ANSYS Mechanical APDL - High Temperature Creep Modeling via Isochronous Curves

[CFDFEAGURU]

ANSYS can be used to model creep in a number of different ways. If you are designing to ASME Section VIII, Div., 2 you might have to verify that your design meets the "shake down to elastic action" criteria. Basically, that means that the strains do not continue to increase over the number of cycles.

One way to do this is to use the TB,MISO command in ANSYS and then fill the table with stress and strain at each temperature. Obtaining the values for the stress and strain as a function of time and temperature is the hardest part however, there are some places they can be obtained and a method that can be used to create them if you are using common place materials.

MISO stands for Multilinear Isotropic Hardening using von Mises or Hill plasticity

If you are using the following materials then the stress and strains can be read from the isochronous curves found in ASME Section III, Division 1, Subsection NH

304 SS to a maximum temperature of 1500 F
316 SS to a maximum temperature of 1500 F
800H to a maximum temperature of 1400 F
2 1/4 Cr. - 1 Mo. to a maximum temperature of 1200 F
9 Cr - 1 Mo. - V to a maximum temperature of 1200 F

Some of these curves can be found in "Design and Analysis of ASME Boiler and Pressure Vessel Components in the Creep Range" by Jawad and Jetter. (Least expensive route)

If you have access to API-579 then you can use the Omega Method. This will require you to write a spreadsheet with the end result being a set of isochronous curves that can be used to load the TB,MISO command.

You will have to have already solved your model for the temperature distribution and then use the LDREAD command on your structural model to read in the appropriate temperature distribution per load step. Depending upon the loading, you will probably have to "fool" with the number of substeps command, NSUBST, to achieve convergence.

A 3D solid model is NOT required for this, 2D shell elements work just fine.

Example

TB,MISO,1,5,34

TBTEMP,1200
TBPT,DEFI,4.3020E-05,912.00
TBPT,DEFI,5.0192E-05,1064.00
TBPT,DEFI,5.7367E-05,1216.00
TBPT,DEFI,6.4550E-05,1368.00
TBPT,DEFI,7.1746E-05,1520.00
TBPT,DEFI,7.8964E-05,1672.00
TBPT,DEFI,8.6221E-05,1824.00
TBPT,DEFI,9.3535E-05,1976.00
TBPT,DEFI,1.0094E-04,2128.00
TBPT,DEFI,1.0846E-04,2280.00
TBPT,DEFI,1.1616E-04,2432.00
TBPT,DEFI,1.2411E-04,2584.00
TBPT,DEFI,1.3237E-04,2736.00
TBPT,DEFI,1.4107E-04,2888.00
TBPT,DEFI,1.5032E-04,3040.00
TBPT,DEFI,1.6030E-04,3192.00
TBPT,DEFI,1.7118E-04,3344.00
TBPT,DEFI,1.8321E-04,3496.00
TBPT,DEFI,1.9665E-04,3648.00
TBPT,DEFI,2.1185E-04,3800.00
TBPT,DEFI,2.2918E-04,3952.00
TBPT,DEFI,2.4911E-04,4104.00
TBPT,DEFI,2.7217E-04,4256.00
TBPT,DEFI,2.9898E-04,4408.00
TBPT,DEFI,3.3025E-04,4560.00
TBPT,DEFI,3.6683E-04,4712.00
TBPT,DEFI,4.0966E-04,4864.00
TBPT,DEFI,4.5984E-04,5016.00
TBPT,DEFI,5.1863E-04,5168.00
TBPT,DEFI,5.8746E-04,5320.00

You will also need stress and strain loading in the low temperature (non-creep) range. This can be done thru use of ASME Section VIII. Div., 2 Annex 3.D Strength Parameters.

 

[Greg Bernhardt]

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?