Wave propagation in granular material: What is the role of particle shape?

Abstract

Wave propagation in granular materials is one of the most fundamental problems in mechanics and physics, with both scientific fascination and practical importance. Whether elastic waves in granular media are affected by particle morphology and, if yes, how they are affected remain open questions. Here we present a novel grain-scale model to address these fundamental questions. Efficient techniques are incorporated in the model to cope with a huge number of non-spherical particles which are randomly packed to propagate elastic waves. The variability of particle shape is mathematically described using the superquadric function and a family of geometric shapes is produced so that a systematic investigation of the effect of particle shape becomes viable. The difficulty with the detection of particle contacts that are represented by nonlinear Hertzian contact law is tackled using an efficient algorithm. A marked finding from the multiple series of simulations using this new model is that: an increase in the aspect ratio of particles (i.e. particles changing from spherical to ellipsoidal) leads to a notable rise in the elastic wave velocity, for both compression and shear waves, whereas for non-spherical particles with a given aspect ratio, an increased particle blockiness causes a moderate reduction in the wave velocity. Moreover, it is found that an assembly of particles with higher aspect ratio is associated with a broader range of transmitted frequencies while an assembly of particles with magnified blockiness holds back the conduction of higher frequencies. Based on statistical analyses, we further show that the transition from spherical to non-spherical particles is associated with a broader range of void ratios and increased coordination numbers whereas inflated blockiness brings an opposite impact, and these changes are linked with the observed effect of particle shape on the characteristics of elastic waves, in both time and frequency domains.