Micromechanical origins of heap formation in sheared granular materials with elongated particles

Granular materials with elongated particles exhibit complex flow patterns under shear deformation. Unlike spherical particles, when assemblies with shape-anisotropic particles are sheared, there is accumulation of material due to secondary convection flows (Fischer et al., 2016). This effect resembles the Weissenberg effect of non-Newtonian fluids, where secondary flows lead to phenomena such as rod climbing. Although a direct comparison between liquid and granular flow is not prudent, due to the discrete particulate nature of the latter, comparisons of the causes behind each phenomenon can be drawn. The split-bottom shear cell geometry, and linear shear cell geometry with a middle split, are employed in discrete element simulations to study the micromechanical origins of the effect. Systems of spherical and anisotropic particles are simulated in both shear cell setups, to facilitate comparisons between materials that do not exhibit heaping and systems that do. The mechanical response is characterized in terms of stress measurements, such as shear stress, normal stress and the differences between the normal components of the stress tensor. Our results reveal that a granular Weissenberg effect occurs, which is attributed to the components of the normal stress differences; this is also the driving factor for the classical Weissenberg effect. Consequently, this work brings an analogy between the Weissenberg effect observed in non-Newtonian fluids and heap formation in granular fluids composed of discrete particles.