In the current study, a series of laboratory triaxial loading and unloading experiments were conducted for the purpose of comprehensively understanding the effects on sandstone of loading processes under axial stress conditions and unloading processes under confining pressure conditions. The mechanical properties and crack propagation behaviors of sandstone specimens were analyzed. In addition, a new damage variable based on crack evolution was proposed. The experimental results showed that the strain-softening modulus of the sandstone specimens under triaxial loading and unloading compression conditions decreased with the increases in the loading rate. Meanwhile, the elastic modulus was only observed to undergo small changes. This study found that with the increases in the loading rates of axial stress, the deformation and strength levels of the sandstone specimens also increased. In addition, the crack initiation stress showed a linear increasing trend with the increases in the loading rates of axial stress. The peak crack volumetric strain displayed a slight increase with the loading rate of axial stress, which indicated that the larger the loading rate of axial stress on the sandstone specimens was, the smaller crack propagation strain levels at the peak stress levels would be. The crack propagation velocities were observed to be larger when the loading rates of axial stress were higher. Also, at the same loading rate of axial stress, the crack propagation velocity corresponding to the peak stress of the volumetric cracks was the largest, followed by radial cracks. The crack propagation velocity of the axial cracks was determined to be the smallest. In this study, a new crack damage variable based on crack volumetric strain was proposed. According to the evolution of the crack damage variables of the sandstone specimens, the following four stages of the failure process were obtained: original damage stage; elastic stage; linear damage stage; and accelerated damage stage. The results obtained in this research study provided an important scientific basis for the support of future designs and safety protection measures in underground mining operations.

本研究开展了一系列室内三轴加卸载实验，旨在全面了解轴向应力条件下的加载过程和围压条件下的卸载过程对砂岩的影响。分析了砂岩试件的力学性能和裂纹扩展行为。此外，提出了一种基于裂纹演化的新损伤变量。试验结果表明，砂岩试件在三轴加载和卸载压缩条件下的应变软化模量随着加载速率的增加而降低。同时，仅观察到弹性模量发生很小的变化。本研究发现，随着轴向应力加载速率的增加，砂岩试样的变形和强度水平也增加了。此外，随着轴向应力加载速率的增加，裂纹萌生应力呈线性增加趋势。峰值裂纹体积应变随着轴向应力加载速率的增加而略有增加，这表明砂岩试样的轴向应力加载速率越大，峰值应力水平下的裂纹扩展应变水平越小。当轴向应力加载速率较高时，观察到裂纹扩展速度较大。此外，在轴向应力加载速率相同的情况下，体积裂纹峰值应力对应的裂纹扩展速度最大，其次是径向裂纹。轴向裂纹的裂纹扩展速度被确定为最小。在这项研究中，提出了一种基于裂纹体积应变的新裂纹损伤变量。根据砂岩试件裂纹损伤变量的演变，得到破坏过程的四个阶段：原始损伤阶段；弹性阶段；线性损伤阶段；和加速损坏阶段。本研究获得的结果为支持未来地下采矿作业的设计和安全保护措施提供了重要的科学依据。原始损坏阶段；弹性阶段；线性损伤阶段；和加速损坏阶段。本研究获得的结果为支持未来地下采矿作业的设计和安全保护措施提供了重要的科学依据。原始损坏阶段；弹性阶段；线性损伤阶段；和加速损坏阶段。本研究获得的结果为支持未来地下采矿作业的设计和安全保护措施提供了重要的科学依据。