Dynamic crack propagation of granite subjected to biaxial confining pressure and blast loading

Abstract To great understanding the dynamic crack propagation of rock under in-situ stress and blasting, the static-dynamic loading experimental platform was built. With different confining pressure (0MPa, 2MPa, 4MPa, 6MPa, 8MPa) and different ratios (K) of horizontal-to-vertical pressure (0, 0.25, 0.5, 0.75, 1), nine texts have been carried out in quadrate granite samples. The dynamic strain gauges and high-speed (HS) camera were used to measure strain field and observe crack growth. According to experimental results, the centrosymmetric damage zone is generated around borehole when K=1, the circumference compressive stress is formed by confining pressure, and it reduces the circumference tensile failure by blast loading. The number and size of the broken radius significantly reduce with confining pressure increases. As K decrease from 1 to 0, the tensile stress concentration is obviously around borehole in the vertical direction, and radial cracks grow from the stress concentration zone and develop into boundary under tensile stress wave from blast loading. From the numerical analysis based on Johnson-Holmquist in AUTODYN, crack branching appear near free boundary obviously, meanwhile the radial cracks grow and run through sample by reflected wave at last. Compared to the experimental results, the evolution and final shape of cracks are agree with the experimental results, the central damage area is less than latter because JWL equation neglect the gas loading in numerical calculation.摘要 为深入理解原位应力（in-situ stress）与爆破作用下岩石的动态裂纹扩展（dynamic crack propagation）规律，本研究搭建了静动载加载实验平台（static-dynamic loading experimental platform）。针对方形花岗岩试样，设置了5组围压（confining pressure）：0MPa、2MPa、4MPa、6MPa、8MPa，以及5组水平-垂直压力比（K）：0、0.25、0.5、0.75、1，共开展9组实验。采用动态应变片（dynamic strain gauges）与高速摄像（high-speed, HS）系统采集应变场并观测裂纹扩展过程。实验结果表明：当K=1时，钻孔周围会形成中心对称的损伤区（centrosymmetric damage zone）；围压可产生环向压应力，抑制爆破荷载引发的环向拉伸破坏。破碎区的半径范围与破碎规模随围压升高显著减小。当K从1降至0时，钻孔周围垂直方向会出现明显的拉应力集中，径向裂纹从应力集中区萌生，并在爆破荷载产生的拉伸应力波作用下向试样边界扩展。基于AUTODYN平台、采用Johnson-Holmquist本构模型开展的数值分析结果显示：自由边界附近会出现明显的裂纹分叉现象，径向裂纹最终在反射应力波作用下扩展并贯穿试样。与实验结果对比可知，裂纹的演化过程与最终形态与实验结果吻合良好，但由于数值计算中采用的JWL方程（JWL equation）未考虑气体加载效应，数值模拟得到的中心损伤区面积小于实验结果。