a network, or otherwise moved around.

Hint: Make sure that those who will decompress the files in future will know how to do it!

Tip 50: Organizing Large FEA Models

Examining the results of an FEA model, selec�ng and modifying por�ons of the model, and keeping a record of

what MAT (material) and REAL (shell thickness, beam size, etc.) values were used for various parts of a model

becomes very difficult with large FEA models. A very large structure represented with hundreds or thousands of

individual beam elements or areas meshed with shell elements, will require that the iden��es, materials and

REAL seEngs for large numbers of parts be organized and recorded.

There is no one way to do this. Individual parts, or groups of parts, can be defined to be components that are

accessed by names of up to 8 characters. These parts can be geometric en��es, elements, or nodes. Macros can

step through all the components using *GET commands. Collec�ons of components can be grouped into

component assemblies. An individual assembly could be created for those components that are to be selected

under certain circumstances, for analysis or for results review. The use of components makes it possible to refer

to either a part or a subassembly by one name, and easy to select it. The crea�on of a component can save a set


40 of 54 1/17/2009 11:41 AM

of en��es that were selected with a certain sequence of select logic, and be used in the enhancement of the

ANSYS select logic process. The database component commands are: CM, CMDELE, CMEDIT, CMGRP, CMLIST,

and CMSEL. A macro can be wri5en that will step through all components, ploEng them, including the

component name and informa�on on it in the plot �tle, or an annota�on.

Either REAL values for elements and en��es, or MAT values, can be used to iden�fy parts in a model with

numbers. (The element type will have to support a REAL value if a REAL is to be created for that element type.

However, a REAL value can be forced on an element even if the element type does not admit assignment of a

REAL. This can be done when crea�ng an element, applied to the geometric en�ty that is to be meshed with the

element, or forced a2er the fact with EMODIF. When EMODIF is used, be cau�oned that in a re‐meshing the REAL

assigned to the geometric en�ty will be used. When the element type does not accept a REAL seEng, the R

seEng can simply be le2 blank. In that case, the commands NUMMRG,ALL and NUMCMP,ALL can make a mess

and should not be used in this all‐inclusive form ‐‐ s�ck to specific forms such as NUMMRG,KP.) In a model made

of shell elements or beam elements, for example, each plate or beam could be described with its own REAL

value, even though there may be many plates or beams of a given thickness or size within the model. Where a

group of parts will always be chosen with the same REAL value, they could share one REAL seEng. This makes

changing the shell thickness or the beam characteris�cs very simple, and provides easy part selec�on with

commands like ESEL, ASEL, or LSEL, as appropriate, according to their REAL value, or a range of REAL values. An

array (see the *DIM command) could correlate REAL values with other informa�on, such as part names (with an 8

character limit). The same approach can be taken with the seEng of MAT values for describing the material

proper�es. Using material numbers for part iden�fica�on, however, could get cumbersome, because there are

such a large number of individual material property seEngs, and they may be temperature dependent in some

models, or include material nonlinearity.

I find it helpful NOT to set any of the geometric en��es or elements in a large model to a MAT or REAL value of

one. One is a default value that is some�mes assigned when no value has been assigned by the user. Geometric

en��es may have a value of zero when nothing has been assigned. I can then select things that have a MAT or

REAL of zero or one to check on whether I have forgo5en to assign a value to any part of the model. Plots with

coloring assigned according to REAL or MAT will help in checking a model.

When REAL or MAT values have been used to differen�ate between different parts of a model, the user must be

careful not to use a NUMCMP,ALL or NUMMRG,ALL command on en�ty numbering, because it will compress or

merge out REAL and MAT values. This will destroy the iden�fica�on scheme. The NUMCMP command will have

to be called with the specific quan��es to be compressed individually iden�fied, such as NODE, as in the manual.

When the parts have been iden�fied by different REAL or MAT numbers, a coloring scheme based on REAL or

MAT can be used during element or geometric en�ty plots, to improve iden�fica�on of the parts of a model, and

the appearance of the FEA plot. Cau�on: ANSYS does not use a "4 color map theorem" when ploEng (can't do

this in 3‐D anyway) so parts of differing REAL or MAT may be adjacent and have the same color.

Arrays could be used to assign numbers to component names, and to keep track of what REAL values were used

by the elements within components. Arrays could assign 8‐character names to the parts described by different

REAL values. Arrays could be used to set several values of a number of shell thicknesses or beam sizes to be

examined in a series of analyses that are to be run automa�cally. As discussed above, this parameter informa�on

can be included in annota�ons during model and results ploEng, making model review easier and less error‐

prone.

Tip 51: Selec�ng Nodes in a Stress or Strain Range

The selec�on of nodes in a certain stress range can be effected with, for example, the command

NSEL,S,S,EQV,40000,9999999 in order to get nodes with EQV (Von Mises equivalent) stresses from 40000 to

9999999. This and similar commands can be used to get at only the por�ons of a full model that are significantly

stressed.

The effec�veness of this command can be compromised somewhat by nodal stress averaging, shell stress surface

selec�on (TOP, MID, or BOT), and other complica�ons. The command would typically be followed by the two

commands ESLN and NSLE to be able to plot the associated elements and their stresses.

If the above part iden�fica�on scheme using REAL values has been employed, the stress level selec�on command


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could be followed by a macro that selects all parts that match the REAL types of the selected elements. This

would make it possible to see all highly stressed parts. This approach is helpful with complex models with parts

visually hidden by other parts.

Tip 52: Selec�ng Nodes that are Subjected to Nodal Coupling

Nodes that are coupled can be selected with commands such as NSEL,S,CP,,1,999999 in order to show only the

coupled nodes, and to have the op�on of using picking to delete nodal coupling, or for other purposes. Once

coupled nodes have been selected, work to evaluate the forces resul�ng from the coupling can begin. Similarly,

nodes can be selected according to their presence in constraint equa�ons (CE), their applied displacement (D),

forces applied, and other criteria ‐‐ see the NSEL command for further informa�on.

