Chapter Four
Postprocessing
4. Postprocessing
Training Manual
– Plot and list your results in ways that make them useful for
engineering decision-making.
• Will my part fail? Is it over designed? How can I improve it?
• Present results to other decision makers (clients, customers,
etc.).
– Verify the validity of your results.
• Are the results correct?
• Nonlinear postprocessing differs in that:
– You review an expanded set of results items, over a range of
“time.”
– Verification includes checking for convergence.
– Results cannot be scaled or superimposed.
Basic Structural Nonlinearities 6.0
What is different about nonlinear postprocessing?
• The same fundamental objectives apply to both linear and
nonlinear analyses.
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…Postprocessing
In this chapter, we will present a brief introduction to the
basics of nonlinear postprocessing via the following topics:
A. Checking for convergence
B. The general postprocessor
C. The results viewer
D. The time-history postprocessor
E. Verification
•
The purpose is to give you an understanding of special
postprocessing techniques for nonlinear analyses.
Basic Structural Nonlinearities 6.0
•
Training Manual
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Postprocessing
A. Checking for convergence
Training Manual
• Before continuing with postprocessing, you must verify that
you have a converged solution.
– If unconverged, find out what went wrong.
– If converged, gain insight into how to make a similar analysis
more efficient next time.
• There are many ways to verify convergence.
– Review the output, error, and monitor files.
– List results summary in the general postprocessor.
– Graph time-history results.
Basic Structural Nonlinearities 6.0
• Obviously, engineering decisions cannot be based on
unconverged results.
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Postprocessing
… Checking for convergence
– If converged – convergence messages displayed for last
substep, at expected “time” at end of last load step.
– If unconverged – restart information is given.
Basic Structural Nonlinearities 6.0
• Review the output file.
Training Manual
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Postprocessing
… Checking for convergence
Training Manual
• Review the monitor file.
– Make sure a solution was completed at the expected “time” at
the end of the last load step.
Basic Structural Nonlinearities 6.0
• Review the error file.
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Postprocessing
… Checking for convergence
– Unconverged results are written for substep 999999.
– Don’t postprocess step 999999, except to debug
nonconvergence.
Basic Structural Nonlinearities 6.0
• List the results summary in the General Postprocessor.
Training Manual
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Postprocessing
… Checking for convergence
Training Manual
– A solution that has converged for several substeps, but that
terminates with an unconverged substep, will usually display a
characteristic straight-line “jump” in time-history graphs.
“Jump”
Smooth curve
Substep 999999
(unconverged)
Last converged
substep
Basic Structural Nonlinearities 6.0
• Graph time-history results.
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Postprocessing
… Checking for convergence
– Structures that become physically unstable:
• Nonlinear buckling.
• Complete plastic section.
F
Convergence at full
load will never be
achieved
Last converged solution will
have engineering significance
Basic Structural Nonlinearities 6.0
• Nonconvergence is sometimes expected.
Training Manual
u
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Postprocessing
B. The general postprocessor
Training Manual
• You can also use the general postprocessor to animate
results over a range of time or results sets.
SEQV plot at time = 2.0
Basic Structural Nonlinearities 6.0
• The general postprocessor allows you to review results for
the entire model, at a single point in time.
Animated response history
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Postprocessing
… The general postprocessor
Training Manual
– Make sure the database is in memory. (RESUME jobname.db if
necessary.)
– Choose the desired results set using the results summary list.
• General Postproc > Results Summary
– Avoid using General Postproc > -Read Results- By Time/Freq
• Can lead to unnecessary interpolation errors.
Results summary list
Basic Structural Nonlinearities 6.0
• To plot or list results at a given point in time:
Reading results by Time
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Postprocessing
… The general postprocessor
Training Manual
•
Use most of the same plot and list operations as for linear
postprocessing.
•
Note that all von Mises (equivalent) strains (elastic, plastic, creep,
etc.) will be calculated using effective Poisson’s ratio.
•
By default:
– EFFNU = user input for EPEL and EPTH
– EFFNU = 0.5 for EPPL and EPCR
– EFFNU = 0.5 if reference material is Hyperelastic
– EFFNU = 0 for line elements (includes beam, link, and pipe elements, as
well as discrete elements)
•
Note: Default EFFNU can be over-ridden in POST1 with the AVPRIN
command if necessary (I.e. EFFNU = 0.495 for nearly incompressible
material )
Basic Structural Nonlinearities 6.0
Plot or list results at a given point in time (cont’d):
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Postprocessing
…The general postprocessor
Training Manual
• Be sure to use the correct animation option:
– PlotCtrls > Animate > Over Time
– PlotCtrls > Animate > Over Results
– No other animation options are appropriate for nonlinear results!
