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Overcoming Convergence Difficulties in ANSYSWorkbench
Mechanical, Part I: Using Newton-Raphson Residual Information
padtinc.com
/blog/the-focus/overcoming-convergence-difficulties-in-ansys-workbench-mechanical-part-i-using-newton-raphson-residual-information
Ted Harris
Unable to converge. Convergence Failure. Failure to Converge.
Never nice words to see when youare trying to get your simulation
done.
If youve encountered convergence failures while running
nonlinear structural analyses in ANSYSWorkbench Mechanical, this
two part series is for you. What is a convergence failure? In a
nutshell itmeans that there is too much imbalance in the system.
The calculated reaction forces do not matchthe applied loads and
even though the program tries hard to make changes to overcome
theimbalances, it hasnt been able to do so and stops. If we look at
the Force residuals under SolutionInformation, we will see that the
solver has been unable to get the force convergence residual,
orimbalance force, to drop below the current criterion
Test model example: Newton Raphson Convergence Failure; Solution
Stops
We wont spend a lot of time here explaining the Newton-Raphson
method, convergence, and residualplots here, since we wrote a Focus
article back in 2002 which discusses them in more detail.
Thearticle begins on p. 7 at this link:
/blog/wp-content/uploads/oldblog/archive/PADT_TheFocus_08.pdf
The context of that article was Mechanical APDL, but the article
is directly relevant since solving in
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Workbench Mechanical is done in Mechanical APDL in batch
mode.
In crayon terms, we want the purple line to drop below the blue
line. When it doesnt and the solver isout of options to keep
trying, the solution stops and we get an error message.
Now what? The traditional knobs to turn are to increase the
number of substeps, decrease contactstiffness if contact is
involved, perhaps add more points to the plasticity curve, etc. But
what ifsomething else is the problem? How can we identify where the
problem is?
In this part I article we will discuss how to plot the
Newton-Raphson residuals as contour plots to seewhere in the model
the highest force imbalances are located. Often this is useful
information to helpus figure out what is going on so we can take
corrective action. First, be aware that we must turn onthe
Newton-Raphson residual plots prior to solving. That means you
either have to turn them on andre-solve after a convergence
failure, knowing that youll get the same failure again, or you need
toclairvoyantly (or perhaps just prudently) turn on the residuals
prior to attempting the initial solve. Whyarent they on all the
time, you ask? Most likely because they slow things down just a bit
and alsorequire a bit more disk space than otherwise, although if
the solution runs to completion no Newton-Raphson residual plots
are saved.
Here is how we turn them on. In the Details view for the
Solution Information branch, change theNewton-Raphson Residuals
setting from the default of zero to a nonzero number such as 3 or
4. Thatwill continuously save the last 3 or 4 Newton-Raphson
residual plots for viewing as contour plots afterthe solution has
stopped due to a convergence failure.
After the solution has stopped, the Newton-Raphson residual
plots will be available underthe Solution Information branch.
The quantity plotted is actually the square rootof the sum of
the squares of the residuals inthe global X, Y, and Z directions.
So, the plotsdont show us direction information, but they doshow
where the residuals and hence the forceimbalances are the largest.
Below is anexample. The region in red shows where theresiduals are
the highest. Since this is a modelinvolving contact between two
bodies,apparently the contact regions and specificallycontact at
the corners of the part on the left is the source of our
convergence difficulties.
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Newton-Raphson Residual Force Plot for the last attempted
equilibrium iteration.
So, how do we use this information? In this case we now suspect
that the contact regions, especiallyat the corners of the smaller
part, are the problematic areas. Using this information we made
twochanges to the model.
First, we changed the Detection Method for the contact elements
from Program Controlled (at theelement Gauss points) to
Nodal-Normal to Target. Many times when contact problems involve
touchingat corners, the robustness of the contact interface can be
improved by changing the detection methodfrom Gauss points to
nodes.
Second, we reduced the contact stiffness by changing Normal
Stiffness from Program Controlled(factor of 1.0) to a Manual
setting of 0.2. Reducing the contact stiffness can help with
contactconvergence for a lot of problems. Too low of a stiffness
value can cause problems too, but in thiscase the resulting
penetration is still small so a value of 0.2 seems reasonable. When
in doubt, asensitivity study can be performed whereby you make
changes to the contact stiffness value whiletracking your results
quantities of interest. As with most inputs you can vary, your
results of interestshould not be sensitive to contact
stiffness.
