In FEA for Thermal Stress problem which is primary variable?. After transient thermal analysis i will apply nodal temperature as input for structural run. I will give thermal property coefficient of thermal expansion. so it will calculate corresponding thermal strain at that node as input. so after solving KX=F; I will get thermal stress/ or elastic and plastic strain(elastic and plastic deformation)? As my primary varibale? can you explain how? Will we get thermal stress than strain(elastic & plastic)? or otherway
An attempt to answer... To get stresses arising from a thermal problem, you need a thermomechanical model. Usually, these models do not take into account some couplings such as the influence of mechanics on temperature (one may nevetheless think of a small colling effect in traction, some boundary conditions such as thermal contact... but just discard them for now on). This is why you can start by a _decoupled_ thermal problem to get the temperature (the primary variable for this problem), then run a mechanical (elastic, anelastic...) problem with a prescribed temperature field. For this last run, you need a mechanical model. For instance thermoelastic: then the primary variable is usually the nodal displacement field. But if you use a thermoelastoplastic model, this will involve internal variables such as plastic strains... You can even think of more complicated models...
What you input is temperature. The thermal strain is of course the function of the temperature. But thermal stress is calculated out, which depends on the distribution of thermal field and mechanical restraints. If you assuming a bulk with uniformed temperature change and no mechanical restraints, then the thermal stress will be zero.
Following David' explanation I can add the next:
There are two ways for solving thermomechanical problems in Ansys:
One way is solving first the thermal problem (using standard thermal elements) and apply the results, temperature, on the mechanical problem (using standard mechanical elements) to obtain the displacements and calculate with these, stresses and deformations. This scheme is not coupled and the primary variable is the temperature for the thermal problem and the displacement for the mechanical one.
The alternative is using element that support displacement and temperatura at the same time (for instance solid 226). Using this element you can calculate the coupled problem but you need to use APDL commands to setup the thermal part of the problem. In this case there are two primary variables: the temperature and the displacement.
Of course, all of this is true if we are talking about standard formulations in FEA. There are several alternatives but this are not the question here.
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