Noncoaxial Behavior of Granular Materials under Principal Stress Rotation: Insights from Stress Probing Analysis

Noncoaxiality refers to the deviation between the principal directions of plastic strain increment and stress. However, in laboratory tests, due to the difficulty of separating elastic and plastic strains, the principal direction of plastic strain increment is commonly approximated by measuring that of total strain increment. In this study, we use a stress probing technique using the discrete element method (DEM) to isolate the plastic component from the total strain response of a three-dimensional granular assembly under pure rotational shear. By subtracting the elastic strain increment, a precise investigation of noncoaxiality is conducted, considering various influential factors such as stress ratio, intermediate principal stress parameter, void ratio, and bedding plane angle of the samples. The test results indicate that the approximation method tends to overestimate noncoaxiality. The degree of noncoaxiality is significantly influenced by stress ratio but exhibits minimal sensitivity to other factors. The phase angle and variation range of noncoaxiality are affected by in-plane and out-plane bedding angles, respectively. Based on the decomposed elastic and plastic strains, the evolution of energy components is analyzed, revealing a substantial portion of plastic work is initially stored within granular assembly, while the majority is dissipated through interparticle friction in subsequent cycles.