Tip 53: /NOPR and /GOPR Speed Up Input Files and Macros

When a long input file or macro is read while running ANSYS interac�vely, text informa�on is wri5en to the

output screen and op�onally to an output file. If a significant number of *GET and similar opera�ons are being

executed, a large quan�ty of text informa�on will be wri5en to output. If the input files and macros are known to

be fully debugged, they may execute faster if they start with /NOPR and end with /GOPR in order to switch off

text output while they are running. If their execu�on is causing geometry, nodes, or elements to be generated, a

speedup may result from temporarily switching off the genera�on of graphics with IMMED,0 and /SHOW,OFF.

They can be re‐ac�vated with IMMED,1 and /SHOW,TERM. You may want to consider the /UIS command also.

Tip 54: Using Commands IMMED and /UIS and /SHOW,OFF to Suppress Plo;ng

I some�mes develop a model interac�vely, seEng up some dimensions as parameters, then manually modify

and add to the log file that is generated. The resul�ng log file becomes an input file that I can use for parametric

genera�on of a model. When I run this input log file, I don't want all of my various plot commands to be

executed, only those for finished model display and results review. This can be implemented with the IMMED,0

(for interac�ve execu�on), /UIS,REPLOT,0 and /SHOW,OFF commands. They can be re‐ac�vated with IMMED,1

and /UIS/REPLOT,1 and /SHOW,TERM. If the graphics output is intended to be sent to a graphics file, the

command /SHOW,FILE can be used for re‐ac�va�on of wri�ng to a file previously designated by a

/SHOW,filename command. If wri�ng to a file, the immediate mode ploEng is off by default. Be warned that if

you change to another output graphics filename with the /SHOW command, then come back to the first

filename, the first file will be overwri5en.

When re‐running a log file using /INPUT the messages that required clicking "OK" will be generated and execu�on

will pause. This means that the /INPUT command will not re‐run all log files una5ended.I have not found the /UIS

command to completely stop this, such as when the /CLEAR command is issued. Running batch is some�mes

desirable.

Tip 55: What's the Bauschinger Effect? Comments on Material Yield

I first wanted to do elas�c/plas�c analysis in ANSYS to get a feel for the onset of failure in an automo�ve part. It

was of value to show that one proposed crossec�on shape was significantly be5er than another. This required

me to use plas�c material proper�es for steel, in nonlinear large deflec�on analysis in ANSYS. Unfortunately, I

had taken neither an academic course in metal forming, nor a5ended an ANSYS course in nonlinear analysis.

Digging into the ANSYS manuals, the first thing one has to decide on is whether to use Kinema�c Hardening or

Isotropic Hardening for the material model. Fortunately, high precision was not needed for what I was doing, so

the exact stress/strain curve and the choice of material yield rules were not a big concern. S�ll, I wanted to know

what I was doing, within reason. The manual men�ons the rela�onship between kinema�c hardening and the

Bauschinger effect. A2er some poking around, I finally found a basic descrip�on of the Bauschinger effect in

Timoshenko's Strength of Materials Part II: Advanced Theory and Problems Third Edi�on, Krieger, Florida, 1976.

Essen�ally, a tension test causing slight yielding permanently deforms (causes slip in) unfavorably oriented

crystals before other crystals in a specimen. Consequently, upon unloading, the permanently deformed crystals

are in some compression. A2er re‐loading with tension, the onset of yield is raised because the deformed crystals

do not reach their new slip stress un�l the load is higher than the first �me. If the material is compressed a2er

tension loading, the deformed crystals reach their compression slip stress before the rest of the crystals, with the

result that compression yielding starts sooner than in a fresh unstrained specimen. Quo�ng Timoshenko, "Thus


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the tensile test cycle raises the elas+c limit in tension, but lowers the elas+c limit in compression." This is the

Bauschinger effect.

One thing that may affect the choice of a yield model in ANSYS will be what is supported by an element type.

Shell 63 does not support nonlinear material proper�es at all. Shell 181 supports isotropic hardening but not

kinema�c hardening. Shell 43 apparently supports both, but it is suggested that Shell 181 is more capable.

It should be remembered that ANSYS requires a true strain curve in material characteriza�on, not the

engineering strain curve, when mul�linear curves are entered. In quick‐and‐dirty checks on the possibility of

failure of a structure, I some�mes consider it sufficient just to use a bilinear model, with the yield por�on of the

curve fairly flat. This wouldn't do for models of metal forming in manufacturing, but can some�mes be used to

assess whether structure failure is a concern when some por�ons of an elas�c model are exceeding yield. It may

be desirable to load the structure beyond the design load in order to observe where significant failure starts, in

order to get a feel for margin of safety. This may require arc‐length analysis. (My use of the word "quick" in

"quick‐and‐dirty" is overly op�mis�c.)

Some design codes have rules for elas�c‐plas�c or for fully plas�c analysis that would have to be used, if such an

analysis was needed to jus�fy or qualify a design legally or to fulfill a contract.

Tip 56: Thought Experiments

Nothing so focuses the mind on the design details of a product as hearing that it failed in tes�ng or in service. You

don't have to be Einstein to perform the following thought experiment: Suppose that you heard that some aspect

of a design had failed in service. The failure could be yielding, buckling, crack growth, fracture, vibra�ng to death,

unacceptable deforma�on, wear or binding, or whatever is appropriate. Brainstorm as to whether it could

happen, what could have caused it, and how analysis could highlight what is or could be wrong. Do this thought

experiment for as many characteris�cs of the product as you can. You may substan�ally extend the number of

things that you consider in the design, and in the FEA work. It may save someone's neck, either figura�vely or

literally.

