To study fracture evolution and peak stress in burst risk coal samples (BRCSs) under true triaxial loading and unloading conditions, experimental and numerical research was applied to BRCSs under true triaxial stress paths entailing “x-direction displacement fixed, y-direction loading, z-direction unloading.” Both the experimental and the numerical results demonstrated that the peak stress borne by the BRCSs was not only affected by the initial stress but also had a negative exponential relationship with the ratio of the unloading rate and the loading rate (RURLR); therefore, peak stress equations of BRCSs under true triaxial loading and unloading conditions were established. The triaxial stress-time curves obtained by experiments and simulations exhibited an “elasticity-yield-destruction” phase, and the characteristics of the yield phase were determined by the RURLR. A typical BRCS was selected for velocity tomographic imaging to analyze the fracture evolution characteristics under true triaxial loading and unloading. The results showed that when the BRCS was subjected to a triaxial state of stress, the high- and low-velocity regions existed alternately due to the presence of the crack; during the elastic phase, the crack closed during loading in the previous phase was reopened upon unloading, so that the velocity of the sample decreased and a wide range of low-velocity regions could be formed; when entering the yield phase, the original crack continued to expand into a hole-through crack, leading to wider extreme values and ranges of these low- and high-velocity regions; at the breaking phase, multiple microcracks were generated around the hole-through cracks, decreasing the overall velocity, and showing point distributions characteristics of high- and low-velocity regions. Overall, many low-velocity regions with similar normal directions to the unloading direction were formed; these correlated well with macrofractures (postfailure).

中文翻译：

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真三轴加卸载应力路径对岩爆风险煤样力学性能和波速的影响

为了研究真三轴加载和卸载条件下爆裂风险煤样 (BRCS) 的裂缝演化和峰值应力，在真三轴应力路径下对 BRCS 进行了实验和数值研究，该路径需要“ x方向位移固定，y方向加载，z——方向卸载。” 实验和数值结果均表明，BRCSs承受的峰值应力不仅受初始应力的影响，而且与卸载率与加载率的比值（RURLR）呈负指数关系；因此，建立了真实三轴加载和卸载条件下BRCSs的峰值应力方程。通过实验和模拟得到的三轴应力-时间曲线呈现出“弹性-屈服-破坏”阶段，屈服阶段的特征由RURLR确定。选择典型的BRCS进行速度断层成像，分析真三轴加载和卸载下的裂缝演化特征。结果表明，当 BRCS 处于三轴应力状态时，由于裂纹的存在，高速区和低速区交替存在；在弹性阶段，前一阶段加载时闭合的裂纹在卸载时重新打开，使试样速度降低，形成大范围的低速区；进入屈服阶段后，原裂纹继续扩展为贯通裂纹，导致这些低速区和高速区的极值和极值范围变宽；在断裂阶段，在通孔裂纹周围产生了多个微裂纹，降低了整体速度，并呈现出高低速区域的点分布特征。总体而言，形成了许多与卸载方向法线方向相似的低速区域；这些与宏观裂缝（故障后）相关性很好。