These two changes allowed our test model to nicely converge for
the full amount of load.
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Other considerations:
The Newton-Raphson Residual plots are always displayed on the
original geometry, not the deflectedgeometry at version 14.0 of
ANSYS Mechanical. If the deflections are large this can make it
harder toascertain what is causing the high residual values. In
those cases, it can be helpful to compare thetotal deformation and
stress plots for the unconverged solution, along with those plots
for the lastconverged solution, with the 1.0 true scale on the
deformation active. This will show the parts in theirdeflected
state, and that can help in determining why the residuals are high
at certain locations.
We recommend creating at least 3 residual plots (set in the
details of Solution Information as describedabove). Sometimes the
location of the imbalance can bounce around a bit from equilibrium
iteration toequilibrium iteration, so having more than one or two
plots to look at can be beneficial in determiningproblem
locations.
Conclusion
Summing it up, the Newton-Raphson residual plots are one piece
of information we can use todetermine why we are having convergence
difficulties. They can give us an indication of where
theconvergence difficulties are occurring in the model, and many
times we can use that information to helpus know what settings
should be modified or what other changes should be made to the
model toimprove the convergence behavior.
In part II of this article, well look at how to quickly use
ANSYS Mechanical APDL to view the elementsthat have undergone too
much deformation.
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padtinc.comhttp://www.padtinc.com/blog/the-focus/overcoming-convergence-difficulties-in-ansys-workbench-mechanical-part-ii-quick-usage-of-mechanical-apdl-to-plot-distorted-elements
By Ted Harris October 18, 2012
Overcoming Convergence Difficulties in ANSYS
WorkbenchMechanical, Part II: Quick Usage of Mechanical APDL to
PlotDistorted Elements
In part I if this series, we saw how to use Newton-Raphson
residual plots as an aid to vanquishing convergencedifficulties in
ANSYS Workbench Mechanical. In part II, we will see how to quickly
launch the ANSYS MechanicalAPDL user interface to plot elements
that have undergone too much distortion, thereby resulting in a
convergencefailure. Several problems can cause convergence
failures, but one that can be particularly frustrating is
elementsthat have undergone too much distortion.
Currently there isnt a way to isolate and view elements that
have triggered a convergence failure due to too muchdistortion
within the Workbench Mechanical user interface. Fortunately we have
access to the older ANSYSMechanical APDL interface, which does
allow us to select and visualize elements that have undergone too
muchdistortion. This can be useful in that it tells us exactly
where in the model the elements are failing. Hopefully wecan use
this information to take corrective action in Mechanical such as
making local mesh modifications, addingmore details to geometry,
etc.
So, how do we do this? Rather than try to give a lesson on how
to use the Mechanical APDL interface, were justgoing to give the
commands needed to be clicked with the mouse or typed in. Were
following the K.I.S.S.principal, meaning Keep It Simple, Silly.
The procedure to follow includes these steps:
1. Identify the directory in which our results file resides.
2. Launch ANSYS Mechanical APDL.
3. Point to the results file identified in step 1.
4. Modify the nodal coordinates so they are in the deflected
state at the point of convergence failure.
5. Plot those error-causing elements.
We will now go into more detail using a model that has
convergence trouble. This model solved successfully forthe first 4
substeps, but on the 5th substep the solution failed to converge.
We get this error in the solver output(Solution Information):
*** ERROR *** CP = 2872.649 TIME= 16:29:51 One or more elements
have become highly distorted. Excessive
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distortion of elements is usually a symptom indicating the need
for corrective action elsewhere. Try incrementing the load more
slowly (increase the number of substeps or decrease the time step
size). You may need to improve your mesh to obtain elements with
better aspect ratios. Also consider the behavior of materials,
contact pairs, and/or constraint equations. If this message appears
in the first iteration of first substep, be sure to perform element
shape checking.
Looking at the model, we see we have an indenter that is being
pressed into a block of material. The indenter issteel and the
block is aluminum. Both have nonlinear material properties
defined.