Possibili�es and "What If's":

What could cause yielding -- are fasteners or welds overloaded? Were their loads even checked -- and for all load

cases, or the bounding load cases? Are the bounding load cases complete? Are stresses above yield over a significant

region? Are surface stresses of shell elements doing something unusual? Were nonlinearities considered? Were all

possible combinations of loading considered? Is there a high load situation that was not considered? Were all

components evaluated in FEA? Is there an unusual boundary condition arrangement that has not been considered? Was

the FEA mesh too coarse? Were relevant details that cause stress concentrations left out of the model? Can what was

discounted as a local stress concentration lead to progressive collapse or crack growth?

Can buckling arise? Have both linear and nonlinear approaches to buckling possibilities been considered? Has a portion

of the model been represented so simplified that buckling possibility is not detected? Has nonlinear buckling been

considered at loads greater than the design loads, so that some sense of the margin of safety is obtained? Can

restraint of thermal expansion cause stress and buckling?

Crack Growth -- what details exist that could possibly be sites for crack growth? Do surface stresses give any

warnings? Where could details be included to reduce crack growth possibilities? Are regions that have geometry that

could lead to crack growth highly stressed and/or cyclically stressed? Is direct tension on welds causing Type I fracture

loading? Is shear, bending or torsional loading (applied forces and moments, and/or applied displacements and

rotations) on structural details causing Type II or Type III fracture loading on welds? Is the loading significant? Is fatigue

an issue? Is fracture analysis warranted? Is there a reliable shortcut guide to what is tolerable? Is such a guide even

possible? Could crack growth be so rapid that it happens between inspections and causes sudden fracture? Are cracks

detectable at a size that does not immediately cause fracture? Should inspection intervals be more frequent when the

product is new?

Vibration -- what loading could stimulate vibration? What frequencies could drive vibration? Is there adequate structural

damping or are there other mechanisms to suppress trouble? Where are the natural frequencies of vibration? Do steady

state responses or random vibration responses need to be evaluated? Is flow induced vibration a possibility? Will sound

and noise cause destructive vibrations? Have all possible boundary condition arrangements been included in assessing

vibration?

Will large deformations go outside of what is acceptable? Is the structure stiffness high enough for the product use? Will

deformation cause loss of function, contact with the surroundings, binding, interference, collision, or excessive wear of

moving parts?

Is the design something that can be manufactured with the quality and uniformity required to avoid structural weakness?

The analyst should extend the above items to everything that needs to be considered, or that could go wrong.


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Tip 57: Control of Meshing

Since I am using ANSYS 5.3, I can't comment on the latest in ANSYS automa�c meshing capabili�es, but a couple

of sugges�ons about the basics may be helpful. You can select the lines that have not yet had mesh density

applied, with the command "LSEL,S,NDIV,,0" as a check that all lines have had mesh density applied, or for

convenience. The same type of command can be used to find the lines with other mesh densi�es.

Basic ANSYS training should have taught you that line and area concatena�on can help you get mapped

meshing, which gives rela�vely neat regular meshes such as all four‐sided area elements, or all six‐sided solid

elements. This can make a big difference in some models.

Tip 58: Four View Plot

When assessing modes of vibra�on or deflec�on of a 3‐D structure, I have found it convenient (though slower) to

generate ANSYS plots showing my model in four views on one sheet of paper or screen plot. The tradi�onal

views: Front Eleva�on (front), Plan (top), Side Eleva�on (right), and Isometric (iso), can be posi�oned in four

windows that are located in the lower le2 quarter, upper le2 quarter, lower right quarter, and upper right

quarter of the plot, respec�vely. (Other standard view layouts can be subs�tuted). A displacement plot of a

mode shape with PLDISP or PLDISP,1 with these four views ac�ve will leave fewer ambigui�es about what is

happening with mode shapes than a single‐view plot. The only shortcoming is that the images are small ‐‐ I prefer

to use 11" x 17" paper in landscape mode for these plots.

An annecdote I heard from a guy I knew: A U.S. ship entered a foreign shipyard needing a new propeller. The

ship's engineer supplied a drawing, and a propeller was cast and installed. The ship was launched and powered

up. When set to go forward, the ship went backward ‐‐ the shipyard used the European standard view

interpreta�on of an American drawing, and the propeller was mirror imaged!

The following code can be put into a macro to generate a four‐view screen. Customize it as you wish ‐‐ I include

commands to turn off PowerGraphics and to use Centroidal sort. This permits clean plots on paper with large

models. NOTE: Users may want to set the /DSCALE value to the same level in all four windows with

"/DSCALE,ALL,value".

! For information only. Use at your own risk.! Put these lines in a macro! Set screen to show four standard views:! User may want to set /DSCALE to the same value in all windows/WIN,1,LTOP ! Window 1 left top/WIN,2,RTOP ! Window 2 right top/WIN,3,LBOT ! Window 3 left bottom/WIN,4,RBOT ! Window 4 right bottom/WIN,5,OFF ! Turn off Window 5/VIEW,1,0,1,0 ! Window 1 top (plan) view/VUP,1,Y ! Reference orientation/VIEW,2,1,1,1 ! Window 2 ISO (isometric projection) view/VUP,2,Y ! Reference orientation/VIEW,3,0,0,1 ! Window 3 front (front elevation) view/VUP,3,Y ! Reference orientation/VIEW,4,1,0,0 ! Window 4 right (side elevation) view/VUP,4,Y ! Reference orientation/AUTO,ALL ! Fit all windows/PLOPTS,INFO,1 ! Include information column/PLOPTS,LEG2,0 ! Don't include view information/TYPE,ALL,2 ! Centroid sort, better print that Z-buffer/CPLANE,0 ! Cutting plane/graphics,full ! NOT PowerGraphics (fewer facets?)! User has to issue the plot command

The next code can be used in a macro to return to a front view in one window. Again, the user may want to

customize some of the lines:

! For information only. Use at your own risk.! Set screen to show one front view in Window 1/WIN,1,SQUA ! Full square Window 1/WIN,1,ON ! Turn on Window 1/WIN,2,OFF ! Turn off Window 2/WIN,3,OFF ! Turn off Window 3


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/WIN,4,OFF ! Turn off Window 4/WIN,5,OFF ! Turn off Window 5/PLOPTS,INFO,1 ! Info on for right column/PLOPTS,LEG2,0 ! Don't show the view information/VIEW,1,0,0,1 ! Front (front elevation) view/VUP,1,Y ! Reference orientation/TYPE,ALL,2 ! Centroidal sort, better print than Z-buffer/CPLANE,0 ! Cutting plane/graphics,full ! Not PowerGraphics (fewer facets?)! User has to issue the plot command

A2er running one of the above view‐genera�ng macros, the user has to issue a plot command to see the result.

