The shell element model gives accurate stresses in most regions. However, through-the-thickness stresses are not as accurate, especially where the reinforcement joins with the nozzle body. Solid elements are used for this analysis to improve the accuracy of through-the-thickness stresses. This problem demonstrates some of the features of the solid layered thermal elements (SOLID278).
For this example, it is assumed that the material behavior is structurally and thermally orthotropic. To represent the material symmetry planes appropriately, it is important to define material properties along certain orthogonal directions within the elements. This underscores the need to define an element coordinate system within each element.
All elements have default element coordinate systems, but these defaults may not always be convenient. Material directions could be misaligned with respect to the element coordinate system (ESYS) and need to be modified. You can typically accomplish this with the following steps:
Define the element coordinate system - Due to rapidly changing curvature, each element in this model must have its own element coordinate system defined. As a result, the element z axis is aligned with the thickness direction, and the element x axis is aligned with the curvature. This makes it very convenient to define material properties along preferred directions.
Adjust the element connectivity - Because solid elements are being used, you must adjust the element connectivity so that the IJKL face is aligned with the element coordinate z axis. This ensures that the layer definition is parallel to face 1 (the IJKL face normal n) of the element and is normal to the ESYS z axis.
Thermal stresses can be obtained using an element with temperature and displacement degrees of freedom (DOFs) that are fully coupled (direct or strong coupling). Alternately, stresses can be obtained using a thermal solution followed by a structural solution (load-transfer or loose coupling). For a discussion of the advantages and disadvantages of these methodologies see Types of Coupled-Field Analysis in the Coupled-Field Analysis Guide.
Since the thermal and structural solutions in this example problem do not significantly affect each other with a high degree of nonlinear interaction, loose coupling is used. The example demonstrates the flexibility that loose coupling offers because the two analyses can be solved independently of each other. For instance, a material could be treated as homogeneous for a thermal solution and layered for a structural solution. This allows you the flexibility to mix and match the solution based on problem requirements, a level of agility that is not possible with a strongly coupled simulation.
In this example a static thermal analysis is solved using element SOLID278, and then temperatures are transferred to a static structural analysis. For the structural solution, thermal SOLID278 is converted to structural SOLID185. For the thermal solution, two different cases are studied, assuming the material to be either homogeneous or layered. A similar assumption is made for the structural solution, resulting in thermal loads from either a homogeneous or layered solution.