On Inception of Instabilities in Granular Media Across Length-Scales with Flexible Boundaries

Emergence of non-uniform deformation modes or instabilities often encountered in laboratory “single element” tests have been examined in this study across various “length-scales” of granular media viz, continuum, discontinuum and laboratory biaxial “element” test level. With inhomogeneity outset, the mechanical response no longer remains a true representation of the material behaviour. It rather portrays the system response with the boundary conditions and the evolving instabilities. Instability onset within a transient undrained continuum elastoplastic framework is found to be a mesh-dependent phenomenon. This “pathological mesh-dependency” of classical Cauchy continua is addressed with the aid of Level Set Discrete Element Modelling of Flexible Boundary (FB) Plane-Strain (PS) tests that takes into account the actual grain morphology into consideration. The micromechanical observations are found to be in good qualitative agreement with the macromechanical “element” response of FB-PS tests. Interestingly, the microstructural arrangement or the grain fabric acts as the triggering mechanism behind the shear strain aggregation in localized zones within the sand specimen. Alternately, the evolution of heterogeneities may be considered as “disturbances” diffused within the sand specimen that later manifest into localized zones of shear strain accumulation on gradual shearing.