Tip 59: Quick Review of Mode Shapes

To start prin�ng plots of mode shapes directly from ANSYS mode shape results, having the hardcopy window pop

up automa�cally, type in an input line such as:

SET,1,1$PLDISP$/UI,COPY

The dollar sign separates the commands that are grouped on one input line. Click the hardcopy OK bu5on to kick

off the hardcopy. You may want to set the print to landscape mode, first. Then, to print plots of the rest of the

mode shapes, type:

SET,NEXT$PLDISP$/UI,COPY

Simply keep repea�ng the second line (to avoid re‐typing, double‐click it in the ANSYS Input window), and

clicking the hardcopy OK bu5on, to get the rest of the modes printed. Note that the SET,NEXT command will loop

back to the first mode shape a2er the number of modes stored in the RST file has been exhausted.

You may see the somewhat odd message:

This comes up because of the /UI,COPY command, and has something to do with the /ZOOM command.

Consequently, this method may not work sa�sfactorily if your zoom is not off. In viewing mode shapes, it will be

typical to have zooming off.

The following code fragment read from an input or macro file can automa�cally plot a set of mode shapes. *GET

commands are used to detect informa�on on what substep and frequency are read. The user does not need to

know how many modes were generated, so automated ploEng to a file is simpler. The displacement plots will

contain substep and frequency informa�on. This code should cope with degenerate eigenvalues or rigid body

displacements. Execute this from within /POST1 a2er a mode case analysis was run, or a2er the database and

RST file for a mode case analysis are loaded. There is no error check, so this must be used properly. The user will

want to test and customize these commands:

! For information only. Use at your own risk.set,1,1 ! set to the first modepldisp ! plot the first mode*do,iii,1,9999999 ! use a very large number set,next ! set to the next mode *get,ntotal,active,0,solu,ncmss ! cumulative substeps -- cycles to 1 if all modes in RST done *get,thefreq,mode,iii,freq ! use this line if desired to get frequency into a parameter *if,ntotal,eq,1,then *exit ! exit do loop if done *endif pldisp ! plot the displaced shape


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*enddo

Tip 60: Using ANSYS Help

When using ANSYS interac�vely, help on any command can be accessed immediately by typing

HELP,commandname into the input window. If the HELP applica�on has been launched independently, the quick

way to get help on a par�cular command is to use the "Navigate" and "Help On..." menu choice. Type the

command name into the "Help On" text box that pops up, then click the Apply bu5on.

This will send the help program to the Commands manual for the command name typed. If you click the "Apply"

bu5on, the "Help On" dialog box remains visible, and can be used to type in other command names.

The online ANSYS help system makes the need for trips to hard‐copy documenta�on much less frequent. The

Help applica�on can be launched while ANSYS is running in the background, so ANSYS documenta�on can be

studied while a large model is solving.

Tip 61: The FEA Job Hunt

Not strictly an ANSYS issue, I want to put in my own two cent's worth on this topic, which is of interest to all of us.

Some �me back when I was job hun�ng during a recession, I was given a pre‐screening interview over the phone.


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The interviewer owned a consul�ng firm. It rapidly became apparent that he had been lied to, many �mes and

by many people, about their FEA experience. I had been using a company's proprietary FEA code and was not

experienced with the major commercial programs, so was immediately suspect. Another �me, I heard of an

applicant using another guy's FEA model images to present in a job interview as "evidence" of his own

experience. I interviewed a guy who claimed experience with "a locally available product I wouldn't know." It

became apparent that he had been coached and knew only the buzz words. When job hun�ng it is challenge

enough to compete with other experienced people ‐‐ misrepresenta�on we don't need.

The FEA job applicant should be able to present evidence of academic and/or post‐academic training. The

applicant should have a por[olio of previous work. The applicant should be instructed to bring these to the

interview. The por[olio informa�on can be a li5le awkward when the products are proprietary. If it would be

illegal to present any images of work done, then I suggest a keen applicant independently develop a set of small

models using ANSYS/ED that illustrate the FEA techniques with which the applicant is familiar. The applicant

should be able to describe the modeling considera�ons, techniques, compromises, pi[alls, and post‐processing

possibili�es of the examples.

Of course, with good references and personal networking, the above situa�ons are less of a concern. S�ll, as with

computer programming, the produc�vity of individuals can vary surprisingly. Consequently, the applicant should

be able to describe what makes FEA produc�vity possible, some of the modeling shortcuts possible, and give an

energe�c, ar�culate and confident presenta�on of self.

Ques�ons of the "how would you model this" and "how would you handle this" type are just as relevant as they

are in other professional interviews.

I have had both excellent and very poor interviewers assessing me. With some, I've had to politely direct the

interview just to be able to point out my range of skills. On the basis of my own small set of experiences I would

say that first impressions are very telling ‐‐ if you get a bad feeling about a place during the interview, it may be

for a good reason. If you see a place as being a welcoming workplace with a healthy environment, and the

people you meet behave well and are socially skilled, the odds are that as long as you func�on as a valuable

employee, life there will be OK. I've had an interviewer keep me wai�ng for an hour and a half past the

appointment �me. I got the job, and found that things were frequently out of control, and the guy was

impossible to see. I had an unpleasant "stress interview", and sure enough, the place was cheap, had an

unhealthy work life, and poorly structured leadership. Another interviewer struck me as highly manipula�ve,

trying to goad me into making nega�ve comments about employers and ethnic groups (of all things‐‐talk about

playing with fire‐‐a personnel manager who apparently fancied himself a psychologist), and giving me inaccurate

data on hiring inten�ons. A friend of mine got the job and detested it, saying the place was poisoned with

poli�cal games. I've had a thoroughly posi�ve interview, got the job, and found I was with great people. Ignore

their hype: What you see is (most likely) what you get.

