Microscopic Modeling of Frictional Response of Smooth Joint Under Normal Cyclic Loading

Normal stress changes occur commonly during fault and rock joint rupture, and play a key role in determining frictional behavior and shear stability of these discontinuities. Previous experimental studies of the direct shear test under cyclic normal loads confirm that there exists a phase shift between peak normal stress and peak shear stress, as well as between peak friction coefficient and peak shear stress with shear stress and friction coefficient lagging. However, the underlying physics of this finding is poorly understood. Here, we present a numerical study to investigate the effect of cyclic normal loads on the friction of smooth joints. Our simulations show reasonable agreements with experimental observations, verifying the capability of the proposed model based on the discrete element method. We also investigate the effect of normal loading rate, dynamic amplitude, static normal stress level, shear velocity, and joint stiffness on the frictional behavior of the joint. From a microscopic point of view, we focus on the underlying processes of the phase shift between the peak shear stress (friction coefficient) and peak normal loads. We find that phase shift is related to the changes of the population of slipping and frozen contacts, and also the evolutions of shear force, shear velocity, and shear displacement of individual contacts, in which the macroscopic shear velocity plays a significant role. Inhomogeneous stress distributions near the joint indicate that damage or failure should occur in some contact-scale regions, although which is not directly verified due to the limitations of the presented model. Our work improves the understanding of the physics on how normal load perturbations affect the shear behavior of smooth rock joints.