Digital speckle correlation method (DSCM) was applied to the observation of the deformation field evolution of granodiorite plate specimen containing surface flaw during biaxial loading. A DSCM post-processing method to quantitatively analyze the crack propagation was developed, by which in-depth investigation on the evolution of several main cracks during specimen fracturing was conducted. The results show that the wing cracks and anti-wing cracks emerged during specimen fracture can be classified into two different types: the former is extensional type, while the latter is extensional and shearing mixed type. Anti-wing crack is actually the trail of petal cracks which initiates and propagates from the interior of the specimen extending to the specimen surface. The anti-wing crack is a major fracture in the specimen containing surface flaw.
Surface flaw refers to a crack with one and extruding the surface while the other yet o cut through the rock mass. Most cracks extruding surface in rock engineering belongs to such a type. Moreover, from a macro-scale perspective, most faults in earth's crust could be regarded as surface flaws. Therefore, experimental research of the crack growth process from a 3D surface flaw is great theoretical and practical significance to the understanding of the mechanical properties of rock mass, and the mechanism of fault movement.
Some foundational achievements existed in earlier experimental research work concerning the propagation of 3D surface flaw in rock. Teng (1987), Yen and Li et al. (1988, 1991) preliminarily in investigated the propagation mechanism of surface flaw by performing the experiment on the plate of glass, PMMA, marble, etc. with a surface flaw. In recent years, a large number of 3D crack propagation experiments have been conducted in the Rock Mechanics Laboratory of the Hong Kong Polytechnical University. Wong et al. (2003, 2004, 2006, 2007) conducted systematic experiments by using specimens which contains 3D surface flaw and are made of various type of materials, such as rock, PMMA, colophony, cement and plaster. They concluded that the mode of crack initiated from the surface flaw depended on the ratio of crack depth (d) to specimen thickness (t). When d/t> 1/3, the crack propagation mode was close to that of 2D (penetration type) crack; when d/t <1/3, a new crack mode (anti-wing crack) was appeared. Recently, Liu at al. (2008) conducted biaxial compression experiment on granodiorite specimens containing surface flaw by using multi observation techniques, including multi-channel stain gauge, acoustic emission 3D location system, digital speckle correlation method (DSCM), etc. They indicated that the crack growth process of the surface flaw could be divided into three basic phases: wing- crack propagation phase, petal-crack activity phase and shell-shaped block formation phase (i.e. anti-wing crack formation phase). All of the research achievements above laid a fundamental and in-depth experimental research of 3D surface flaw propagation. However, the quantitative analysis of the crack propagation from a surface flaw of the specimen and therefore the in-depth analysis on the mechanism of the fracture are lacked.
