Numerical investigation of dynamic fracture in rock specimens containing a pre-existing surface flaw with different dip angles

Abstract

Numerically simulated specimens with a three-dimensional pre-existing surface flaw are subjected to stress waves until failure via finite element analysis. Different types of cracks are reproduced, including wing cracks, anti-wing cracks, shell-like cracks and spalling fractures. The effects of different flaw dip angles on the initiation and propagation of wing cracks, anti-wing cracks and shell-like cracks are analysed under dynamic loading conditions. The numerical simulation results indicate that the dip angle of the flaw plays an important role in the crack behaviour and failure patterns of flawed specimens under dynamic loading. Wing cracks and anti-wing cracks emerge and present similar crack propagation patterns on the surface of the specimen when the pre-existing flaw dip angle is less than 60°. In contrast, only wing cracks appear in specimens with flaw dip angles of 75° and 90°. Shell-like cracks are reproduced inside of the specimens in all cases except the specimen with a flaw dip angle of 90°. In addition, the further propagation of shell-like cracks proceeds after the wing and anti-wing cracks stop propagating. Meanwhile, the numerical simulation also identifies the acoustic emission (AE) counts and energy of the specimen, which are closely related to the flaw dip angle during the dynamic failure process. The flaw dip angle has a slight effect on the dynamic strength of the specimens. Furthermore, the cracking patterns of specimens with a single pre-existing flaw under dynamic and static loading conditions are compared based on numerical investigations. Dynamic loading can produce more cracks and lead to more complex cracking behaviour. Moreover, loading rates can significantly influence crack propagation patterns and AE characteristics of rock specimens. Under high loading rates, the specimens fragment more efficiently. In addition, for specimens subjected to high loading rates, the more shear fractures occur, the greater the cumulative AE energy and the fewer the AE counts. Finally, heterogeneity also plays an important role in the distribution of micro-cracks in the material and the crack propagation patterns. Crack branching occurs in relatively heterogeneous rocks, which affects the further propagation of cracks initiated early. The stress concentration zone is not the only decisive factor and may be replaced by the degree of heterogeneity to determine the crack initiation and propagation. The results of this study can provide a valuable reference for studies on the failure process of heterogeneous brittle materials under dynamic loading.