Fault rocks in the brittle regime are often the result of large-displacement shearing. Traditional direct shear tests that perform small displacements can only study one episode of the whole faulting process. Ring shear apparatuses that can achieve large-displacement shearing are mostly used to study incohesive soil or friction granular materials. The objective of this paper is to study the brittle cataclastic process of fault rocks from the damage of intact rock to the frictional movement of the incohesive cataclastic rocks. A large-displacement direct shear model with a constant contact area of fault walls based on the particle-based discrete element method is proposed to simulate the structural evolution and mechanical behavior of dry brittle fault rocks. The results show that the crushing and rounding of irregular fragments are responsible for the reduction in the friction coefficient. The sharp fall is due to the crushing of rock fragments, while a moderate decline indicates grain rotation in the postpeak stress-displacement curve. This model helps to understand the cataclasis of brittle fault rocks.
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