真三轴压缩下中、小主应力卸载应力路径与速率对砂岩能量演化特征的影响

Effect of intermediate principal stress and minimum principal stress unloading stress path and rate on energy evolution of sandstone under true triaxial compression

  • 摘要: 煤矿深部开采过程中,受开挖卸荷的影响,深部岩石经历的卸载应力路径和卸载速率复杂多变。不同的应力路径和速率可能导致岩石不同的变形和破坏特征。为了理解深部岩石在真三轴卸载过程中,复杂应力环境下应力变化对岩石能量演化和损伤的影响,采用设计的真三轴应力平衡卸载试验分析真三轴卸载主应力试验的能量和损伤。首先,开展不同卸载应力路径与速率下的砂岩真三轴卸载主应力试验(σx-σy-σz试验和σy-σx-σz试验);然后,设计一种无损伤卸载的真三轴应力平衡卸载试验,并采用声发射技术验证卸载过程的无损伤特征,在该试验的基础上,提出真三轴卸载主应力试验的能量分析方法;最后,通过分析卸载完成后试样外表面裂纹开裂程度与能量耗散联系,阐明卸载应力路径与速率对岩石损伤影响。结果表明:在某一主应力方向卸载过程中,其他主应力方向的输入应变能密度、弹性应变能密度和耗散应变能密度均会增大,且随着卸载速率的增大,非卸载方向的输入应变能密度、弹性应变能密度和耗散应变能密度也相应增大。此外,卸载主应力方向释放的岩石储能随着卸载速率的增大而增大,并且沿卸载方向释放的储能明显低于该方向的储能。通过比较σx-σy-σz试验和σy-σx-σz试验的结果,发现耗散应变能密度与砂岩损伤程度呈正相关性,即卸载主应力产生的耗散应变能密度越大,岩石损伤程度越高,且σy-σx-σz试验的卸载应力路径更容易引发岩体损伤。

     

    Abstract: In the process of deep coal mining, the unloading stress path and unloading rate of deep rock are complicated and changeable due to the influence of excavation and unloading. Different stress paths and rates may lead to different deformation and failure characteristics of rocks. To understand the influence of stress changes, energy evolution, and damage in deep rock under true triaxial unloading in complex stress environments, a designed true triaxial stress balance unloading test was employed to analyze the energy and damage in true triaxial unloading principal stress tests. First, true triaxial unloading principal stress tests on sandstone were conducted under different unloading stress paths and rates (σx-σy-σz test and σy-σx-σz test). Then, a damage-free true triaxial stress balance unloading test was designed, and acoustic emission technology was used to verify the damage-free characteristics during the unloading process. Based on this test, an energy analysis method for true triaxial unloading principal stress tests was proposed. Finally, by analyzing the degree of crack opening on the outer surface of the specimen after unloading and its connection with energy dissipation, the impact of unloading stress paths and rates on rock damage was elucidated. The research results indicate that during the unloading process in one principal stress direction, the input strain energy density, elastic strain energy density, and dissipated strain energy density in the other principal stress directions increase, and these densities also increase with the unloading rate. Additionally, the released rock strain energy in the unloading principal stress direction increases with the unloading rate, and the released strain energy along the unloading direction is significantly lower than the stored energy in that direction. By comparing the test results of the σx-σy-σz test and the σy-σx-σz test, it was found that the dissipated strain energy density is positively correlated with the degree of sandstone damage, meaning that the greater the dissipated strain energy density generated by the unloading principal stress, the higher the degree of rock damage, and the unloading stress path of the σy-σx-σz test is more likely to induce rock mass damage.

     

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