Click the desired button
Specify a “time” range
Basic Structural Nonlinearities 6.0
• Animation allows you to review the response history of the
entire model.
Animate Over Time
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Postprocessing
...The general postprocessor
Training Manual
• Because nonlinear response is not proportional, the principle
of superposition is invalid.
– Load case combinations (General Postproc > Load Case) are
invalid.
– Results scaling is invalid.
Superimposed
results are wrong!
F1
+
u1

F2
u2
F1+F2
Basic Structural Nonlinearities 6.0
Important:
u1+u2
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Postprocessing
C. Results Viewer
Training Manual
• The Results Viewer may provide a 10x post-processing
performance improvement. It is a specialized
postprocessing menu and graphic system.
– You can quickly access and compare results sets for different
substeps.
– It provides an easy to use menu system for quick results
viewing.
– It uses the PGR file (a specialized graphics object).
Basic Structural Nonlinearities 6.0
• Post-processing of large models with many time steps may
slow your system performance (memory limitations, large
amounts of disk I/O).
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Postprocessing
… Results Viewer
Training Manual
– Use the POUTRES command before
solving to write a jobname.pgr file
during solution (greatest speed
advantage).
– Solution > Output Cntrls > PGR file…
Hold CTRL key for
multiple selection
Basic Structural Nonlinearities 6.0
• The PGR file can be created two
different ways …
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Postprocessing
… Results Viewer
– General Postproc > Write PGR File…
Basic Structural Nonlinearities 6.0
– Or use the PGWRITE command after solution to write a
jobname.pgr file.
Training Manual
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Postprocessing
… Results Viewer
Training Manual
Basic Structural Nonlinearities 6.0
• Open the Results Viewer from the General Post Processor.
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Postprocessing
... Results Viewer
Training Manual
Query Results
Animate using PNG
files
Element Plot
List Results
Results Set
Locator
Capture/print
image
Basic Structural Nonlinearities 6.0
Nodal/element/vector/trace
results plots
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Postprocessing
... Results Viewer
Training Manual
Basic Structural Nonlinearities 6.0
• The graphics window becomes “Context Sensitive”.
Right Click
on Model
Right Click on
Contour Legend
Right Click on
Color Contour Bar
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Postprocessing
D. The time-history postprocessor
• You can graph:
– Deflection vs. time.
– Stress vs. strain.
– Load vs. deflection.
– And many other items.
Basic Structural Nonlinearities 6.0
• The time-history postprocessor allows you to display
analysis results at specific points in the model in X-Y
response history graphs.
Training Manual
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Postprocessing
… The time-history postprocessor
Training Manual
– The program identifies these variables by index numbers.
– The program automatically defines “Time” as variable 1.
• “Time” is used for the X-axis by default.
– You can assign other results items to variable 2, etc.
– You can specify a different variable for the X-axis.
• Open Results File: TimeHist Postpro> Variables Viewer
Basic Structural Nonlinearities 6.0
• To create a time-history graph, you must first store results
items as temporary variables.
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Postprocessing
… The time-history postprocessor
• Assign unique variable name
– in this case “DISPLACEMENT” for UY DOF solution
Basic Structural Nonlinearities 6.0
• Add Variable to Viewer Table
Training Manual
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Postprocessing
… The time-history postprocessor
Select the applicable node by graphical picking or via command
input window
Pick node (in element
or node plot)
• Repeat for other variables, as desired.
Basic Structural Nonlinearities 6.0
•
Training Manual
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Postprocessing
… The time-history postprocessor
UY displacement
at node 1
Total force at
node 1
Calculator allows user
to perform many
mathematical
operations on variables
Basic Structural Nonlinearities 6.0
• Each variable represents one results item at one node.
Training Manual
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Postprocessing
… The time-history postprocessor
Training Manual
– By default, the X-Axis will be variable 1, or “Time.” You can
specify a different variable by changing the selection in the
X-Axis column
3. Select
the graph
button
2.
Highlight
the
FORCE
variable
1. Specify the
DISPL variable for
the X-axis
Basic Structural Nonlinearities 6.0
• Graph the force vs deflection curve
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Postprocessing
… The time-history postprocessor
Training Manual
Load-Deflection graph
Basic Structural Nonlinearities 6.0
• With variables defined, and the desired X-Axis variable
specified, you can plot the X-Y graph by selecting the graph
button
on the variable viewer menu
• Let’s see how to improve the quality of this graph.
