高静荷载下卸载速率对岩石动力学特性及破坏模式的影响

Effects of the unloading rate on dynamic characteristic and failure modes of rock under high static loads

为研究深部岩石的动力学特征及破坏模式受卸载速率影响而变化的规律,基于深部岩石开挖工程面临的"高静载卸荷过程中受冲击扰动"的力学环境,利用改进的动静组合加载SHPB试验系统开展高轴压卸荷频繁冲击扰动试验。试验一个完整循环过程为:先参考深部蛇纹岩的单轴抗压强度施加轴向荷载,再以一定速率卸载至预加轴压值的50%时施加0.4 MPa的冲击荷载,最后停止卸荷。试验研究结果表明:岩石的动态应力–应变曲线及其包络线都经历短暂的直线段进入非线性发展阶段,说明卸载冲击扰动过程中岩样主要发生塑性变形;卸载速率的增加导致岩样对冲击荷载的延缓让力效应明显,造成岩石均值强度及承受的累计扰动冲击次数增加,且引起岩石发生脆性、延性互相转化的现象;预加载轴压、卸荷速率的增加,导致破坏岩块块度呈增大的趋势发展,且前者促使岩样以拉伸破坏为主,后者促使岩样以剪切破坏为主。

In order to study effects of the unloading rate on the dynamic characteristics and failure modes of deep rock,the improved SHPB test system combined with dynamic and static loadings,which can simulate the mechanical environment of deep rock excavations subjected to impact disturbance during high static unloading process,was used to carry out the frequent disturbance test under high axial pressure and unloading. The samples were applied by a complete cycle loading. Specifically,a sample was firstly applied with a given axial load referring to the uniaxial compressive strength of deep serpentine,then unloaded till to 50% of the axial load at a certain rate and at the moment,an impact load of 0.4 MPa was exerted,and finally the unloading process terminated. The test results show that the dynamic stress-strain curve of rock and its envelope have a stage of nonlinear development following a short straight line stage,which indicates that plastic deformation of rock specimen plays a dominate role during impact disturbance and unloading. The increase of the unloading rate leads to obvious delaying effect of rock specimen on the impact load,resulting in an increase of rock′s mean strength and the cumulative number of impact disturbance,and causing the interconvertible phenomenon of brittle and ductility. With increasing the pre-axial stress and the unloading rate,the fragmentation of damaged rock mass increases,and they lead to tensile and shear failures,respectively.