Study of the fracture behavior of mode I and mixed mode I/II cracks in tunnel under impact loads

Abstract

Blast excavation may induce radial cracks around a tunnel, and these cracks may initiate and propagate under nearby mining activities. In order to study the dynamic fracture behavior of mode I and mixed mode I/II cracks in a tunnel under impact loads, a new large sample, i.e. a tunnel with a radial crack (TWSRC) was proposed. Impact tests by using the TWSRC samples and a drop-weight impact testing device were also implemented. The advantage by using the TWSRC samples is that the corresponding results could be directly employed to tunnel engineering practice, and could lead tunnel designers to improve tunnel dynamic stability and to prevent tunnel hazards. The pre-crack was designed in a tunnel roof and was parallel to the symmetrical axis of the tunnel. The ratio α of the distance between the crack and the symmetrical axis to the radius of tunnel roof was in the range between 0 and 1. The crack initiation time and the crack propagation characteristic were measured by strain gauges and crack propagation gauges. A finite difference commercial code AUTODYN was used in the simulation of crack propagation paths and the fracture mechanism was analyzed. A finite element commercial code ABAQUS was employed to calculate crack dynamic stress intensity factors (DSIFs), and the experimental-numerical method was used to measure crack initiation toughness. The results showed that for mode I cracks, the cracks propagate along the pre- exiting crack direction, whereas the mixed mode I/II cracks, they propagate with a certain angle to form a wing crack at the early stage, and finally they propagate along the major principal stress direction. For mixed mode I/II cracks, the critical mode I SIF increases slightly with the distance to the symmetrical axis, whereas the critical mode II SIF increases largely as compared to the mode I SIF.