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FEA vs hand calcs of a simple rod

Question asked by Charles Lee on Jan 25, 2012
Latest reply on Jan 26, 2012 by Paul Kellner

So, I've been using SimulationXpress for a while and one study made me question the results.  Anyway, I went back and created a simple square rod 1"x1"x5" and applied a 2000lb tensile load to one of the square ends.  The other end is fixed.  Simple hand calculations (F/A) say the stress should be 2000 psi ( 2000lbs/1in^2). 
However, the SimulationXpress results (static study) yeild something vastly different.  I was wondering if someone could tell me what's going on here?  Am I missing something?   Then when I started playing with the mesh sizes (global size and tolerance), the stress values start going higher for smaller meshes.  Also, I changed the length to 20" and the stress values

 

Couple things that I don't understand here:

1. As the mesh element size decreases, the stress increases.

2.  The stress is 5-80% over what it should be.... way outside an acceptable range of values compared to simple hand calcs

3.  The length of the rod shouldn't influence the stress results.  Similar element sizes for a 5" rod compared to a 20" rod shouldn't yeild different stresses.  Some are right on and some are 8% off.  Why is this?  Should be the same calculations with an increase in the number of equations?

4.  I've also tried this on a different machine which had an older license.  The results are different for same restraints, loads, and geometry but it's possible I can scratch this off to different compilers.  I'm not sure though, so I included the information here.

5.  I've also tried this on a circular rod which gave me similar results, but the 20" rod was a lot closer to the hand calcs.  Is there reason to think that the separation of the load and the fixture would influence the results?

 

Anyway, I've attached the results and any help would be appreciated.

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