Abstract
Previous work by the authors using the discrete-element method (DEM) has used the octahedral shear stress within a sphere together with a Weibull distribution of strengths and a size effect on average strength, to determine whether fracture occurs or not. This leads to fractal particle size distributions and a normal compression line which are consistent with experimental data. However, there is no agreement in the literature as to what the fracture criterion should be, and as yet it is not clear whether other criteria could lead to the correct evolution of voids ratio and particle size distribution under increasing stress. Various possibilities for the criterion have been studied in detail here to ascertain whether these other criteria may give the correct behaviour under normal compression. The use of the major principal stress within a particle, the mean stress and the stress calculated from the maximum contact force on a particle are each investigated as alternatives to the octahedral shear stress. Only the criterion based on the maximum contact force is shown to give behaviour observed experimentally and the simulations shed further insight into the micro mechanics of normal compression.
Highlights
Particle crushing is usually modelled using the discrete-element method (DEM) and employing one of two methods: agglomerates or replaceable particles
It is hoped that further micro mechanics of normal compression can be exposed and that a suitable breakage criterion can be established, based on comparison with existing experimental data
This work has sought to clarify the suitability of various breakage criteria when modelling particle breakage in DEM, which was achieved by investigating the use of four different common measures of stress to determine whether or not a particle should break
Summary
Particle crushing is usually modelled using the discrete-element method (DEM) and employing one of two methods: agglomerates or replaceable particles. Following McDowell & Bolton (1998), the emergence of a fractal PSD implies that any suitable breakage regime must take into consideration the coordination number, whereby smaller particles (which have higher strengths but fewer contacts) suffer higher stresses than comparatively larger particles (lower strengths but more contacts) – otherwise, if it were the weakest particles that are most likely to crush, the result would be a uniform matrix of fine particles, behaviour which is not evident in the geotechnical literature In their previous work, the present authors’ used the octahedral shear stress, q, as the characteristic particle stress (and to determine whether a particle should break), defined as q. The focus of this paper is to examine alternative criteria for crushing to ascertain whether each gives rise to (a) a realistic NCL and (b) a fractal distribution of particle sizes with the correct fractal dimension In this way, it is hoped that further micro mechanics of normal compression can be exposed and that a suitable breakage criterion can be established, based on comparison with existing experimental data
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