Ansys Polyflow can also compute the residual deformations and stresses due to a nonuniform temperature distribution for the domain. Consider a fluid parison or a sheet, which has a nonuniform temperature distribution after a cooling phase. During the cooling phase, it is assumed that the stresses are relaxed, and therefore, the material is stress free at this stage. However, the temperature distribution will lead to a deformation/stress at ambient temperature.
The model is based on the linearity of the elasticity equations (Equation 27–1 – Equation 27–3), for which the superposition principle can be applied. Starting from the assumption that the domain is stress free for a given nonuniform temperature field, stresses and deformations that would exist at ambient temperature are calculated. With such an assumption, the results of the computation are qualitative, since the real residual stresses and deformations are time dependent, and also depend upon the temperature history. But, for a first approximation, this approach allows you to ascertain which parts of the product are in traction, compression, or shear, as well as how the product is deformed.
The computation for the residual deformations and stresses is not only applicable for blow molding or thermoforming simulations, but can also be used for applications like extrusion.
The thermoelastic problem requires boundary conditions for the displacements. The boundary conditions used for the fluid-structure interaction remains valid, except for the interface between the fluid and solid. All other boundary conditions (such as normal/tangential displacement/force imposed, or plane of symmetry) can be applied. See Elasticity Boundary Conditions for more details.
In order to compute the residual stresses and deformations, there are several steps:
The first step is to simulate the blow molding, thermoforming, or extrusion. This first step may or may not be isothermal.
The second step is only required if the first step is performed with shell elements. With shell elements, there is only one temperature unknown across the thickness. Since there is no temperature gradient across this thickness, it is not possible to simulate the cooling phase. Thus, you have to convert the shell element geometry and the related results into a real 3D geometry. See Converting a Shell Mesh and Results for details on how to convert the mesh and results.
The third step is a cooling phase, for which you need to define a transient heat transfer task. If the previous simulation already took into account the temperature, you have to restart from this result, in order to initialize the temperature field. Otherwise, you have to initialize it with a constant value (for example, the average temperature item). The transient task may be interrupted when a criterion like the minimum of temperature reaches a limit. See Interrupting Evolution for details on interrupting the transient task.
Finally, the fourth step is the computation of the residual stresses and deformations.