Numerical analyses of discontinuous material bifurcation: strong and weak discontinuities

In this paper an algorithmic formulation for numerical analyses of material bifurcation is presented. Conditions for the onset of both weak discontinuities (discontinuous strain rates) and strong discontinuities (discontinuous velocity fields) are summarized. Based on a recently proposed plasticity model formulated within the logarithmic strain space, the condition for the formation of strong discontinuities is extended to anisotropic finite strain plasticity theory. The resulting equations associated with the mode of bifurcation are solved numerically. For that purpose, an equivalent optimization problem is considered. The algorithmic formulation is based on Newton's method using a consistent linearization. To enlarge the radius of convergence, a line search strategy is applied. The applicability of the proposed implementation as well as its performance and numerical robustness is investigated by means of three-dimensional numerical bifurcation analyses of a Drucker-Prager type plasticity model.