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Stress recovery in Optistruct

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Hi all,


I have recently started to learn solving with optistruct. While solving for a simple 2D shell static analysis of a plate with a hole under tensile stress, I noticed something strange. I applied tensile loads on opposite ends of the plate and did not apply any constraints at all. Optistruct solved it without errors. The displacements were obviously very large (10^9 range) owing to lack of constraints BUT the stresses were acurately calculated! Does anyone know how can optistruct recover stresses accurately with wrong displacement results? 

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Stresses are calculated (via the material model) using the calculated strains.  The strains are calculated using the relative displacements of the nodes, along with the initial element size.  Consider a simple example of a 1D element that is 10 mm long.  If the left end displaces 1,000,000,000 mm and the right end displaces 1,000,000,001 mm the relative displacement is only 1 mm.  The strain would then be (change in length)/(initial length) = (1mm)/(10mm) = 0.1.  As long as the strains can be accurately calculated the stresses should be OK.


For your example it sounds like OptiStruct was able to get the correct strains and subsequent stresses despite the lack of constraint.


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