To end this on a few posi�ve notes: Employers want to hire someone who will be a success for them. Your boss

will be very happy if you make everyone's life easier through your contribu�ons. Prepare yourself to give a

picture of your range of skills, energy, confidence, communica�on skill, ability to work with others, range of past

experience, and ability to �me‐manage a set of responsibili�es. Some employers begrudge every penny, but

others are pleased to compensate you a5rac�vely if you produce well. Size up the poten�al employer carefully,

for your �me is valuable. Best of luck.

Tip 62: *VPUT and DESOL

I have no idea why the commands manual entry for *VPUT describes the parameter ParR as "The name of the

resul�ng vector array parameter." The parameter ParR is the source of data, NOT what is changed by *VPUT.

Note that *VPUT can write to node results, and to an element ETABLE. There is a difference between wri�ng to

"nodal degree of freedom results", and what the manual calls "element nodal results" with *VPUT.

The *VPUT command can write informa�on to the node results which can then be plo5ed as if it was the nodal

results, using the PLNSOL command. The command manual tells us that the effect is permanent for degree of

freedom results (changing the database), but temporary for all others (derived results, not changing the

underlying database). Wri�ng stress data with *VPUT does not affect element plots with PLESOL,S,op+on. If you

use *VPUT to write to "element nodal stress results", immediately do a PLNSOL plot to see the effect, do a PLDISP


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plot (seeing unaffected degree of freedom data), and then do another PLNSOL stress plot, the la5er PLNSOL plot

shows original data that is unaffected by *VPUT. The temporary modified PLNSOL stress plot effect does not

cooperate with PowerGraphics to give a plot with contour discon�nui�es. Before using the *VPUT temporary

effect in ploEng nodal stress (or other derived) results, test the method carefully for errors, and for any errors in

what I have just said! Warning: In a shell model, the plot of temporary *VPUT derived data may make the plot

legend indicator for the TOP, MID or BOT surface of the shell elements meaningless ‐‐ annotate the plot to inform

the reviewer.

The DESOL command does write derived informa�on into the database to the nodes of elements, on a

permanent basis. The command is powerful, and poten�ally dangerous. Annotate plots and change �tles to

inform the reviewer. It can be painfully slow to apply DESOL to every node of every element, element by

element, in a large model.

Tip 63: How to Divide One Element Table Column by Another

To divide one column of an element table (ETABLE) by another column, use the SEXP command. Make sure the

denominator is nonzero! Per the commands manual, SEXP "forms an element table item by exponen�a�ng and

mul�plying." The result of SEXP is formed from (ABS(Lab1)**EXP1)*(ABS(Lab2)**EXP2). Because of the absolute

value opera�ons that protect ANSYS from complex numbers being generated, you will get the absolute value of

the answer you want. To divide with SEXP, use a posi�ve exponent EXP1=+1 for the numerator Lab1, and a

nega�ve exponent EXP2=‐1 for the denominator Lab2.

If you must have the ETABLE column answer with its posi�ve or nega�ve sign, one way to get it would be to use a

blend of SEXP and SMULT. Given ETABLE columns A and B, you want a column of A/B values with their signs. Use

SMULT to form C=A*B which has the same sign as A/B. Use SEXP to form D=1/(ABS(B)**2) which is posi�ve. Use

SMULT to form E=C*D. Now the column E=A/B with its correct sign.

Tip 64: Element Tables (ETABLE) and Array Data Exchange ‐‐ An Example

Note that *VGET and *VPUT can communicate with an ETABLE and with an Array. In this way, informa�on that

can only be obtained in either an ETABLE or an Array can be moved back and forth for manipula�on, evalua�on,

and display. You have to be inside /POST1 to use ETABLE. SOLVE has to have been executed in order for ETABLE

data to be available. In brief tes�ng, I found par�al solve execu�on not to be sufficient to make any data

available for an element table, for example, element volume. In order to use *VPUT on an ETABLE, the array and

ETABLE column will have to already exist. Create the ETABLE column by making a copy of an exis�ng ETABLE

column (you can use the SADD command, for example), or by crea�ng a new dummy column from model

informa�on. Give the new column an appropriate name for the data that will come from the array. If selec�on of

only a subset of your elements is in place, or if there are gaps in the element numbering, it may be preferable to

use a *VMASK during the *VGET and *VPUT calls to avoid warning messages. The *VMASK array contents can be

based on a test of element selec�on. Array size can be based on MIN and MAX values for selected element

numbering, if you use an offset during data movement. Minimizing array size is a good reason for compressing

node and element numbers when developing a large model, before load cases, solu�ons and array dimensions

are prepared.

An example with shell elements: put shell element volume in an ETABLE, shell element area in an array, move

the area array into an ETABLE column, divide volume by area to get shell element thickness for each element in a

new ETABLE column, and do a colored ETABLE contour plot of your model's shell element thicknesses (do not

average the values in the ETABLE plot). If the shell element is of varying thickness, this process generates an

average for the element. Print a colored picture of this plot. Augment this with an element plot using /ESHAPE,1

and coloring based on REAL constant values. This should help with "pesky visitors" who are always wondering

how thick certain parts of a complex shell model are. It may help you catch some modeling errors. Here is a

macro to generate the ETABLE thickness column and plot the model colored by element thickness. This macro is

wri5en to process Shell63 elements in the selected set of elements. It is a basic macro with no tes�ng to prevent

error condi�ons, or "cleanup" a2er execu�on. It illustrates movement of an array column into an element table

column, use of masks, offset of the transferred data to minimize array and element table size, and division of one

ETABLE column by another using the SEXP command. The macro has to be re‐executed if the model is changed ‐‐

an ETABLE update would not be sufficient. The denominator that contains element area in the divide opera�on

should automa�cally be nonzero because all shell elements have areas.


