Incremental (visco)plasticity: additive decomposition

The implementation of incremental plasticity for linear geometrical calculations in CalculiX follows the algorithms in [19], section 5.3 and is based on an additive decomposition of the strain tensor into an elastic and a plastic part. In CalculiX, it is used in the absence of the NLGEOM parameter on the *STEP card. The internal variables are the same as for the multiplicative decomposition (cf. previous section) in their infinitesimal limit. It seems that the additive decomposition exhibits less convergence issues than the multiplicative decomposition (although this may also be attributable to the general nonlinear geometrical setup).