Total deformation for the last converged substep looks like
this:
The unconverged results show that we have some elements that
have large nodal deflections:
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So, our error message tells us that one or more elements have
become highly distorted. Which elements arethey? The following
procedure will show us how to view those for sure, using Mechanical
APDL.
Here are each of the 6 steps mentioned above, in detail:
1. Identify the directory in which our results file resides:
We do this from the Workbench window, by clicking on View >
Files. Scroll down in the resulting list of files untilyou find
file.rst, the ANSYS Result file. The location will be listed in the
resulting information, but the text is notselectable. To make it
easier, right click on the file.rst row and select Open Containing
Folder.
From the top of the resulting Windows Explorer window, select
the folder path and right click > copy.
2. Launch ANSYS Mechanical APDL:
Click Start > All Programs > ANSYS 14.0 >
ANSYSMechanical APDL Product Launcher. In the resultingwindow,
paste in the directory path in the WorkingDirectory box:
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Click the Run button at the bottom of the window. The Mechanical
APDL user interface will start.
3. Point to the results file identified in step 1:
Click on General Postproc on the left, then Data & File
Opts. In the resulting Data and File Options window, clickon the []
button below Read single result file:
You should see the result file, file.rst, available in the
resulting window. Click on that file, then click Open. ClickOK in
the Data and File Options window.
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We need to read in one set of results to load the model into the
Mechanical APDL database. Click GeneralPostproc > Read Results
> Last Set.
4. Modify the nodal coordinates so they are in the deflected
state at the point of convergence failure:
Lets plot the elements so we can see the model (this will show
the elements with nodes in the original,undeflected positions).
Well just have you type in the command to make the element plot: in
the input line nearthe top of the window, type eplot, then
return.
The plot will show in the default front view, looking down the
global Z axis. Note that if weak springs are on inWorkbench
Mechanical, you will see these as line elements pointing away from
the model in a few places.
The nodal modification is performed in the preprocessor. Click
on the Preprocessor command on the left side ofthe window. Type in
this command in the input line to modify the nodal positions to
those of the unconverged (lastset) of results:
upgeom,,,,file,rst
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Plot the elements again. You should now see the deflected nodal
positions.
Using the view controls over on the right side, we can rotate
and zoom in. A short cut is to use the right mousebutton to box
zoom and Ctrl + Right Mouse Button to rotate the model. Now we can
better see where thedeformations are occurring. We still have all
elements selected and plotted, so the next step will be to filter
theplot to show the error-causing elements.
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5. Plot those error-causing elements:
Shape checking of elements consists of two levels, warning and
error. The solver will not continue if any elementsexceed the error
level. Shape checking is discussed in detail in section 13.1 of the
Theory Reference in theANSYS Help. We have the ability to plot both
warning level elements and error level elements, using
thisprocedure:
On the left side of the window, click on Meshing > Check Mesh
> Individual Elm > Plot Warning/Error Messages.
With all boxed checked, this is the resulting plot in the front
view. Good elements are displayed in blue, warning elements in
yellow,and error or failed elements are shown in red.
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When the elements are very highly distorted, their surfaces cant
always be displayed and it looks like there is ahole in the model.
This wont always happen depending on how highly distorted the
elements are, viewingdirection, etc..
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If we uncheck the Good Elements (blue) box, then only the
warning and error elements are displayed.
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When you are done viewing the elements, click on the Quit button
near the top, and exit without saving to get outof Mechanical
APDL.
So what does all this tell us? For this model, the elements
below the indenter body are experiencing too muchdeformation (red
elements). Some elements in the indenter body are at the warning
level but not the error level(yellow elements). The fix could be to
apply the load more gradually (more substeps), refine the mesh at
thislocation, or maybe a combination of both. In this case we also
changed the Workbench Mechanical shapechecking from Standard to
Aggressive Mechanical.
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ANSYS Penetration Model
Overcoming Convergence Difficulties in ANSYS Workbench Mechanical,
Part I: Using Newton-Raphson Residual InformationOther
considerations:ConclusionOvercoming Convergence Difficulties in
ANSYS Workbench Mechanical, Part II: Quick Usage of Mechanical APDL
to Plot Distorted Elements