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! ETABLE and Array usage and interaction example.! For illustration only. Use at your own risk.! This example is used on SHELL63 elements.! An array is created called "aaa", element selection may be reduced,! and element tables are used. A plot results.! Run from within /POST1! Put element volume in an ETABLE, and element area in an array.! Move the area array data into the ETABLE! Divide element volume by element area to get an element thickness column.! Element thickness value should be the average for variable thickness elements.! Plot the ETABLE thickness data for a view of the Shell 63 elements! that are contour colored according to the element thickness.!esel,r,ename,,63 ! re-select only the SHELL63 elementsaaa= ! kill the array to be used*get,xmax,elem,,num,max ! what is the highest element number selected? ! element number compression will be desirable*get,xmin,elem,,num,min ! minimum element number*dim,aaa,array,xmax-xmin+1 ! array to hold areas has to be this big*vget,aaa(1),elem,xmin,esel ! fill array with info on whether element is selected ! -1=not selected, 0=undefined, 1=selected ! offset with xmin (see the manual)*vmask,aaa(1) ! use element selection info as a mask*vget,aaa(1),elem,xmin,geom ! fill the array with geom info on the elements ! for shell elements this is AREA. Offset with xminetable,volu,volu ! create element table column with element volumesadd,geom,volu, ,0,1 ! create dummy column to contain other data*vmask,aaa(1) ! use array as a mask (geom data is positive or zero)*vput,aaa(1),elem,xmin,etab,geom ! put data into ETABLE "geom" column. Offset with xminsexp,thick,volu,geom,1,-1 ! divide volume by area, get avg. shell element thick./title,ETABLE Plot of Shell 63 Element Thickness Valuespletab,thick,noav

You can then select a few elements of interest, and list the element table for the column containing the

thickness, to get a numerical thickness value for those elements. You could use *VGET to put the REAL values for

the shell elements into an array, and transfer that data into an ETABLE column. Then when you list ETABLE

informa�on for a selected element, you could see the REAL and the thickness value side‐by‐side. Contour colors

could be explicitly assigned to the different thicknesses found in the ETABLE column, if the number of thicknesses

was not too great. You could place a bu5on on the toolbar that calls a macro that (1) checks that the user is in

/POST1, (2) checks that there is results data in the database, (3) asks you to pick elements with the mouse, then

(4) generates and prints the ETABLE values for the REAL, thickness, and stress values of those elements, and (5)

finally cleans up and restores the original element selec�on.

Another use: Put shell element mid‐plane Sx, Sy, and Sxy into columns, and mul�ply them by the element

thickness column. The resul�ng data would be similar to TX, TY, and TXY data that can be obtained directly, but

now would be in the direc�on defined by the ac�ve coordinate system. A macro could (1) have the user pick

nodes or keypoints to define a local coordinate system, (2) make it ac�ve, (3) develop this ETABLE data and

generate plots colored by load‐per‐unit‐length in the known direc�ons, then (4) clean up. This could be done for

SINT or SEQV if desired.

The possibili�es are endless.

Tip 65: Error Es�ma�on, PowerGraphics, and ERNORM

Error Es�ma�on will not be available when you enter /POST1 if PowerGraphics is ac�ve. If you turn off

PowerGraphics, then the "Op�ons for Output" in the GUI will offer the ERNORM seEng that ac�vates error

es�ma�on. If PowerGraphics is not ac�ve, then the "Op�ons for Output" in the GUI will not offer the AVRES

seEng that controls discon�nuity of contours at changes of material and REAL value for elements. The first �me I

encountered this was when I couldn't get an error report, and couldn't imagine why, un�l I had poked around for

a while. I had entered /POST1 with /PowerGraphics ac�ve. ERNORM is ON by default when you enter /POST1,

but only if PowerGraphics is OFF with /GRAPHICS,FULL.

Tip 66: Concatenate and Mesh Last

One of the things I have seen go wrong in model development is: I have concatenated lines and/or areas, then

performed a boolean opera�on on them. Model problems resulted and I had to start from scratch (I re‐ran most


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of the log file); I suggest that boolean opera�ons happen first ‐‐ concatenate and mesh last. (This was with ANSYS

5.3; solid modeling problems are reduced with each version.)

I have found that when three or more areas are concatenated, the lines that are implicitly concatenated have to

be concatenated manually before successful mapped meshing will proceed. When only two areas are

concatenated, the lines concatenate automa�cally.

When concatenated areas are used to map mesh a volume, it may happen that an adjacent volume defined by

an area that is part of the concatenated set will not mesh un�l the "pseudo‐area" that results from the

concatena�on is deleted. I've had occasions when this did and did not happen. The concatenated lines may

need cleanup, also. It is possible that if the volumes that do not require concatenated areas in order to be

meshed, are meshed first, that the remaining volumes can have area and line concatena�on created a2er, and

then be meshed themselves, without error messages. This considera�on may compromise easy mesh refinement

and adap�ve meshing. It may be necessary to go to tetrahedral elements for easy meshing, unless ANSYS

revisions have fixed up mapped meshing concatena�on.

Trivia: I first encountered the word "contatenate" when using an IBM 370 a long �me back. A neighbour told me

that "concatenate" is based on the La�n word for "chain".