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Postprocessing
… The time-history postprocessor
Training Manual
PlotCtrls > Style > Graphs > Modify Axes
– Axis labels.
Labels
– X and Y ranges.
– Font size.
– Reverse X and Y axes.
– And more.
X range
Y range
Font size
Reverse
Y-axis
Basic Structural Nonlinearities 6.0
• You can modify many aspects of the graph axes.
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Postprocessing
… The time-history postprocessor
PlotCtrls > Style > Graphs > Modify Grid
Basic Structural Nonlinearities 6.0
• You can add a grid background to the graph.
Training Manual
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Postprocessing
… The time-history postprocessor
Training Manual
– Take absolute value
– Differentiate.
– Integrate.
– And more.
• However, be careful not to scale or superimpose results!
– Remember, for nonlinear analyses, the principle of
superposition is invalid.
Basic Structural Nonlinearities 6.0
• You can also perform math operations on variables using
the variable viewer calculator.
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Postprocessing
… The time-history postprocessor
Training Manual
Basic Structural Nonlinearities 6.0
• You can also change graph colors.
PlotCtrls > Style > Colors > Reverse Video
PlotCtrls > Style > Colors > Graph Colors
– Curve color, etc.
Improved Load-Deflection graph
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Postprocessing
E. Verification
Training Manual
• Various modeling problems can cause a seemingly wellbehaved solution to be incorrect.
– Mesh discretization (too coarse a mesh).
– Mesh distortion (poor element shapes after first iteration).
– Errors in material characterization.
– Failure to identify potential contact regions.
– Improper boundary conditions.
• You must always verify that your results are correct.
Basic Structural Nonlinearities 6.0
• You cannot simply assume that an analysis that converged
with no error messages will give valid results.
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Postprocessing
… Verification
– Are your results “reasonable?”
Do these
results make
sense?
– Do they conform to your expectations?
• Good judgment is based on:
– Experience.
– Benchmark studies.
– Physical testing or known behavior of prototype parts.
– A skeptical, vigilant frame of mind.
• Good practice always requires that you reconcile
analytical results with rational expectations.
Basic Structural Nonlinearities 6.0
• Engineering judgment is your first and best
verification tool.
Training Manual
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Postprocessing
… Verification
Training Manual
– Discontinuities and missing contours are common indicators of
a mesh that is too coarse.
• Path plots can also be a useful way to check the mesh
discretization density.
Basic Structural Nonlinearities 6.0
• Element contour plots can help reveal mesh discretization
error.
– A smooth path plot is usually expected. Jagged plots may
indicate that the mesh is too coarse.
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Postprocessing
… Verification
Training Manual
– Most shape checking is active only at the start of solution.
– In large-strain analyses, elements might distort into unviable
shapes after the first iteration.
– Plot the displaced shape to check distorted element shapes.
Originally quad-shaped
element deforms to 1800
corner angle - results are
questionable.
Basic Structural Nonlinearities 6.0
• Deformed shape plots can reveal highly distorted elements.
Poor element shapes may yield inaccurate results.
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Postprocessing
… Verification
Training Manual
– Does the known stress-strain data extend beyond the range of
strain experienced in the model?
– Do stress-strain results match the input stress-strain data?
Input data
Basic Structural Nonlinearities 6.0
• Stress-strain plots can help verify the material response.
Time-history graph
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Postprocessing
… Verification
Training Manual
• Animating the deformed shape over time will show up any
overlapping regions where you failed to identify potential
contact surfaces.
Basic Structural Nonlinearities 6.0
• Deformed shape plots will reveal if any contact surfaces have
excessive over-penetration.
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Postprocessing
… Verification
Training Manual
– A jagged time-history plot may have a valid physical explanation.
• Such as, windshield wiper blade “chatter.”
– Without a good physical explanation, jagged results are
questionable.
• Might indicate that time step size is too large.
• Animations should similarly exhibit smooth motion.
Basic Structural Nonlinearities 6.0
• Time-history plots should generally be smooth.
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Postprocessing
… Verification
Training Manual
– Both over- and under-constrained conditions will give incorrect
displacements and erroneous stresses.
• Careful review of deformed shape plots, stress contour plots,
and other postprocessing data will help you answer the
question, “Do these results make sense?”
Basic Structural Nonlinearities 6.0
• Unrealistic boundary conditions will ruin the validity of your
analysis.
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Postprocessing
F. Workshop
Training Manual
• W7. Postprocessing – Tensile Specimen
• W8. Postprocessing - Results Viewer
Basic Structural Nonlinearities 6.0
Please refer to your Workshop Supplement for instructions on:
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