One Step Transient Thermal Stress Analysis

Linear or Nonlinear Transient Heat Transfer Analysis is run first, and the entire temperature history is applied as thermal load to subsequent static analysis. The static analysis can be a linear or nonlinear subcase.

For a linear OSTTS solution, after the transient heat transfer run, multiple linear static subcases are internally spawned (equal to the number of thermal loads from transient heat transfer) and the stress state is calculated for each output time-step based on the applied temperature loads.
For a nonlinear OSTTS run, the temperature from the preceding transient heat transfer analysis is mapped linearly to the nonlinear static analysis subcase in the time domain as thermal load.

Linear and nonlinear OSTTS runs provide displacement and stress history for the duration of transient heat transfer. To capture the stress history accurately, you do not need as many time steps as Transient Heat Transfer Analysis. It is recommended to keep the number of output time steps (internal subcases) less than 50. When MUMPS solver is used, the upper limit for the number of boundary conditions is 200.

Fixed Initial and Final Time Temperature Mapping (HTIME=ALL)

A transient heat transfer subcase and a static subcase should be defined to perform a one-step transient thermal stress analysis. The static subcase can be either a linear or nonlinear subcase. TEMPERATURE case control entries with the HTIME keyword can be used in the static subcase to select specific or all time steps to perform the stress analysis. Below is a sample input deck:

SUBCASE 100
   TITLE = Transient thermal subcase
   ANALYSIS = HEAT
   THERMAL(sort1, punch)=ALL 
   FLUX=ALL  
   TSTEP = 100
   IC = 20
   DLOAD = 200
$
SUBCASE 200
   TITLE = One step transient thermal stress subcase
   SPC = 1
   TEMP(LOAD,HTIME=ALL) = 100
   DISPLACEMENT=ALL
   STRESS=ALL

Temperature history and peak stress history from OSTTS are shown in Figure 1. The peak temperature is 43.27 degrees Celsius at 145 seconds, while the stress peak is 42.35 Mpa at 165 seconds.

Note: The peak stress is not required to occur at the same time as the peak temperature or at the initial or final time step.

This illustrates that OSTTS is useful in capturing the stress peak over the duration of the transient analysis.

Flexible Initial and Final Time Temperature Mapping (TEMPT)

This format supports nonlinear OSTTS only. In this format, the source of temperature load in a nonlinear static analysis is taken from one of the following two options:

An internal transient thermal subcase (HSUB). The thermal load is mapped into the nonlinear static analysis linearly from the transient thermal analysis in time domain. The mapping rule is defined in TEMPT card. Below is a sample input deck for nonlinear OSTTS in ‘TEMPT’ format with HSUB.
```
SUBCASE 100
TITLE = Transient thermal subcase
ANALYSIS = HEAT
THERMAL(sort1, punch)=ALL
FLUX=ALL
TSTEP = 100
IC = 20
DLOAD = 200
$
SUBCASE 200
TITLE = Nonlinear One step transient thermal stress subcase
SPC = 1
NLPARM = 50
TEMP(LOAD,TEMPT) = 300
TEMP(INIT)  =  60
DISPLACEMENT=ALL
STRESS=ALL

BEGIN BULK
TEMPT,300,HSUB,100
TEMPD,60,10.0
```
An external file (HFILE). The thermal load is mapped into the nonlinear static analysis linearly from the external file in time domain. Currently, only PUNCH files are supported as external temperature sources. The mapping rule is defined in TEMPT card. A transient heat transfer subcase is not required for the case where an external file (HFILE option) provides the thermal loads for mapping. Below is a sample input deck for nonlinear OSTTS in ‘TEMPT’ format with HFILE.
```
ASSIGN,HFILE,203,temp_results.pch
SUBCASE 200
TITLE = Nonlinear One step transient thermal stress subcase
SPC = 1
NLPARM = 50
TEMP(LOAD,TEMPT) = 300
TEMP(INIT)  =  60
DISPLACEMENT=ALL
STRESS=ALL

BEGIN BULK
TEMPT,300,HFILE,203
+,HTINI,0.2,HTFIN,0.8
TEMPD,60,10.0
```

Initial Temperature

In OSTTS analysis, initial temperature is determined by TEMP(INIT) entry. If TEMP(INIT) is not specified in an input deck, the following process is used to define initial temperature.

For Linear OSTTS and Nonlinear OSTTS in ‘HTIME=ALL’ format, the elemental initial temperature is taken from the reference temperature in its material card. Nodal initial temperature is the average of the initial temperature in the elements supporting the grid.
For Nonlinear OSTTS in ‘TEMPT’ format, the nodal initial temperature is taken from the referenced TEMPT card (based on HTINI).

Note:

If OSTTS analysis (HTIME=ALL format) is used to perform either linear or nonlinear static analysis at all output time steps of Transient heat transfer analysis. Transient heat transfer analysis outputs temperature results for every time step by default. This can lead to high run times and possibly generate large result files. It is recommended to use the skip factor (NO# field) on the TSTEP entry to control the number of time steps at which temperature results are saved for output, such that OSTTS analysis can still capture the stress history without significant computational cost.
For Nonlinear OSTTS, the incremental step in nonlinear static analysis and the output step of transient heat transfer analysis do not need to match each other. The incremental output of nonlinear static analysis is controlled by the NLOUT entry.
DLOAD entry in Nonlinear Static Analysis is not supported for Nonlinear OSTTS.