Tip 67: ANSYS Output of Data to Files for Use by Other Programs

Numerical data contained in parameters can be output into ASCII files using the *CFOPEN, the *VWRITE, and the

*CFCLOS commands. The *VWRITE command only works when called from an input file that includes a format

statement similar to FORTRAN. The following simple macro makes the *VWRITE command easy to use:

! Put this code into a macro file called "writer.mac"! call with: writer,data! write data in arg1 to a file previously opened with *CFOPEN! later on, close the file with *CFCLOSE*vwrite,arg1(E16.8)

The following ANSYS /INPUT data test will demonstrate the use of the above macro. In this test, the macro has

been called "writer.mac" and it is in the current directory. Either numbers or parameters that evaluate to

numbers can be used in the following commands:

! Example of data output to file from ANSYS*cfopen,myoutput,datwriter,123writer,234/clearwriter,345writer,456*cfclos

Execu�ng the above commands from within ANSYS generates the following data in the file "myoutput.dat", and

demonstrates that the output file remains open in spite of the /CLEAR command. The following is the content of

the file "myoutput.dat":

.12300000E+03 .23400000E+03 .34500000E+03 .45600000E+03

The filename used in the demo ended in ".dat" so that the data would be accessible to the MathCad program

from MathSo2, Inc. The above procedure makes it possible to get ANSYS informa�on out into another program

without errors in manual transcrip�on. If you can get the ANSYS informa�on into a parameter, it can be moved

to an external file. Data can be read back into ANSYS with the *VREAD command. Similar methods can be used

to move arrays full of data. Note that the *VGET and *VPUT commands can move data between element tables

and arrays, and the arrays can be used to put data into external text files, so significant automated data

movement is possible. This approach can help to reduce data errors in reports.

Here is an example of temporarily switching the ANSYS /OUTPUT informa�on from the default, to a file. Note


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that certain list informa�on will not go to the file when the GUI is in use (read the manual).

! The following code switches /OUTPUT to a file,! writes two comments, writes PRSECT information on! linearized stresses along a previously defined path,! then returns /OUTPUT to the default./output,lin_path,out/COM,Linearized Path Results from PRSECT/COM,Compare Results with Code AllowablesPRSECT, ,0 /output

This gives a permanent record that is independent of plo5ed results.

Tip 68: Wri�ng Array Columns to Output or to Files

The *VWRITE command can be used to output an array column, in addi�on to scalar parameters. The array

posi�on from which the prin�ng will start must be indicated when execu�ng the *VWRITE command. As

men�oned above, the *VWRITE command cannot be executed inside the GUI, it has to be executed from an

input file or macro. The *VWRITE command prints the data from the star�ng posi�on on down to the end of the

column. The output that results can be re‐directed with the /OUTPUT or with the *CFOPEN and *CFCLOS

commands. The following two macros can be used to make calling the *VWRITE command easy. The array must

exist, having been created with a *DIM command. The first macro works on a 1‐dimensional array parameter.

Note the instruc�on on how to call the macro, with the array parameter name surrounded by single quotes in

order to delay the evalua�on.

! This macro will print a 1-dimensional array! according to the starting position indicated.! If this macro is called WRITEAR1.MAC and an! array called COL1DATA is to be printed from! position COL1DATA(1) to the end of the array! then call this macro with the statement:! WRITEAR1,'COL1DATA',1! setting the name of the array in single quotes.! The user may wish to change the FORMAT statement.

*vwrite,arg1(arg2)(E16.8)

The second works on a 2‐dimensional array parameter. The macro call will include the row and column posi�on

from which to start. The *VWRITE statement will cause prin�ng of a column of the 2‐dimensional array. When

calling the macro, the array parameter name is, as above, enclosed with single quotes to delay evalua�on.

! This macro will print a column of a 2-dimensional array! according to the starting position indicated.! If this macro is called WRITEAR2.MAC and an! array called MYDATA2D is to be printed from! position MYDATA2D(1,2) to the end of column 2 then call! this macro with the statement:! WRITEAR2,'MYDATA2D',1,2! setting the name of the array in single quotes.! The user may wish to change the FORMAT statement.

*vwrite,arg1(arg2,arg3)(E16.8)

Given that all ANSYS arrays are implicitly 3‐dimensional, the second macro above could be used to print out a

1‐dimensional array if the second calling parameter is set to one. A similar macro can be wri5en to print a

"column" of a 3‐dimensional array. If a term in the array is MYARRAY(III,JJJ,KKK) then the *VWRITE command will

cycle through the values of the III index when prin�ng out data. The macro for a 3‐dimensional array could be

wri5en so that it tests ARG2, ARG3, and ARG4 to see if they are zero. If they are zero, then they presumably were

not entered, and the correct form of a *VWRITE command could be used to print a scalar, 1‐D array, 2‐D array, or

3‐D array, as appropriate. Such a macro is illustrated below. Its use would be very error prone without error

checking code. A scalar need not have its name enclosed in single quotes in calling this macro, but an array

would have to be enclosed in single quotes as in the above examples. A user may want to customize this macro

to change the FORMAT statements, or to remove the /NOPR and /GOPR commands.


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! Macro to write a scalar or an array column, as appropriate.! Indicate the starting position for *VWRITE if an array is used.! Enclose an array parameter name in single quotes. #################! Examples, if this macro is called WRITER.MAC:! writer,aaa ! if aaa is a scalar! writer,'bbb',1,3,2 ! if bbb is a 3-D array parameter! Note the /NOPR and /GOPR commands. They will overwrite user settings.

/nopr ! reduce the amount printed to /OUTPUT*if,arg2,ne,0,then ! if nonzero an array is used *if,arg3,eq,0,then ! if not a 2-D array arg3=1 *endif *if,arg4,eq,0,then ! if not a 3-D array arg4=1 *endif *vwrite,arg1(arg2,arg3,arg4) (E16.8)*else ! if a scalar *vwrite,arg1 (E16.8)*endif/gopr ! switch on /OUTPUT

Test these macros thoroughly before use. Note that they contain no error handling code. Warning: It is

par�cularly difficult to remember to surround the name of the array parameter with single quotes.

