三轴加卸载下花岗岩脆性破坏及应力跌落规律

基于2种卸荷应力路径和常规三轴压缩试验,研究了加卸载条件下花岗岩的变形破坏及应力脆性跌落特征。卸荷条件下岩石变形主要是向卸荷（主）方向回弹或拉伸变形为主,而非或次卸荷方向的塑性变形很小,峰后应力应变曲线呈现明显的脆性特征。而加载条件下岩石以轴向压缩变形为主,且压缩塑性变形随围压增大而增大;卸荷条件下破坏岩石各种级别的张拉裂缝较多,张裂面一般垂直于卸荷主方向,高初始围压时双向卸荷甚至在次卸荷方向也可产生环形张拉裂缝。破坏围压较高时破裂面剪性特征相对明显,但剪性裂面一般追踪张性破裂面发展而成,并在剪切面两侧发育较多微张裂缝。而相对较高围压下常规三轴压缩岩石一般为剪切破坏,张性裂缝很少;常规三轴压缩岩石的应力脆性跌落系数随围压的增大而增大,而在卸荷条件下却随初始围压的增大而减小。相同初始围压时,卸荷条件下比加载时的应力脆性跌落系数小得多,方案Ⅱ在初始围压达到30 MPa时甚至出现负值,应力脆性跌落系数R依次为：RⅢ〉RⅠ〉RⅡ。

Characteristics of Brittle Failure and Stress Drop Under Triaxial Loading and Unloading

The characteristics of deformation failure and stress brittle drop of granite under loading and unloading conditions are studied respectively based on two unloading stress path cases and conventional triaxial compression experiments.The experiments show that the primary deformation of rock is rebound or tension in the（main） unloading direction,but plastic deformation in the non-or sub-unloading direction is not significant.And the curves of stress-strain after peak strength present obvious brittle.Whilst the primary deformation of rock under loading is axial pressure deformation,and plastic deformation increases with the raise of confining pressure.The fracture is characterized by strong tensile,and all tensile cracks are fully developed.Furthermore,when the rock samples are unloaded in two directions,and even normal to unloading direction,sub-unloading annular tensile cracks are produced.The shear fractures is obvious comparatively when confining pressure becomes higher,and it develops following tensile cracks with more micro-tension cracks formed in two sides along shear plane.But the fracture is characterized by shear under conventional triaxial compression with little tensile cracks;the coefficients of brittles stress drop of granite is increased with the growth of confining pressure under conventional triaxial compression and reduced under unloading.When the initial confining pressure is the same,the coefficients of brittles stress drop under unloading is less than that under loading.In SchemeⅡ,the coefficients of brittles stress drop reaches negative when initial confining pressure is up to 30 MPa,and the one（R）is RⅢRⅠRⅡ in three test cases.