Discrete element simulation on failure mechanical behavior of transversely isotropic shale under two kinds of unloading paths

Abstract

Based on the discrete element method, a two-dimensional numerical model of shale with different bedding inclinations was established. Using this model, the macro-and meso-mechanical properties of bedded shale under different unloading paths and bedding inclination were studied. In this study, the damage mechanism of bedded shale during the unloading process is revealed. The simulation uses the displacement-controlled loading method, with a loading speed of 0.05 m∙s−1. The results indicate that the bedding inclination of the shale has a significant impact on both mechanical parameters and failure modes under unloading. The peak strength and the ultimate bearing capacity show a “U” shape trend under both unloading paths. Observations of maximum values are observed at β = 0° or β = 90° while observing the minimum values at β = 30°or β = 45°. The failure mode of shale specimens with β = 0° at lower initial confining pressure, the failure mode is splitting damage along the multiple bedding plans, which gradually changes to shear damage with increasing initial confining pressures crossing the weak surface. The failure mode of the specimens with β = 15–45° at different initial confining pressure is shear-slip damage along the bedding planes. And shale at β = 60–90° failure modes are all conjugate shear damage intersecting with the weak surface. The damage to the specimen caused by unloading and increasing the axial stress is greater than that caused by the unloading and constant axial strain. The effects of β = 0° and 90° on microcracking and stress distribution within the specimen are significantly smaller than for the other bedding inclinations.