The effects of confining compression on fracture coalesence in rock-like material

This paper primarily studies the effects of confining pressure on the fracture coalescence using modelling rock-like material containing three flaws (cracks). The pre-existing flaw angle α, bridge angle β (angle between two flaws), frictional coefficient μ along flaw surface and the confining pressure were varied. The flaw length and bridge length were fixed. In addition, the failure behavior has been simulated numerically by using the Rock Failure Process Analysis (RFPA2D) code to clarify our experimental studies. Based on our experimental observation and numerical simulations, three effects were observed under biaxial compression loading. (1) The growth of tensile crack decreases with the increase of confining pressure. (2) With the increase of confining pressure, mixed mode coalescence (tensile + shear) is dominant, that is shear and tensile crack appear simultaneously within the bridge area. (3) Second coalescence is observed. This second coalescence occurs after the peak strength and after the appearance of the first coalescence. The second coalescence is primarily dominated by the mixed mode coalescence. This phenomenon was not observed under the uniaxial compression test of three-flaw specimens. For effect of confining pressure on peak strength, it is found that peak strength increases with the applied confining pressure. Furthermore, the prediction by the crack model of Ashby and Hallam (1986) agrees with our experimental results only for the case of low confining pressure. For the high confining pressure, Ashby and Hallam's (1986) crack mode should be modified because shear crack was also induced between the bridge area (not just tensile cracks as assumed by Ashby and Hallam, 1986). The numerical simulation using RFPA2D basically coincides with the experimental results for both case of low and high confining pressure.