A numerical parameter-sensitivity analysis has been conducted to evaluate the effect of heterogeneity on the fracture processes and strength characterization of rock under uniaxial compression loadings. The deformation mechanisms of rock under different constant confining pressures was briefly analyzed based on Continuum damage mechanics and the effect of confining pressure on deformation, strength and macroscopic fracture patterns of model rock specimens are also studied using Rock Failure Process Analysis (RFPA) code, that show the nucleation and growth of macrocracks in relatively heterogeneous specimens under uniaxial loading. In this simulator, the heterogeneity of rock is considered by assuming that the material properties of elements conform to Weibull distribution, an elastic damage-based law that considered the strain-rate dependency is used to describe the constitutive law at mesoscopic scale, and finite element program is employed as a basic stress analysis tool. The theoretical analysis and numerically obtained results duplicate the deformation, strength (such as Young's modulus, compressive strength, etc) and macroscopic fracture patterns observed in laboratory. While the details of macrocrack formation varied from specimen to specimen, a number of features were consistently obtained in the numerical simulations. The theoretical studies and numerical simulations are extremely instructive and indicative for investigating some catastrophic hazard phenomena such as rock bursts, instability induced by excavation. Splitting and faulting failure modes often observed in experiments are also observed in the simulations under uniaxial compression. It is found that tension fractures are the dominant failure mechanism in both splitting and faulting processes. The numerical simulation shows that faulting is mainly a process of tensile fractures, often en echelon fractures, developed in a highly stressed shear band, just is as observed in actual Uniaxial compression tests. In these simulations, the same diffused AE events or micro fractures but with higher count number also appeared in the early stage of loading.
The uniaxial compressive strength of a rock is one of the simplest measures of strength. It may be regarded as the largest stress that a rock specimen can carry when a unidirectional stress is applied to the ends of a specimen. In other words, the unconfined compressive strength represents the maximum load supported by the specimen during the test divided by the cross sectional area of the specimen. Although the utility of the compressive strength value is limited, the unconfined compressive strength allows comparisons to be made between rocks and provides some indications of rack behavior under more complex stress systems.
Experimentally, researchers have undertaken the task of loading specimens to obtain better knowledge of the compressive failure mechanisms and considerable discussion has been devoted in the literature to this test method (Pells,1993; Wawersik etc,1970; Wawersik & Brace,1971; Lockner DA, et al.,1992; Lockner & Byerlee,1991; Cox & Meredith,1993; Blair & Cook,1998;etc.). Though this mode of failure has been studied in detail for decades, the details of the failure mechanisms, including the microfracture initiation, propagation, coalescence, axial splitting, shearing, etc., are not fully understood and still remain the subject of considerable scientific interest.