I need to get the mid-side node stresses on plate and volume quadratic elements for isotropic elastic material (mainly Q8 and H20 elements). I know that these values are not provided by NX NASTRAN. Does anyone has a program to compute these stresses by using the node displacement for example?
In general the strains and stresses are recovered at element integration points and extrapolated to grids in the parametric space of the element. Strains are spatial derivatives of displacements and are hence one order lower than the element interpolation functions for displacements. Hence the strains for quadratic elements vary linearly (approximately) in the parametric coordinates. After all is said and done the strains and stresse at the mid edge locations are the average of the corner grids on that edge. NX Nastran saves output volume by not reporting the mid edge values since there is no additional information gained by doing so.
From one code to the next there are variations on this but none of them do a direct evaluation of displacement derivates (hence strain and stress) at grids.