岩石裂纹扩展微观机制声发射定量反演

Microcracking mechanisms of sandstone from acoustic emission source inversion

岩石受载内部微裂纹扩展及其震源机制反演有助于认识宏观裂纹扩展过程的非线性断裂力学行为.借助声发射监测手段,本文建立了仅涉及微裂纹张开/闭合和剪切滑移的位移不连续震源模型,通过各位置处传感器耦合质量标定及点源远场P波矩张量反演获得了含预制裂纹砂岩受载过程的震源机制解及时变响应特征,在全局坐标系下分析了微裂纹的三种断裂力学行为.结果表明:在位移不连续模型中,震源矩张量特征值与试样泊松比之间必须满足特定约束条件,该约束条件下的优化问题可采用拉格朗日乘子和Levenberg-Marquardt迭代法求解;受载砂岩裂纹扩展过程中,声发射震源以剪切滑移机制占优,微裂纹空间取向及运动方向与试样宏观主裂纹夹角平均值分别为40.9°和17.7°;对微裂纹体积分解表明岩体微观破裂机制以沿X方向Ⅰ型张开为主,而沿Y方向的Ⅱ型断裂滑移方向与试样全局变形方向相一致,由于试样内部晶粒分布非均质性造成了少量沿Z方向的Ⅲ型平面外微裂纹滑移行为;受载砂岩裂纹扩展过程中微裂纹模式角与震源极性值变化趋势一致、利用震源震级评估的局部应力降值与实验观测结果相吻合,这两者均表明了位移不连续模型在震源机制定量反演中的适用性.

Inversion of microcracking activities and corresponding micromechanisms in the fracture process of rock containing pre-existing macrocracks contributes to further understanding of the nonlinear fracture mechanical behavior through acoustic emission(AE)monitoring.The microcracking source was modeled as a displacement discontinuity(DD)involving only the microcrack opening/closing and shear-sliding mechanisms,which can describe the fracture modes anticipated inside a loaded specimen.By means of the absolute calibration of AE sensors and the moment tensor of P-wave amplitude,the focal mechanism solutions of the DD source model and their time-varying responses were obtained.Then,the three-type fracture behaviors of the microcracks were analyzed by decomposing the microcrack volume into the global coordinates.Results show that the crack only DD source model involves the moment tensor M,where the intermediate eigenvalue M2 must follow a specific relation with M1 and M3,which provides a necessary constraint.Such a constrained optimization problem can be solved by combining the Lagrange multiplier and Levenberg-Marquardt iterative approach.During the macrocrack propagation of loaded sandstone,the microcracking events with shear-sliding mechanism dominate.The average included angles between microcrack and motion directions with macrocrack orientations are 40.9°and 17.7°,respectively.The microcrack volume decomposition under global coordinate indicates a dominate mode I opening mechanism in X-direction,while the direction of mode II in-plane sliding is consistent with the global deformation direction of the loaded specimen.Due to the heterogeneity of grain distribution,a small amount out-of-plane sliding behaviors along the Z-axis are also observed.The variation of crack mode angles from the DD model and source polarity values are consistent,and the local stress drop evaluated by source magnitude coincides well with the experimental data,which indicate the feasibility of the DD model in quantitative inversion of microcracking source mechanism.