SPH as a nonlocal regularisation method: Solution for instabilities due to strain-softening

• The strain-softening process in the FEM is localised in a single element and cannot propagate.
• FE results in the presence of strain-softening are highly mesh sensitive.
• Localisation effects related to material strain-softening are not present with the SPH method.
• Size of the softening zone was defined by the smoothing length and was increasing with the increasing smoothing length.
• For a fixed smoothing length hh, the stain softening was independent of the particle density.

Within the framework of continuum damage mechanics (CDM), mechanical loading leads to material damage and consequent degradation of material properties. This can result in strain-softening behaviour, which when implemented as a local model in the finite element (FE) method, leads to an ill-posed boundary value problem, resulting in significant mesh sensitivity of the solution. It is well-known that the addition of a characteristic length scale to CDM models, a non-local approach, maintains the character of the governing equations. In this paper, the similarities between the Smooth Particle Hydrodynamic (SPH) method and non-local integral regularisation methods are discussed. A 1D dynamic strain-softening problem is used as the test problem for a series of numerical experiments, to investigate the behaviour of SPH. An analytical solution for the test problem is derived, following the solution for a 1D stress state derived by Bažant and Belytschko in 1985. An equivalent SPH model of the problem is developed, using the stable Total-Lagrange form of the method, combined with a local bi-linear elastic-damage strength model. A series of numerical experiments, using both SPH and FE solvers, demonstrate that the width of the strain-softening region is controlled by the element size in FE, but in SPH it is controlled by the smoothing length rather than the inter-particle distance, which is the analogous to the element size in the FE method. This investigation indicates that the SPH method is inherently non-local numerical method and suggests that the SPH smoothing length should be linked to the material characteristic length scale in solid mechanics simulations.