Crack growth and coalescence mechanism in granite material containing two surface cracks under uniaxial compression

The main purpose of this study is to investigate the crack growth process and the coalescence mechanisms of two parallel pre-existing 3-D surface flaws under uniaxial compression in real rocks. During the experiment, only the bride angle (β, the relative inclination between two surface flaws) was varied from 90° to 45° with other parameters were fixed. Two observation systems were used during the experiment containing CCD camera and acoustic emission (AE) system. In this study, petal cracks (Mode III crack) initiated along the interior surface flaw front, and propagated in three-dimensional curve shape towards to the surface of the specimen and to the bridge area. The coalescence crack and anti-wing crack are formed by the mixed mode crack of petal crack (mode III) and tensile crack (mode I). It is found that the crack growth mechanism was strongly affected by bridge angle and bridge length. Crack initiated at higher stress level when the two flaws were arranged in co-planar type (bridge angle of 45°). For the bridge length near to 2c (flaw length), coalescence occurred in step type flaws rather than in the co-planar type flaws.