Dear all,
I have been working on modeling of elastoplastic reponse of a metal during last months in Comsol 5.5. Since I ran into a confliction in equation view of related nodes, I was wondering wether I am wrong or its a bug. Based on weak expression in elastic node and structural mechanics module user's guide, by selecting include geometry nonlinearity button in step node, 2nd Piola stresses are replaced in the formulations instead of Cauchy stresses. On the other hand, by adding a plasticity sub-node in linear elastic material node of solid mechanics module, assuming that plastic strains are small and the formulation is based on decomposition of strains, it is seen that stress-based variables in equation view of plasticity node are defined based on ** Cauchy stresses** (solid.sl11, solid.sl12, solid.sl13, ..). Another proof for usage of Cauchy stresses in plasticity is comparing solid.sl11-equivalent plastic strain and solid.Sl11-equivalent plastic strain diagrams (in uniaxial modeling) with given stress-equivalent plastic strain response of an uniaxial tensile test . Knowing that in elastoplastic formulations, exmployed stress types (1st Piola, 2nd Piola, or Cauchy) must be the same, why elastic node is employing 2nd Piola stresses while the plastic one is using Cauchy stresses?
Thanks for your responses