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

A6A-1ANSYS, Inc. Proprietary

2010 ANSYS, Inc. All rights reserved.Release 13.0

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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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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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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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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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Customer Training Material... Procedure

If launching MAPDL fro the WB Project Schematic, highlight

Analysis and RMB=> Open Mechanical APDL



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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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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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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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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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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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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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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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When using Save & Close, the curve-fitting data is written into the

MAPDL database for the selected material property number.

,



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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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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.

Mechanical-Nonlin_13.0_App6A_Creep Curve Fitting in MAPDL

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