压剪荷载作用下红砂岩裂纹尖端应变分布特征及起裂机理研究

Study on strain distribution characteristics and fracture initiation mechanism at crack tip of red sandstone under compressive shear load

岩石在纯剪切荷载作用下,断裂类型都为Ⅰ型或者复合型断裂,为了研究Ⅱ型断裂,有必要探索压剪荷载共同作用下的断裂机理。本文首先利用剪切盒装置对双边切口的红砂岩试样进行加载试验,然后通过数字图像相关(DIC)技术对试样切口尖端及岩桥上的应变状态进行分析,最后对裂纹起裂机理做出判断。试验结果显示,剪切角为30°~60°时,试样均从切口尖端起裂,并且起裂裂纹沿岩桥所在平面有不同程度偏转;剪切角为70°时,试样从岩桥中部起裂,起裂裂纹沿着岩桥所在平面扩展。分析试样断裂特征及DIC试验数据后得到的主要结论如下:随着剪切角的增大,起裂平面张拉位移最大值和滑移位移最大值之比逐渐增大,起裂机理逐渐由剪切断裂转变为张拉断裂;当剪切角为30°、40°时,试样岩桥在初始裂缝平面上的应力状态始终处于压剪状态,起裂机理为剪切主导的拉剪复合型断裂;当剪切角为50°、60°时,在加载前期,试样岩桥在初始裂缝平面上的应力状态处于压剪状态,但随着荷载进一步增大,岩桥应力状态逐渐转变为拉剪状态,起裂机理为拉剪复合型断裂;当剪切角为70°时,试样岩桥上的应力状态在加载过程中始终存在拉应力,并且随着荷载增大,切口尖端也出现拉应力,不能达到压剪状态,裂纹起裂机理为张拉断裂。

In the pure shear state of rock,the fractures are all mode I or mixed mode.In order to study the mode II fracture,it is necessary to explore the fracture mechanism under the combined action of compressive shear load.In this paper,the shear box device was used to load the doubled-edge notched red sandstone samples.The strain state on the sample's notch tip and rock ligament was analyzed using the digital image correlation(DIC)technique,and the crack initiation mechanism was determined.The test results revealed that when the shear angle was 30°to 60°,all samples cracked from the notch tip,with varying degrees of fracture deflection along the rock bridge plane.When the shear angle was 70°,the sample cracked from the middle of the rock bridge,and the fracture propagated along the rock bridge plane.By analyzing the fracture characteristics of samples and DIC test data,the main conclusions were drawn:with the increase of shear angle,the ratio of maximum tensile displacement and maximum sliding displacement gradually increased,and the fracture mechanism changed from shear fracture to tensile fracture.when the shear angle was 30°and 40°,the stress state of the sample rock bridge at the initial fracture plane remained in the compressive shear state,with the fracture mechanism dominated by shear-induced tensile shear mode.When the shear angle was 50°and 60°,the stress state of the rock bridge initially remained in the compressive shear state,but with further load increase,it gradually transitioned to a tensile shear state,resulting in a mixed fracture mechanism of tensile shear.When the shear angle was 70°,tensile stress always existed in the loading process of the specimen rock bridge.Moreover,with increased load,tensile stress also appeared along the initial crack plane of the notch tip,which cannot reach the compression shear state,and the crack initiation mechanism is a tensile fracture.