岩石静态与动态断裂韧性的宏细观试验

Macro-micro experimental study of rock static and dynamic fracture toughness

基于直切槽半圆盘弯曲(notched semi-circle bend,NSCB)试件及断裂韧性测试方法,分别对北京房山花岗岩样品实施了静态和动态断裂韧性试验,获得了其静态与动态断裂韧度的定量关系,发现在文中采用的中高应变率条件下,动态断裂韧度值为静态断裂韧度值的1.3~2.6倍。采用不同表面形貌刻画技术(SEM、激光共聚焦显微镜、高速摄像机等)对破坏岩样的表面形貌进行观测与表征,获得了岩石表面的三维重构及其粗糙度,其静态和动态断裂都是呈I型裂纹拉伸破坏模式,动态与静态裂纹扩展差异的原因在于应力波在岩样内部界面处的来回反射,诱导微裂纹的萌生、汇合与贯通;裂纹传播过程均经历了加速阶段和减速阶段,裂纹传播速度与其表面三维形貌重构的相对高度变化趋势一致,同样与表面粗糙度变化规律相吻合。岩石静态与动态行为的本质区别是:材料在动态加载时所表现出来的率效应(惯性效应),与材料自身由物理、几何引起的结构效应,此两类效应存在相互抵消、此消彼长的本质属性。

The notched semi-circle bend(NSCB) specimen and fracture toughness test method was used to measure the static and dynamic fracture toughness of Beijing Fangshan granite samples. The results showed the dynamic fracture toughness is generally 1.3~2.6 times the static fracture toughness for medium and high strain rates. The surface morphologies of the damaged rock samples were then characterized using SEM and a laser confocal microscope for three-dimensional reconstructions of the fractures and surface roughnesses. The static and dynamic fracture modes were both mode I crack tensile failures. The difference between the dynamic and static crack growth was that the stress wave in the dynamic tests reflected back and forth at the internal interface of rock sample which induced micro-cracks and accelerated the convergence and penetration of existing and new cracks. The crack propagation had acceleration and deceleration stages with the changes in the rock crack propagation speed consistent with the relative height of the reconstructed three-dimensional rock surface morphology, which also corresponded to the rock surface roughness variations. The key difference between the static and dynamic behavior is that the rate effect(inertia effect) during dynamic loading tends to counteract the structural effects of the physical and geometric properties of the rock material itself.