Ansys Mechanical's Logic for Meshing

If a lower-order (linear) element is specified, how does Mechanical choose the appropriate element type? 

Mechanical meshes on a part-by-part basis. When a lower-order structural element is requested, the mesher asks two questions before meshing each part:

1. Will the entire part be meshed only with tetrahedral elements?
2. Is a nonlinear material defined for any body in that part?

If the answer to both of those questions is yes, then the mesher will use SOLID285 for the entire part. If the answer to either of those questions is no, the mesher will use SOLID185 for the entire part. Also, if the entire part is meshed with only tetrahedral elements, other nonlinear options (e.g., NLAD) may trigger the mesher to use SOLID285 rather than SOLID185.

Note: Tetrahedral SOLID185
elements are not recommended, particularly for non-linear applications.