Mineral heterogeneity dominates the tensile failure mode of Eastern Himalayan Gneiss

Abstract

As two fundamental fracture types in rock engineering, tension and shear fractures are commonly considered to operate under different stress conditions. However, abundant recent research shows that the microscale heterogeneity of rocks leads to the possibility of microscale tensile and shear fracturing under any stress state. In particular, localised stress concentrations and tensile fractures caused by mineral heterogeneity under compressive stress have been widely studied. However, the heterogeneity-induced processes involved in the shear fracture under direct tension conditions remain ambiguous. In this study, a micro-direct tensile test was used to comprehensively reveal the process and mechanism by which mineral heterogeneity leads to shear fracture in the gneiss of the eastern Himalayas under direct tension. Microscopic observation of the failure process under direct tension, normalised crack initiation and propagation stress, and transverse strain dilation caused by shear behaviour indicate that gneiss with an exceptional anisotropy angle may exhibit more pronounced shear sliding during progressive tensile failure processes. Micro shear sliding significantly increased the strength of the gneiss and changed its tensile strain, elastic modulus, and progressive failure process, which were determined using a direct tension test. Depending on the degree of frictional sliding, the tensile failure mode of gneiss can be divided into three modes: tensile, hybrid, and shear. Mineral heterogeneity influences the pattern of crack propagation through potential fracture planes, altering the crack propagation stress, significantly affecting the mobilisation of frictional strength, and resulting in a progressive transition between the three modes. The observation of shear sliding under direct tension caused by heterogeneity emphasises the importance of mineral-scale heterogeneity in evaluating the tensile strength of anisotropic rocks.