Microscopic analysis of granular material behaviour from small to large strains

Abstract

The Discrete Element Method (DEM) has been widely used to study the macro–micro behaviour of granular materials at large strains (>1%). However, investigations over a wider strain range are lacking. This study conducts DEM triaxial tests on specimens with different particle physical properties to examine their influence on macro–micro behaviour from small strains (below 1 %) to large strains. Small-strain behaviour is characterised by the maximum shear modulus, elastic range and stiffness degradation rate. Large-strain behaviour is analysed through the peak stress ratio, critical state stress ratio and void ratio. Then, the micro-mechanisms underlying these results are examined using the Stress-Force-Fabric (SFF) relationship, which links the (macro) stress ratio and (micro) anisotropy source. This study is the first to apply the SFF relationship to small strain behaviour. Results reveal the qualitative relationship between particle physical properties and macro-behaviour at different strains: increasing particle Young's modulus enhances the maximum shear modulus but accelerates stiffness degradation; increasing shearing and rolling friction significantly reduces the stiffness degradation at small strains and enhances strength and dilation at large strains. This study also highlights the limitation of the Hertz contact model in capturing both small-strain and large-strain behaviour quantitatively using a single set of parameters. Hence, modellers should calibrate model parameters based on whether their focus is on large-strain or small-strain behaviour. For micro-behaviour, the relative importance of anisotropy sources depends on strain level rather than particle physical properties. At small strains, the mechanical anisotropy source (both normal and tangential forces) primarily controls stiffness and its degradation. At large strains, material strength is influenced by both mechanical and geometrical anisotropy sources, with anisotropy from the normal force being the most significant, followed by contact normal, tangential forces, and branch vector.