Fracturing process of granite inferred from measurements of spatial and temporal variations in velocity during triaxial deformations

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Abstract
Fracturing in fine‐grained homogeneous granite during triaxial deformation was investigated by measuring the spatial and temporal variations of elastic wave velocities. Velocities along 20 ray paths were measured with 12 piezoelectric transducers attached to a sample under confining pressures up to 400 MPa. Systematic velocity differences observed among the 20 paths apparently indicate the formation of a dilatant region near the surface of a sample. The damaged region formed near the surface due to asymmetric development of dilatancy seemed to play an important role in controlling ultimate failure. No evidence of the formation of a crack concentration zone adjacent to the ultimate fracture plane could be found before failure, insofar as could be estimated from the velocity variations of elastic waves propagating along its plane. However, a highly concentrated region of cracks in which the fracture nucleated was generated in the damaged region at stresses of more than 97–99% of the ultimate strength in all the specimens based on tomographic images computed using an algebraic reconstruction technique. The elastic waves were strongly affected by the damaged region even under the high confining pressures at which cracks are considered to close.