Influence of circular hole defect on dynamic crack propagation behavior under impact loads

Cavities and crack defects usually coexist in deep earth rock mass structures, which together affect the structural safety and stability of rock masses. In order to study the effect of circular cavity on crack propagation behavior in rock mass under dynamic loads, circular opening specimens with straight crack cavity (COSSCC) specimen were proposed in this study, and a large-scale drop hammer impact device was applied to conduct impact tests. Crack propagation gauges were implemented to measure fracture mechanics parameters, such as dynamic crack initiation time and crack propagation velocity. Then a modified finite difference code Autodyn was applied to carry out the numerical simulation analysis of crack propagation path and stress field around the circular hole. The traditional finite element code Abaqus was also used to calculate the dynamic initiation toughness and dynamic propagation toughness. The results indicate that: (1) when the inclination θ of the circle hole is less than 10°, the crack propagation path deflects and passes through the surface of the circle hole; when the inclination θ of circle hole is 20° and 30°, the crack propagation paths deflects in the direction of the hole but does not pass through the circular hole, indicating that the circular hole has obvious guiding effect on crack propagation; when the inclination θ of circle hole is 40° and 50°, crack propagation path does not deflect, and the guiding effect of the circular hole is obvious weaken. (2) When the crack propagation path reaches the vicinity of the circular hole, the tensile stress zone at the crack tip coincides with the tensile stress zone at the edge of the circular hole. At this time, the crack propagation speed increases significantly, and the dynamic fracture toughness of the crack decreases significantly. (3) The deflection direction of the crack is basically the same as the direction of the maximum circumferential stress at the crack tip. (4) The dynamic fracture toughness of the crack is always smaller than the crack initiation toughness, and the dynamic fracture toughness of the crack has a linear relationship with the dynamic crack growth rate. The larger the dynamic crack growth rate, the smaller the dynamic fracture toughness of the crack.