Is it possible to do FEM using hexahedral elements in Mathematica? If it possible, is there any help to do that?
The free Imtek Mathematica Supplement IMS (available here or here) does include hexahedron elements for FEM. As far as I know it has Not been updated in several years, therefore I do not expect all of it to work in Mathematica version 8 or 9. Here is an example of a calculation with tetrahedral elements. As you can see in that example, IMS does not use Mathematica for meshing, but other external meshing programs. Therefore, even though the hexahedron element does exist in IMS, you further need to create the mesh either with another program or to implement that yourself in Mathematica. If you install IMS in your computer, you will find the IMS documentation in the "Installed add-ons" section of Mathematica's documentation center. Then, the documentation about the hexahedral element can be found in ComputationalGeometry->Graphs->MeshElementLibrary, at the bottom of the page. If you decided to use IMS, I suggest you review carefully the application examples of FEM named "Quick Example(Mixed Elements 2D)" and "Quick Example(Periodic Boundaries 2D)", also in IMS documentation