We present a novel rotary shear experiment that was conducted under X-ray micro-computed tomography (µ??) scans. This combination of methodologies allows for the measurement of friction during shearing and provides the opportunity to directly observe the rock joint evolution. Our results demonstrated that the initial phase of slipping was significantly influenced by asperity interlocking and breakdown, and the asperity breakdowns created secondary fractures. We present direct observations of the real contact area with progressive wearing of the shear surface. The real contact area on the joint surface consisted of numerous contacted asperities that accounted for about 11.7-17.6% of the nominal joint surface area. The slip distance required for friction behavior to be stable was similar to the sizes of the largest contact asperities. These observations helped improve our understanding of joint slip behavior and demonstrated that the µCT technology is a powerful tool for the study of joint evolution.

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