Macroscopic and microscopic analysis of particle flow in pre-peak cyclic direct shear test of rock joint

The pre-peak cyclic numerical direct shear test of rock joints considering second-order asperity was imple- mented using two-dimensional granular flow (PFC2D) program. The cumulative damage characteristics of rock joints and variation law of shear strength were respectively analyzed from the macroscopic and mesoscopic view. The reliabili- ty of the calculation method was verified by comparing and analyzing the existing laboratory test results. The results are as follows:① as the first-order asperity or the normal stress increases,the failure mode of the rock joint has the tendency to change from climbing to climbing-cutting to cutting,and the shear strength of rock joint shows the trend to in- crease generally. ② The effect of low-rising value of cyclic loading on the joint shear strength is extremely limited and can be neglected,while the propagation and interpenetration extent and number of cracks in the rock asperities are largely increased as the value of cyclic loading increases,which indicates that the value of cyclic loading is an impor- tant factor determining the rate of cumulative damage. ③ As the pre-peak cyclic loading times increases,the number of cracks in the rock sample increases and so does the increase rate of it,and the surge phenomenon of cracks occurs when the number of cyclic loading increases to a certain level,showing that the propagation and interpenetration of cracks have the characteristic of mutation after accumulating. As the cyclic loading times increase,the peak value of the joint shear strength has the variation trend of increasing firstly and decreasing latterly,which shows that the limited number of cyclic loading is beneficial to increase the peak shear strength of joints,but the cyclic loading will eventual- ly cause the degradation of shear strength due to the accumulation of joint damage. This study enriches the indoor test results and helps to further understand the accumulated damage and strength deterioration rule of rock joints under the action of frequent microseisms.