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n ANSYS Release 10.0, you can use the PRSECT command to
linearize the total stress along a
path. Linearizing decomposes the total stress into the membrane,
bending, and peak stress
components. The stress intensity for each of these components
can be compared to the appropriate
ASME Code limit. PRSECT will not, however, identify the
appropriate stress category. Per Section
VIII, Division 2 of the ASME Boiler and Pressure Vessel Code,
stresses may be primary or
secondary depending on the loading conditions that produced them
and/or their location. It is the
user`s responsibility to determine the appropriate stress
category (and thus allowable limit).
On Mar 11, 2009, at 12:06 PM, Hothi, Jaspreet (E F PR GT EN 151)
wrote:
> ses at the critical location to check with the
> allowable stresses. However I realised that linearising the
stresses
> over a line gives me very conservative results. My question
is
>
> Is there a proven technique to linearize the stresses over
a surface
> instead of a line?
Read up on the reasons behind 'stress linearization.' There's a
lot
of mis-understanding about the topic. The process is part of the
ASME
Code methodology for separating stress arising from
statically
determinate load components from stresses resulting from
indeterminate loading and from peak stresses. If it's done
properly
it's never conservative.
Typically you consider the maximum calculated stress at a point
of
stress concentration as a peak stress, subject to fatigue
allowables.
By figuring the net bending moment and direct load from a
through
thickness stress profile you can determine the equivalent
bending and
direct stress resulting from combined determinate and
indeterminate
loading, called secondary stress which is subject to lower
allowables
than peak stresses but higher allowables than primary stress
resulting from statically determinate loads.
If you're going to use 'linearization' it needs to be done
properly.
It's easy in a lot of cases, not so easy in others. For example
a
corner detail in the neighborhood of the joint between a
pressure
vessel head and an elliptical shell carries the primary
membrane
stress, PR/h; a secondary stress due to statically
indeterminate
loading developed by the deformation incompatibility between
the
shell and the head and a localized peak stress at the corner
detail.
The various components are easy to sort out. Not so easy is
the
stress distribution at a lift point, since the determinate
components
are not easily sorted out from the indeterminate loading.
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Thermal stress is another matter because it has characteristics
of
determinant and indeterminate loading--probably a good idea is
to
consult the literature or the ASME Code for guidance. There's
plenty
of literature, too.
Linearized Stress Using Nodal Locations for Path Results in
Workbench Mechanical 14.5 Posted on July 8, 2013 by Ted Harris
Postprocessing results along a path has been part of the
Workbench Mechanical capability for several revs now. We
need to define a path as construction geometry on which to map
the results unless we happen to have an edge in the
model exactly where we want the path to be or can use an X axis
intersection with our model. You have the option to
snap the path results to nodal locations, but what if you want
to use nodal locations to define the path in the first
place? Well see how to do this below.
For more information on picking your nodes, see the Focus blog
entry written by Jeff Strain last
year:http://www.padtinc.com/blog/the-focus/node-interaction-in-mechanical-part-1-picking-your-nodes
The top level process for postprocessing result along a path
is:
Define a Path as construction geometry
Insert a Linearized Stress result
Calculate the desired results along the path using the
Linearized Stress item
The key here is to define the path using existing nodes. Why do
that? Sometimes its easier to figure out where the
path should start and stop using nodal locations rather than
figure out the coordinates some other way. So, lets see
how we might do that.
First, turn on the mesh via the Show Mesh button so that its
visible for the path creation
From the Model branch in Mechanical, insert Construction
Geometry
From the new Construction Geometry branch, insert a Path
Note that the Path must be totally contained by the finite
element model, unlike in MAPDL.
If you know the starting and ending points of the path, enter
them in the Start and End fields in the Details view
for the Path.
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Otherwise, click on the Hit Point Coordinate button:
Pick the node location for the start point, click apply
Pick the node location for the end point, click apply
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In the Solution branch, insert Linearized Stress (Normal Stress
in this case); set the details:
Scoping method=Path
Select the Path just created
Set the Orientation and Coordinate System values as needed
Define Time value for results if needed
Results are displayed graphically along the pathK
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Kas well is in an X-Y plot and a table
Besides normal stresses, membrane and bending, etc. results can
be accessed using these techniques. So, the next
time you need to list or plot results along a path, remember
that it can be done in Mechanical, and you can use nodal
locations to define the starting and ending points of the
path.
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Linearized Stress Value Calculation
Mechanica calculates the linearized stress values with respect
to a local coordinate system
with the X axis aligned with the line from location 1 to
location 2 and the origin at the
midpoint of the line from location 1 to location 2.
Mechanica first calculates the total local coordinate stress
components at each point. It then
calculates membrane, bending stress, peak stress, and total
stress as follows:
Membrane and bending stress values are obtained from numerical
integration along the line
between location 1 and location 2 as follows:
where:
is any local stress component
L is the distance from location 1 to location 2
Total stress is the value calculated by Mechanica, and the peak
stress is defined by:
Peak = Total (Membrane + Bending)
Peak, Total, and Bending Stresses vary along the line from
location 1 to location 2;
however, membrane stress remains constant.
Mechanica then processes the component values of these stresses
at each point to obtain
principal and von Mises stresses, using the standard formula for
principal and von Mises
stress.
Note: The formula for peak and total stress applies for each
component of stress, but not
for the principal or von Mises stress.
For axisymmetric models, similar formulas are used, with
correction terms to account for
the offset of the neutral bending axis from the midpoint.
Return to Linearized Stress Report.