Constraints and Thermal Expansion
When a structure is undergoes thermal expansion, the fact that the motion is restricted by constraints will in general cause stresses. There are three types of such effects:
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The global expansion is restricted by constraints at different locations, so that internal forces are introduced throughout the structure.
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At a boundary which is constrained, local stresses can appear if the boundary is not free to expand in the tangential direction.
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Internally, the same type of local constraint effects will be caused by rigid objects, such as Rigid Domains.
In many cases, not only the structure which actually is modeled deforms due to the changes in temperature, but also the surroundings (which are approximated by constraints) will deform. You can take this effect into account by adding a Thermal Expansion subnode to the constraints. The constraints will then provide an extra displacement based on a given temperature field. For thermal strains which have a simple variation in space (for example linear temperature variations), it is possible to completely offset the constraint stresses using this method. For more general cases, the stresses caused by the constraint can be significantly reduced.
The thermal expansions of the constraints are independent of that of the material in the adjacent domain, so that the surrounding structure can be made from another material, or have a different temperature distribution.
You can however also inherit temperature and thermal expansion coefficient from the domain being constrained. This is useful for the common case that the temperature and materials are the same over the modeled structure body and its surroundings.
The spatial variation of the temperature and coefficient of thermal expansion must be explicit functions of the material frame coordinates. It is not possible to use a computed temperature distribution for the thermal expansion of the constraints.
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Thermal Expansion (for constraints)
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Thermal Expansion of Constraints in the theory section.
For an example showing how to relive the stress at constraints in a heated structure, see Thermal Expansion in a MEMS Device: Application Library path Structural_Mechanics_Module/Thermal-Structure Interaction/thermal_expansion.