Abstract: The damage of brittle rock is induced by its microcrack growth which significantly affects the deformation and strength of rocks subjected to the compressive loadings. However, the relationships between microcrack growth and the variable shear properties (cohesion, internal friction angle, and shear stress) are rarely studied. In this study, the relationships between internal friction angle, cohesion, shear strength and micro-parameters at the peak point of stress-strain relationship are proposed, according to the stress-strain relationship triggered by crack growth and the Mohr-Coulomb failure criterion. By introducing the Mohr-Coulomb yield criterion, the theoretical expression is established for dynamic variations of cohesion, internal friction angle and shear stress along with microcrack growth or strain at plastic deformation phase of brittle rocks. Cohesion, internal friction angle, and shear stress firstly increase until to maximum values, and then decrease with the increasing crack growth or axial strain in brittle rocks under compressive loading, which is corresponding to the stress-strain constitutive curves in triaxial compressive tests. Finally, the rationality of the proposed theoretical expression is verified by comparing with the experimental results. Besides, the effects of friction coefficient between microcrack interfaces on the relationships between cohesion, internal friction angle, shear stress and crack growth or axial strain are discussed.

Key words: rock mechanics, micro-mechanism, triaxial compression, variable shear properties, damage