Tensile Strength and Fracture Surface Morphology of Granite Under Confined Direct Tension Test

The tensile strength of rock is a crucial parameter for the stability assessment and reinforcement design of rock engineering, which is commonly determined by indirect tension test or uniaxial direct tension test. However, the rock in slopes and underground caverns is often in a direct tension stress state under confining pressure, due to the stress redistribution induced by excavation or mining. In this study, the direct tension tests of granite samples under both different confining pressures and tensile loading rates were carried out by an auxiliary apparatus installed in pressure chamber, to investigate the confined tensile strength and fracture surface morphology. High-precision 3D laser scanning was used to obtain the morphology data and the geometrical statistics of asperities on fracture surface, containing the distributions of height, dip angle and dip direction of the asperities. The tensile strength first increases and then decreases with the increase in confining pressure, indicating that the maximum tensile strength of tested granite is not the uniaxial tensile strength and appears in the condition of a certain confining pressure. The tensile strength slightly increases with the increase in tensile loading rate. The fracture surface is smoother as confining pressure increases, while is rougher with the increase in tensile loading rate. As confining pressure goes up, from a micro-point of view, the friction force and the locking force between grains of granite have a similar changing rule to the tensile strength. The roughness of fracture is closely related with both micro-fracture pattern between grains and macro-fracture pattern concerning tensile and shear properties. These provide a reasonable explanation for the correlation of the tensile strength and fracture roughness with confining pressure and loading rate.