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Recover CHEXA stresses at corners with a non-positive Jacobian - #343
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A valid CHEXA can have a zero or negative Jacobian at a corner while it is positive at every Gauss point. Stress recovery evaluates the strain- displacement matrix at the corners, so it stopped with a fatal error even though the element stiffness was fine. The MacNeal-Harder distorted patch test is one such case. At those corners the strains are now extrapolated from the 2x2x2 Gauss points, which is exact for constant strain. Regular elements are unaffected. Fixes MystranSolver#225.
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A valid CHEXA can have a zero or negative Jacobian at a corner while it is positive at every Gauss point. Stress recovery evaluates the strain- displacement matrix at the corners, so it stopped with a fatal error even though the element stiffness was fine. The MacNeal-Harder distorted patch test is one such case.
At those corners the strains are now extrapolated from the 2x2x2 Gauss points, which is exact for constant strain. Regular elements are unaffected.
Fixes #225.
This change made use of various AI tools. I've reviewed it and believe it's correct and worthwhile, and am submitting this myself.