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Reading the thread I would suspect that AutoCAD Mechanical does not do 3D FEA, you would need Inventor Professional for that functionality (or Autodesk Simulation).
I suspect you are correct JD but I watched a video by Peter that implied 3D was possible in AC Mechanical.
On a related subject I just posted a question regarding the inadequacy of the materials list in AC Mechanical FEA 2D. If you look at it you will see that the choices for aluminum alloys are byzantine.
Cheers