To investigate the process of stick‐slip shear failure, small‐scale poly methyl methacrylate samples with friction faults and a high‐speed recording experiment system are used. The evolution of shear stress and slip displacement clearly shows both the propagation of stick‐slip failures and the local slip‐weakening behavior of the fault. The distribution of local stress ratio τyx/σy along the fault is of great significance to determining the nucleation location. The interrelations among local shear stresses, slip displacements, slip velocities, and slip accelerations show the details of local deformations and motions of breakdown near the crack tip. Four distinctive phases of local breakdown process are found in both the nucleation zone and the dynamic rupture propagation zone. Experiments with three prefabricated roughnesses of the fault surfaces are conducted at normal stresses varying from 2 MPa to 4 MPa. The critical crack length decreases and the rupture velocity increases with increasing normal stress or decreasing the roughness. Both the local dynamic stress drop Δτd and local breakdown stress drop Δτb generally increase with increasing normal stresses and decrease with increasing the roughness. Although the critical slip‐weakening displacement uc seems to be insensitive to normal stress, it decreases as the fault becomes smoother. The shear fracture energy Gc, which is determined by Δτb and uc, increases with an increase in the normal stress and increases with increasing the roughness. In addition, the linear relations of Δτd, Δτb, and uc to the static friction coefficient μs are found, and a quadratic function relating Gc and μs is established.

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This paper studies the internal morphology of cracking of two 3-D pre-existing cross-embedded flaws in poly(methyl methacrylate) specimens under uniaxial compression. In this study, observable cracking behaviors include the crack initiation, growth, wrapping and coalescence, and the ultimate fracture of the specimen. The experimental results show six modes of crack initiation and two types of crack wrapping. The initiation of silky cracks in 3-D cracking behaviors is reported first, as well as the closed-mode wrapping of petal cracks. Substantial wrapping of petal cracks, which plays an important role in crack coalescence, appears to be the major difference between the 2-D and 3-D cracking behaviors of pre-existing flaws. Moreover, three modes of crack coalescence, which include the coalescence of petal cracks, antiwing cracks, and oblique secondary cracks, are found. Crack coalescence is predominantly created by the linkage of petal cracks, which is further classified into six categories. Note that only limited vertical growth of petal cracks is observed, and the ultimate failure of specimens is induced by the continued growth of the propagated wing crack. It was found that the ultimate fracture face of the specimens consists of continuous wing cracks and fragmented silky cracks. This paper provides the internal morphology basis for the 3-D fracture mechanism of specimens containing two 3-D pre-existing cross-embedded flaws.

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