Tip 69: Synthesizing Parameter Names and Manipula�ng Jobnames and Long Strings in APDL

Although I haven't found documenta�on reference to the following tricks, they work in ANSYS 5.3, and

presumably will in future. The ability to synthesize parameter names, and to do other text manipula�on, could

lead to some very crea�ve ac�vi�es in "programming" ANSYS methods and in macro wri�ng. Parameter names

themselves can be synthesized by chaining text strings together. Remember that there is a limit on the number of

parameters in a model ‐‐ arrays must be used to get around this, if it is a problem. Try these statements in ANSYS

‐‐ manually enter these lines one at a �me in the ANSYS Input window, and check what turns up when the

*STATUS command is issued (scroll down to the bo5om of the STATUS text window that pops up):

aaa='qwer'bbb='tyui'%aaa%%bbb%=12345*status%aaa%1234=5*status*do,iii,1,9$abc%iii%=iii$*enddo*status

For what it's worth, you can even store ANSYS commands in parameters, and execute them. I encountered some

trouble execu�ng commands with commas embedded inside the character parameters, but the following

worked. Give them a try:

aaa='nplo'%aaa%a='*get'b='xx'c='active'd='0'e='time'f='wall'%a%,%b%,%c%,%d%,%e%,%f%*status,%b%thetime=%b%/title,%a%,%b%,%c%,%d%,%e%,%f% yields %thetime%/repl

Parameters that hold text data are limited to 8 characters. Several text variables can be chained together

(concatenated) using the "percent" sign. The user should experiment to see how blanks are truncated. The

following example illustrates chaining. The strings are concatenated in the /TITLE command, and the /REPL

command shows the result in the plot �tle.


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xxx='The 1st'yyy=' & 2nd'/TITLE,%xxx%%yyy%/REPL

Situa�ons when this trick might be used could be to save and employ long �tle strings, annota�on strings, or job

names for the /FILNAME command. The method can use several parameters, or several terms of an array

parameter. A jobname can be up to 32 characters long, so up to four parameters, or 4 terms in an array

parameter, would be needed. Note that the *GET command can read in a jobname, with the *GET statement

poin�ng to the character in the jobname where the parameter will begin to read the string. An example of array

use is:

*DIM,aaa,char,4aaa(1)='First te'aaa(2)='rm & 2nd'/TITLE,%aaa(1)%%aaa(2)%/repl

A database could be saved with statements such as these, in which parameter "aaa" is text and "num" is an

integer. The contents of "num" could be the results of a load step "num" and the file name would iden�fy the

load step number:

aaa='model'num='456'SAVE,%aaa%%num%,dbSAVE,%aaa%123,dbSAVE,myjob%num%,db

Another use is with a parameter inside percent signs grouped with text with all inside single quotes. Consider this

example, which was used to delete load step files in a complex applica�on:

! Parameter "compname" contains the Jobname of a load step file*do,jjj,st1,st2,st3 *if,jjj,lt,10,then /delete,compname,'s0%jjj%' *else /delete,compname,'s%jjj%' *endif*enddo

Both the RESUME command and the /CLEAR command can destroy the counter used in a *DO loop. I have used

the PARSAV command before RESUME, and the PARRES command immediately a2er, in order to recover the

counter and other looping parameters when placing RESUME inside a *DO loop. The user should test this

technique before using ‐‐ the parameters saved and restored may undesirably overwrite parameters in the file

that is resumed. Look at this /CLEAR example; it illustrates what can be done:

! test of loopingfini ! exit whatever is active*do,iii,1,3 parsav,all,xxx,parm /clear ! be prepared to hit OK button parres,new,xxx,parm*enddo

These methods can make it possible to program considerable automa�on into ANSYS, using the ability to

assemble parameter names, parameter contents, commands, and file names from le5ers and numbers.

Tip 70: Solid Elements 95 and 92 ‐‐ Efficiency and Interconnec�on

These two solid elements have mid‐side nodes, so they follow curved surfaces nicely. Fewer elements are needed

than with Solid45 8‐node bricks, for equivalent accuracy. Note the ANSYS manual comments that the simpler

flat‐sided elements may be preferred for material nonlinearity. However, nonlinearity is supported by Solid92

and Solid95. The Solid45 tetrahedral element is "not recommended" in the ANSYS manuals because of its low

accuracy in predic�ng stresses ‐‐ the higher order Solid92 and 95 tetrahedral op�on elements do not have these

warnings, and may be preferred in some cases.


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Solid element 95 is a 20‐node brick element. It also supports a prism, pyramid, and tetrahedral element shape

op�on. At revision 5.3 of ANSYS, my tes�ng suggests that the prism and pyramid forms are not generated by the

automa�c volume mesher. At the 5.5 revision, pyramids may be generated by the mesher at the interface

between the brick form, and the tetrahedral form. Brief tes�ng I did suggests that meshing of volumes can

successfully interface the brick and tetrahedral forms. The tetrahedra formed at the interface with brick elements

do not have mid‐side nodes where they would not exist on the matching brick elements, so there should not be a

big mismatch problem at the interface. These elements that have a mid‐side node missing are considered to be

degenerate forms in the ANSYS manuals, and are not recommended in regions with high stress gradients, or

where exace stress values are important.

The Solid92 element is a 10 note tetrahedron ‐‐ it should act the same as the 10 node version of the Solid95

element in producing results. However, it executes significantly faster than the 10 node Solid95 element,

presumably because the so2ware is not dealing with the redundant extra 10 nodes. In large models, this

speedup will be of value to users, so Solid92 elements should be considered where Solid95 tetrahedral forms

would otherwise be employed in large structural models.

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© 1998, 1999 by Peter C. Budgell -- You are welcome to print and photocopy these pages (don't plagiarize or sell the contents).

May 27, 1999

E‐mail Address: Please see my main page.

Link to: The ANSYS® Home Page at www.ansys.com

Link to: ANSYS Technical Overview Recommended Reading

For more links, Return to Links on Main Page.


54 of 54 1/17/2009 11:41 AM

10673282 Ansys Tips and Ansys Tricks

Documents

analysis tip

lines tip

ansys users

use annotations tip

ansys manuals

fea work

fea methods

teaching of fea