(1. Key Laboratory of In-Situ Property-Improving Mining of Ministry of Education，Taiyuan University of Technology，Taiyuan，Shanxi 030024，China；2. College of Mining Engineering，Taiyuan University of Technology，Taiyuan，Shanxi 030024，China)
Abstract：The stratification and bedding characteristics of the coal-bearing construction make the surrounding rock more of a shear disaster. To further investigate the shear fracture behaviour of coal rock masses under cyclic loading conditions，uniaxial compression tests on sandy mudstone were carried out using constant amplitude cyclic and stepwise linear cyclic loading and unloading methods in conjunction with CT scanning. The “Domino” structure of shear rupture of rock samples and its evolutionary characteristics were analysed by means of the rupture morphology and the stress-strain curve of the whole process. The results show that：under uniaxial compression and constant amplitude cycles and stepwise linear cycles，shear failure occurs in all rock samples. Moreover，the shear fracture zone formed by the failure of rock samples under stepwise linear loading and unloading cycles includes several strain localization zones with a “Domino” structure，and the stress-strain curve corresponding to the failure process shows a special “hysteresis-reciprocation-hysteresis-reciprocation” oscillation fluctuation. The strain localization zone presents the evolution process of nucleation，cracking，expansion，decay and conduction. The rotating-gyration motion of the fragmented block with the “Domino” structure causes multi-stage dissipation of the input energy and elastic storage energy after the peak in the friction of the block，which prolongs the damage time and has a significant “slow release” effect. The shear crack morphological evolution and stress–strain concealment information cooperate with each other to divide the shear failure process into six failure stages，namely：crack initiation micropropagation(I)，crack initial propagation(II)，primary shear crack propagation(III)，Crack propagation metastable(IV)，rapid crack propagation(V)，accelerated failure(VI). The “Domino” structure is a unique form of shear failure，and the energy “slow release” effect it plays may be a new idea for coal mine roadways to use the surrounding rock structure to relieve pressure and protect the roadway.
