Elastoplasticity with softening as a state-dependent sweeping process: non-uniqueness of solutions and emergence of shear bands in lattices of springs

TL;DR

This paper models elastoplastic lattices with softening as a state-dependent sweeping process, revealing non-uniqueness of solutions and shear band formation due to strain localization.

Contribution

It introduces a novel mathematical framework for elastoplasticity with softening, capturing non-uniqueness and bifurcations in lattice models.

Findings

Shear bands emerge in numerical simulations of softening lattices.

Multiple co-existing solutions are analytically derived in toy models.

Discontinuous bifurcations with stability exchange are observed.

Abstract

Plasticity with softening and fracture mechanics lead to ill-posed mathematical problems due to the loss of monotonicity. Multiple co-existing solutions are possible when softening elements are coupled together, and solutions cannot be continued beyond the point of complete degradation of the set of admissible stresses. We present a state-dependent sweeping process which solves the evolution of elasto-plastic Lattice Spring Models with arbitrary placement of softening, hardening and perfectly plastic springs. Using numerical simulations of regular grid lattices with softening we demonstrate the emergence of non-symmetric shear bands with strain localization. At the same time, in toy examples it is easy to analytically derive multiple co-existing solutions. These solutions correspond to fixed points in the implicit catch-up algorithm and we observe a discontinuous bifurcation with the exchange of stability of those fixed points.