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