How to perform a thermal stress analysis

The temperature distribution in a part can cause thermal stress effects (stresses caused by thermal expansion or contraction of the material).  Examples of this phenomenon include interference fit processes (also called shrink fits), where parts are mated by heating one part and keeping the other part cool for easy assembly.  Another example is thermal creep, which is permanent deformation resulting from prolonged application of a stress below the elastic limit.  An example of this is the behavior of metals exposed to mechanical loads and elevated temperatures over time.

Thermal stress effects can be simulated by coupling a heat transfer analysis (steady-state or transient) and a structural analysis (static stress with linear or nonlinear material models or Mechanical Event Simulation [MES]).  The process consists of two basic steps:

1. Perform a heat transfer analysis to determine the temperature distribution.
2. Directly input the temperature results as a load in a structural analysis to determine the stress and displacement caused by the temperature loads.

For example, a thermal stress analysis of a transistor and heat sink assembly was performed as follows:

• A steady-state heat transfer analysis was performed to obtain the temperature distribution (see Figure 1).

Figure 1:  A transistor and heat sink model with temperature distribution results from a steady-state heat transfer analysis.

• In the FEA Editor environment of Autodesk Simulation Mechanical, the analysis type was changed for a structural analysis.  In this case, static stress with linear material models was used.
• Constraints were specified for the structural analysis by fully fixing the two bottom surfaces of the model.  Additional structural loads (such as forces, pressures, and gravity) could have been added if desired; however, for this example, the only loads were the temperatures from the heat transfer analysis.
• On the "Multipliers" tab of the "Analysis Parameters" dialog, a load case multiplier of "1" was specified in the "Thermal" column so that thermal effects would be included in the structural analysis (see Figure 2).

Figure 2:   A load case multiplier was specified to include thermal effects in the structural analysis.

• On the "Thermal" tab of the "Analysis Parameters" dialog, "Another Design Scenario in loaded file" was chosen from the pull-down menu of options in the "Source of temperatures" field.  "1 – Design Scenario 1" was chosen from the pull-down menu of options in the "Use temperatures from Design Scenario" field was used to specify the location of the temperature results file from the previous steady-state heat transfer analysis (see Figure 3).

Figure 3:   The "Thermal" tab of the "Analysis Parameters" dialog was used to specify the Design Scenario that would be used as input to the static stress analysis with linear material models.

• The static stress analysis with linear material models was run and then the results, including thermal stress effects, were displayed in the Results environment (see Figure 4).

Figure 4:   Displacements (left) and stresses (right) in the transistor and heat sink assembly due to temperature loads, displayed in the Results environment.

Thus, the ability to couple heat transfer and structural analysis capabilities provides an easy and convenient way to simulate thermal stress effects.