Numerical aspects of nonlocal plasticity with strain softening

A regularized plasticity model for the strain-softening behavior of concrete under compression is proposed, the corresponding numerical algorithms are developed, and numerical examples are presented. The plasticity model of the integral nonlocal type is an extension of the classical Drucker-Prager model, which is based on a pressure-dependent yield criterion and a non-associated flow rule. Regularization is achieved by replacing the local softening variable in the softening law by a linear combination of the local softening variable and its nonlocal average. The stress-return algorithm of this model leads to a nonlinear complementarity problem with coupling among the Gauss points, which can be solved in an iterative fashion. Numerical examples of a compression test under plane strain show no spurious dependence on the computational grid. The angle under which a shear band develops agrees well with analytical predictions. The loading process up to complete failure can be simulated without spurious locking effects.