压剪应力条件下砂岩开裂扩展过程的试验研究

TEST STUDY OF SANDSTONE CRACKING AND PROPAGATION PROCESS UNDER COMPRESSIVE-SHEAR STRESS

利用自主研发的含瓦斯煤岩细观剪切试验装置及PCI–2型声发射(AE)测试分析系统,开展法向应力分别为0.0,1.5,3.0,4.5,6.0 MPa条件下砂岩的压剪试验,对砂岩在压剪应力条件下的变形特性、声发射特性及其细观开裂扩展特征等进行系统研究。研究结果表明:随着法向应力的增加,砂岩抗剪强度增大,峰值剪切力及其相应的峰值位移均近似呈线性增大;不同法向应力条件下岩石的最终断裂破坏形态虽有所不同,但其开裂扩展趋势基本都是沿预定剪切面不断向前延伸扩展;法向应力越大,开裂扩展的时间越晚,岩石越不容易发生失稳破坏;受法向应力的影响,AE事件率并未随着剪应力达到峰值而达到最大值,而是在峰值剪应力后急剧增大,微裂纹急剧扩展演化,最终AE事件率达到最大值,裂纹贯通导致岩石失稳破坏;随着法向应力的增加,剪切面两侧颗粒之间的摩擦力增大,同时受应力分布不均及岩石内部结构的影响,主裂纹的宽度越大,开裂扩展形态更加复杂。

Using self-developed meso-shear test equipment for coal and rock containing gas and PCI-2 acoustic emission testing and analytical system,the compressive-shear tests of sandstone at normal stresses with 0.0,1.5, 3.0,4.5,6.0 MPa under the shear load condition were carried out. The deformation and AE characteristics were studied,and the mesoscopic feature of the cracks′evolution was analyzed. The data shows that as normal stress increases,the shear strength increases,and the peak shear force and corresponding peak displacement increase linearly. Under the condition of different normal stresses,the rock has some distinctions on shapes or appearances. However,they basically extend along the expected shearing surface. As normal stress increases,the crack initiation and propagation will get later,the instability and failure of the rock become more difficult. By the impacts of normal stresses,the AE event rate doesn′t reach the maximum value along with the shearing stress increases to the peak value. The fact is that the AE event rate increases rapidly after the shear stress reaches the peak value,also with the tiny cracks spread suddenly. With these two conditions,the AE event rate finally reaches the maximum value,then the rock unstable failure had been destroyed after those cracks went through it. As normal stress increases,the friction of particles between two sides of shear surface also increases. Meanwhile,the major crack is wider and the shapes of cracks become more complicated due to effects of the unbalanced stress distributions and also the internal structures heterogeneity of materials.