A criterion for crack initiation and propagation of intact rocks under creep condition

Abstract

The mechanism and criterion of crack initiation and propagation of rocks were investigated by many researchers. And the creep behaviour of rocks was also theoretically and experimentally studied by some scientists and engineers.The characteristics of crack initiation and propagation of rocks under creep condition, however, are very important for rock engineering and still not paid enough attention by researchers. In this paper, the criterion and mechanism ofcrack initiation and propagation under creep condition were investigated using specimens collected from sandstone rock formations outcropping in the Emei Mountain, the Sichuan Province of China. Cuboid specimens under three point bendingwere used in this investigation. All specimens were classified into four sorts and used for Mode-I fracture or creep fracture tests. The experimental result shows that due to creep deformation,rock crack will inevitably initiate and propagate under a load of K I, which is less than fracture toughness K ICbut not less than a constant (marked as K IC2). K IC2indicates the ability of rock to resist crack initiation and propagation under creep conditions and is less than fracture toughness K IC, defined as creep fracture toughness in this paper.K IC2 should be considered as an important parameter on design and computation of rock engineering. The microstructural mechanism for crack initiation and propagation of rock materials undercreep condition was introduced based on competitive model between softening effect and hardening effect, and the validity of test result was explained. The test result was also verified in rheological theory. When K Iis more than K IC2 but less than K IC, rock crack will initiate and propagate after a time interval of sustained loading under creep condition.In order to find the relation between duration of sustained loading, which can lead to crack initiation and propagation, and the initial stress intensity factor K I, an unequal-interval time sequence forecasting andpredicting model was introduced, and the relation was obtained for homogeneous and isotropic fine-grained red sandstone. Finally a modified fracture toughness formula was given, in which the influence of fracture process zone(FPZ) was fully considered.