Experiments of vertical fracture propagation based on the digital speckle technology

The vertical crack propagation behavior for sandstone/mudstone layered specimen made by the similar material was analyzed. The stress and strain field was obtained using Digital Speckle Correlation Method to analyze the propagation characteristics of vertical cracks. The test shows that the higher the strength of the interface layer, the more susceptible the crack to deflect from the side of the low-strength than from the side of the high-strength of the material. It is easy for the interface to unglue when the interfacial adhesion strength is weak, which results in the dislocation propagation after the crack passing the interface whether crack extending from the side of high-strength material or the side of low-strength material. The longer the expansion path is before the crack passing the interface, the more likely it deflects after passing the interface. Similarly, the shorter the initial prefabricated crack is, the more susceptible to deflect after the crack passing the interface. The main factor of the deflection is the difference in the properties of the composite materials, resulting in a larger shear strain of the interfacial layer. The shear strain can induce type II shear fracture, so it is the main factor leading to crack deflection. The bigger the shear deformation is, the more intensive the crack deflection is.