Ali Tarokh

Scaling of the fracture process zone in rock

The zone of microcracks surrounding a notch tip—the process zone—is a phenomenon observed in fracture of quasi-brittle materials, and the characterization of the process zone is the topic of the paper. Specimens of different sizes with a center notch fabricated from a granite of large grain (Rockville granite, average grain size of 10 mm), were tested in three-point bending. Acoustic emissions were recorded and locations of microcracks were determined up to peak load. The results show that both the length and width of the process zone increase with the increase of the specimen size. Furthermore, the suitability of a proposed theoretical relationship between the length and width of the process zone and specimen size was studied experimentally and numerically. The discrete element method with a tension softening contact bond model was used to investigate the development of the process zone with the specimen size. A synthetic rock composed of rigid circular particles that interact through normal and shear springs was tested in the numerical simulations. It was shown that the limiting specimen size, beyond which no further noticeable increase in the length of the process zone is observed, is significantly larger than the limiting specimen size beyond which the width of the process zone shows no size effect.

Dimensions and Brittleness Effect on the Size of Process Zone in Rock-like Material Characterized by Bonded Particle Model

Process zone is recognized as one of the dominant variable in the fracture process of rock, which is a region of damage around the crack tip mainly effected by character of rock such as size and brittleness. This zone has a key influence on the strength of rock and deformation pattern of a structure. Therefore, investigate size of the process zone based on different rock properties is a very feasible method in study of rock fracture. In this paper, a new bonded particle model (BPM) was used for simulation of the rock material. Macroscopic and microcosmic parameters were calibrated by uniaxial compressive, tensile and three point bending test. To examine the item of structure composed of rock with different size and brittleness, few groups of four point bending simulation tests are presented, with a detailed measurement of process zone. As the significant factors influencing the size effect, the length and width of the process zone were determined by a numeric statistics method. Base on analysis of size of process zone, it is shown that when the brittleness is low, seems there is no size effect exits, but as the material turns brittleness to ductile, the length and width of the process zone is become more dependent on the specimen size. The analysis of the numerical results indicated that the discrete element method with a bonded particle model is able to mimic the problem of rock fracture.