Titulo: What is failure? The Answer from Discrete Modelling
Conferenciante Prof. Felix Darve
Area Tematica: Computational material failure and fracture
Fecha, lugar y evento: 26/10/2011, Barcelona, II International Conference on particle-based Methods Fundamentals and Applications PARTICLES 2011
The classical definition of failure for rate-independent materials is related to the existence of some limit stress states, which can not be passed by the material. By gathering altogether these limit stress states, the so-called ?failure surface? is obtained in the six-dimensional stress space. However for non-associate materials, like the granular materials, some failure states are obtained in classical lab experiments strictly inside this failure surface in the plastic hardening regime, as for example in the undrained triaxial tests on loose sands with an axial force control . While the failure surface corresponds to a singularity of the elasto-plastic matrix ( the plastic limit condition), it has been shown that these other failure states observed with a regular elasto-plastic matrix correspond to a loss a definite-positiveness of this matrix, which is the so-called ?second order work criterion? . For non-associate materials, characterised by a non-symmetric elasto-plastic matrix, both these criteria are strictly different, the second one being met before the first one. Thus a bifurcation domain appears in the stress space delimited by both these criteria. Besides, because the second order work criterion is a quadratic form, there are some cones of unstable stress directions and a stress-strain loading path (mixed loading) has to be used to obtain an effective failure, characterised by a burst of kinetic energy and by plastic strains suddenly increasing as a power law versus time. For granular materials, discrete element method (DEM) is well adapted since these direct numerical simulations allow to observe the details of failure modes , including the post-failure regime. So it is proposed to check the above theoretical results (the existences of a bifurcation domain and of instability cones, mixed loading required for an effective failure and burst of kinetic energy in this case) on a numerical cubical specimen of spherical grains, whose mechanical behaviour is simulated by a DEM. In conclusion a new viewpoint of failure phenomenon emerges from these results.