The pre-peak microstructure evolution of the material is crucial in determining the strength and failure scheme of the material, but is not captured by the existing shear strength parameters models which focus on post-peak stage information. Novel concepts of pseudo cohesion and pseudo internal friction angle that account for both the pre-peak and post-peak stage information were proposed in this study, based on the experimentally observed response of the rock mechanics parameters to an external load (the maximum principal stress). These two parameters were analytically derived as functions of maximum principal strain, with coefficients to be determined (i.e. by applying the Mohr-Coulomb criterion with these parameters to experimental data). The dependence of pseudo cohesion and pseudo internal friction angle on the axial strain and confining pressure were investigated. The evolution of the strength parameters was found to relate to the propagation of micro crack developed in the loading process, as revealed by PFC 2D simulations reproducing experiments. The results show that (1) analytical model characterizing the pseudo cohesion and internal friction angle as the quadratic function of axial strain accurately reflects the mechanical response characteristics of the specimen; (2) before the residual stage, the pseudo cohesion first increases and then decreases with increasing strain, while the pseudo internal friction angle increases continuously; (3) for the sandstone specimens studied, the pseudo cohesion at the peak stress increases by a gradually decreasing rate with increasing confining pressure, while the pseudo internal friction angle at the peak stress decreases by a decreasing rate with the increasing confining pressure; (4) CPM well characterized the mechanical properties of rock under multiple confining pressures, as the total stress-strain curves and failure modes of numerical specimens reproduced the experimental results; (5) the evolution of rock pseudo shear strength parameters is closely related to the external load and the generation, propagation and penetration process of rock cracks. The rock pseudo shear strength parameters are gradually mobilized with the increasing load, and the propagating of cracks owing to the decrease of mobilizable maximum cohesion and the increase of mobilizable maximum internal friction angle.

The pre-peak microstructure evolution of the material is crucial in determining the strength and failure scheme of the material, but is not captured by the existing shear strength parameters models which focus on post-peak stage information. Novel concepts of pseudo cohesion and pseudo internal friction angle that account for both the pre-peak and post-peak stage information were proposed in this study, based on the experimentally observed response of the rock mechanics parameters to an external load (the maximum principal stress). These two parameters were analytically derived as functions of maximum principal strain, with coefficients to be determined (i.e. by applying the Mohr-Coulomb criterion with these parameters to experimental data). The dependence of pseudo cohesion and pseudo internal friction angle on the axial strain and confining pressure were investigated. The evolution of the strength parameters was found to relate to the propagation of micro crack developed in the loading process, as revealed by PFC 2D simulations reproducing experiments. The results show that (1) analytical model characterizing the pseudo cohesion and internal friction angle as the quadratic function of axial strain accurately reflects the mechanical response characteristics of the specimen; (2) before the residual stage, the pseudo cohesion first increases and then decreases with increasing strain, while the pseudo internal friction angle increases continuously; (3) for the sandstone specimens studied, the pseudo cohesion at the peak stress increases by a gradually decreasing rate with increasing confining pressure, while the pseudo internal friction angle at the peak stress decreases by a decreasing rate with the increasing confining pressure; (4) CPM well characterized the mechanical properties of rock under multiple confining pressures, as the total stress-strain curves and failure modes of numerical specimens reproduced the experimental results; (5) the evolution of rock pseudo shear strength parameters is closely related to the external load and the generation, propagation and penetration process of rock cracks. The rock pseudo shear strength parameters are gradually mobilize d with the increasing load, and the propagating of cracks owing to the decrease of mobilizable maximum cohesion and the increase of mobilizable maximum internal friction angle.