Granular Plasticity in Triaxial Compression Experiments: Nonuniversal Stress Fluctuations and Particle Shape Dependence

A disordered network of particles in a granular material, when compressed past yielding, can quietly rearrange or violently erupt into restructuring cascades. By varying particle shape we explore a wide range of plastic deformation behavior in triaxial compression experiments on 3D-printed and laser cut particles. Sudden stress drops are shown via X-ray imaging to be localized particle slip events. We calculate from stress-strain data each particle shape’s friction angle, a dimensionless measure of shear strength, and volatility, a method for quantifying fluctuations in financial mathematics. Qualitative regions emerge in the parameter space suggesting distinct microstructural origins of particle rearrangement. For all shapes the magnitude of relaxation events appears to follow a truncated power law distribution, with particle shape driving both the power law exponent and the location of the exponential roll off. We find a nonuniversal range of exponents which are correlated with the particle shape’s ratio of the radii of circumscribed and inscribed spheres. Finally, we discuss extensions to interface depinning and other popular frameworks which better explain our results in the broader context of amorphous plasticity.