Running Stress Test on AutoCad 3D object in Inventor

[Anonymous]

I have a fully editable 3D skull file in autocad.  I am trying to run stress and force tests on it in inventor.  When I open it in inventor it seems like it doesn't even recognize it is there.  I can't edit it or place forces/ constraints on the skull. Any help? Would Nascad work better for this? 

I attatched the file. 

[swalton]

What are you trying to learn by analyzing this file?

At first glance, I see two major issues with your file and project.

1. The AutoCAD file is a 3d Triangle Mesh of the exterior of the skull, not a watertight solid.
   1. Inventor Pros's FEA will need a 3d Solid in order to generate the FEA mesh. Do you have a scan of the interior surfaces of this skull?  Inventor's FEA can use shell elements, but it needs a solid as a basis to create the shell.  I suspect that the skull does not have a uniform thickness, so you will need to get an actual watertight solid for useful results.
   2. Nastran InCAD should be able to run a study using shell elements, but without thickness information for each element, I doubt you will get relevant results.  
2. Bone is an anisotropic material.  Its material properties vary.
   1. Inventor Pro's built in FEA assumes that the material properties are uniform at all positions and all times for a solid body.  Think a steel bar, not a wood plank or rubber band.
      1. This code also assumes that the loading of the material is in the linear elastic portion of the stress-strain curve.  Think compressing a spring, not forming sheet metal.
   2. I don't see any evidence of the different skull bones in your file.  Those bones may have different mechanical properties.  Is that level of detail necessary for your question?
   3. Nastran InCAD should be able to run anisotropic materials. Do you know the bone properties over the volume of the skull?

I was able to import your file in Inventor Pro 2018.  

1. I used the STEPOUT command in AutoCAD Mechanical 2018 to convert the dwg. to a .stp file.  Inventor seemed to import the .stp better than the .dwg.
2. I imported the .stp file and got a composite surface body. 
3. The built-in FEA did not understand the surface. 
4. Nastran InCAD would allow me to select each triangle and assign a shell element, but I did not know the thickness for each element.
   1. I tried window-selecting the surface body, but gave up waiting for Nastran InCAD to respond after 5 minutes.

I found this Master's Thesis "Finite Element Analysis of Infant Skull Trauma using CT Images" by ARNA ÓSKARSDÓTTIR (2012) via a Bing search for "Skull bone fea studies" (http://www.diva-portal.org/smash/get/diva2:560435/FULLTEXT01.pdf).  It describes using a CT scan to get the skull geometry, processing the scan data with Matlab, and running simulations with LS-Dyna. A deeper scientific literature search may be in order for your problem.