mechanical-nonlin_13.0_app6a_creep curve fitting in mapdl
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Customer Training Material
Appendix 6A
Creep Curve Fitting in
MAPDL
Structural Nonlinearities
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training MaterialChapter Overview
This appendix presents an optional lecture on Creep Curve Fitting in
MAPDL
Prerequisite to this Appendix is Chapters 2 and 6 and a basic knowledge
of the MAPDL GUI
The following topics will be covered:
A. OverviewB. Preparing the Test Data
C. Procedure
.
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training MaterialA. Overview
The thirteen creep equations discussed in Chapter 6 relate creep
strain or creep strain rate to stress, temperature, and creep strain or
time
The required test data is dependent on the creep equation used.
Conversely, one can select a given creep equation to fit to based on
available test data and observed creep behavior (primary, secondary, or
both)
Curve-fittin tools are ver useful to calculate the coefficients used in
the creep equations from test data. MAPDL provides curve-fitting
routines for all thirteen creep equations.
e es a a s ou e prepare n a separa e ex e. s can en eread into the MAPDL creep curve-fitting module
Calculated and experimental data can then be plotted against each other
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to compare the fit of the curves
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training MaterialB. Preparing Test Data
Text file(s) containing test data should be prepared first:
The file should contain columns that are space- or tab-delimited
The header (first few lines of the file) should contain information on what
variables are present. The variable notation is below:
Abbreviation used in text data file
Time time
Equivalent Creep Strain creq
Equivalent Creep Strain Rate dcreq
Equivalent Stress seqvTemperature temp
For each column of data available, put a /#,var in the header to indicate
which variable is associated with which column
If a variable is assumed to be constant, use /var,value in the header to
indicate the value for that variable
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Preparing Test Data
For example, lets assume that equivalent stress and temperature
were held constant for a given creep test with values of 4000 and 100,
respectively.
Data for creep strain will be entered incolumn 1, and data for creep strain rate /seqv,4000
/temp,100 .
The header information (first four lines)
would contain the information on
/1,creq
/2,dcreq
0.00215869 0.000203055
constant variables and column variables
The resulting text file is shown on the
. .
0.00664691 0.000165303
0.0102068 0.000152217
0.0151416 0.000140946.
0.0220102 0.000130945
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
If launching MAPDL fro the WB Project Schematic, highlight
Analysis and RMB=> Open Mechanical APDL
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
Navigate to the Creep Curve-fitting tools in MAPDL:
Main Menu > Preprocessor > Material Props > Material Models
Structural > Nonlinear > Inelastic > Rate Dependent > Creep > Curve Fitting
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
After selecting the Creep Curve-Fitting module, specify the prepared
text file containing the test data
Use Add DataSet to enable reading multiple test data files
(for example, creep tests performed at multiple temperatures)
After selecting the file,
the contents will be
displayed in the dialog
box. Select Next to
continue.
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
Specify the creep law to curve-fit to
The thirteen creep equations are categorized by primary and/or
.
constant creep strain rate. Use the guidelines discussed in Chapter 6 toselect an appropriate creep law
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
Each creep law has a different number of
coefficients C1-Ci. Since the curve-fitting
procedure is nonlinear, initializing the creep
coefficients is important to the curve-fitting
process
to aid in the curve-fitting routine. Some
coefficients can be initialized and fixed to.
TheANSYS Structural Analysis Guide online help provides some detailed
guidelines in curve-fitting for various creep laws under Material Curve
Fitting > Tips for Curve-Fitting Creep Models.
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
The error norm can be normalized or unnormalized. The normalized
method is default and gives all points equal weight.
,
methodwill emphasize that range of creep strain (rate) values with the mostdata points.
If the unnormalized option is used, the data points will fit the larger creep
strain (rate) range better, as the points with larger value have more weight.
( )=
=N
i
experiment
i
trial
iedunnormalizE1
2
&&
( )( )
=
=
N
iexperiment
i
experimenti
triali
normalizedE1
2
2
&
&&
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
Regression settings, which include the maximum number of iterations
and the residual and coefficient tolerances for convergence can be
specified
If temperature-dependency is present, this can be accounted directly inthe creep equation (Arrhenius term) or by requesting temperature-
de endent coefficients to be solved for.
After specification is complete, select the Solve button to perform the
curve-fit. The message Solution is Completed will indicate when the ,
button
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
Compare the plot of experimental and fitted data
The x-axis will always be the first column of the test data, so the test data
.
All of the other variables (columns) will be plotted as the y-axis
If the cree fittin is suitable select Save&Close
The left column
indicates creep laws
The graph window is
right-click sensitive,
so various graph
curve- ng was
performed on. To
return to curve-fitting,
select this tree.
changed.
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
When using Save & Close, the curve-fitting data is written into the
MAPDL database for the selected material property number.
,
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training Material... Procedure
Please note that the curve-fitting information is available during the
MAPDL session, but it is not saved with the MAPDL database.
- ,
fitting parameters as many times as he/she likes. Be sure to keep the test data files if that information will be required
later, as the test data is not saved in the MAPDL database, either.
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ANSYS Mechanical Creep Curve Fitting in MAPDL
Customer Training MaterialD. Curve-Fitting Tips
Some basic creep curve-fitting tips:
If a creep law is chosen but a variable is not used, fix the coefficient to
a value to make that term equal to one
For example, some creep laws contain the Arrhenius equation. If notemperature-dependent data exist, fix that coefficient to zero (making the
.
For temperature-dependent data, users can
solve the Arrhenius term or request fortemperature-dependent coefficients
Have an understanding of the range of values of the coefficients:
, . ,
depending on your choice of units
Some coefficients are multipliers and should have very tiny ranges. Unless
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these coefficients are initialized correctly, the curve-fitting procedure may
encounter problems.