Fracture process zone (FPZ) in quasi-brittle materials: Review and new insights from flawed granite subjected to uniaxial stress

In the failure of quasi-brittle materials such as rocks, microstuctural damage always precedes macroscopic fracture and manifests itself in the formation of fracture process zones (FPZs). In-vestigations of the FPZ are of importance to a better understanding of quasi-brittle fracture behavior along with the size effect and failure forecast. The sensitivities of the FPZ to material microstructures, specimen geometries, loading conditions, ambient factors and monitoring techniques have been investigated extensively, as well as the effects of the FPZ on crack damage and strength/fracture toughness. The first purpose in this study is to discuss the developments and contributions of experimental studies on the FPZ in quasi-brittle materials. Besides, there is limited advance knowledge of the correlations of cracking levels with the FPZ evolution patterns due to the fact that researchers concern cracking levels seldom from the FPZ perspective. This study is also to establish a better understanding of stable and unstable crack growth in quasi -brittle materials from the FPZ perspective. Acousto-optic-mechanical data from uniaxial compression tests faithfully show that the existence of apparent FPZs is a material basis for un-stable crack growth in flawed granite.