Figure. Quadratic tetrahedrons should typically be replaced by linear tetrahedrons.
Although ProTOp can run optimization with various FE types, there might be good reasons to convert all finite elements to linear tetrahedrons. These reasons are related to execution efficiency and tools availability.
Topology optimization typically requires very fine meshes in order to get nice cut surfaces. Engagement of a computationally efficient element type is therefore of paramount importance.
In this scope the quadratic tetrahedron element is typically not a good choice. Namely, compared to linear tetrahedron it consumes an order of magnitude more CPU time and RAM during the FEA analysis. Therefore, in a typical situation quadratic tetrahedrons should be replaced by linear tetrahedrons.
Figure. Quadratic tetrahedrons should typically be replaced by linear tetrahedrons.
Some ProTOp tools like the mesh refinement or mesh stripping tools are available only for meshes containing exclusively linear tetrahedrons. Typically, such tools are quite important in order to get a good result. Effectively this means that, although linear hexahedrons are well suited for topology optimization, in ProTOp it is highly recommended to convert them to linear tetrahedrons.
Figure. Hexahedrons need to be converted to linear tetrahedrons in order to make more tools available.