Experimental micro–macromechanics: particle shape effect on the biaxial shear response of particulate systems

The effect of particle shape on micro and macroscopic behaviour is investigated experimentally through a series of biaxial compression tests on round and angular rods. Dual-size assemblies of thousands of circular (round) or hexagonal (angular) particles were sheared under three confining pressures. The displacement, rotation and evolution of coordination number of all the particles during the shearing process were identified through image analysis. The macroscopic results show that the angular assemblies exhibit higher strengths and dilations. Angular particles tend to be more dilative and have higher critical state strengths. The microscale features show that the magnitude of particles' rotation is higher in circular assemblies. Particle rotations are normally distributed within both samples. However, rotating clusters are observed in angular samples. Angular particles are more resistant to rotations due to interlocking and clustering, causing higher strength at the macroscale. The absolute mean cumulative rotation for the circular assemblies at the end of shearing was approximately twice that of the angular assemblies. In circular assemblies, small particles rotate almost twice as much as big particles, which initiates a ball-bearing effect that further contributes to strength reduction. Particle rotation further governs the average coordination number, with hexagonal aggregates having higher coordination numbers.