A numerical failure model of rock, RFPA(Rock Failure Process Analysis), is used to simulate the process of rock failure under different confining pressures, including acoustic emission produced by rock failure. We find that the spatial distribution of acoustic emission (A E) event locations is fractal and has very good statistical self-similarity in different confined conditions. The fractal dimension D decreases progressively in the rock failure process. In other words, the AE event locations become more and more ordered. The fractal dimension D becomes eventually smaller in the specimen under higher confining pressure.
Acousticemission (AE), as a transientelastic wave generated by the rapid release of energy within a material, is a ubiquitous phenomenonassociated with rock fracture and has provided a wealth of Information regarding the failure process In rock (Lockner). In other words, the AE activities in rocks represent the damage process of rock failure. Fractal Geometry has been widely used for the description of irregular phenomena in various scientific fields recently. It is a powerful tool not only for studying complex shapes, but also for describing damage phenomena with statistical characteristics. Hirata et al.conducted a creep test on Oshima granite at 40 MPa confining pressure and about 85% fracture strength. He found that the spatial distribution of AE in rocks is a fractal, and that the fractal dimension decreases with the evolution of rock fracturing (Hirata, 1987). Patience A. Cowie et al presented a numerical rupture model to simulate the growth of faults in a tectonic plate. They explored in detail the time evolution of the capacity (D0), information (D1), and correlation (D2) fractal dimensions (Cowie, 1995, I 993). recently, a numerical approach for simulating damage initation and related propagation of seismic energy during brittle rock failure, RFPA 2d (Rock Failure Process Analysis), was presented by Tang C. A. et al (Tang. Chen & Tang, 2000). However, almost the whole research work on rock failure aspects using RFPA has been concentrated on the qualitative analysis on fault propagation, the spatial distribution of AE and the location of deformation. In this paper, fractal concepts will be used to evaluate quantitively the evolving degree of disorder in the spatial distribution of AE event locations and the propagation of faults in rock failure under different confining pressures. lt is found that the defined fractal dimension decreases through the loading step.Inother words, it turns out thatthe distribution of AE becomesprogressively more ordered with the development of faults,f rom the initial loading to the main failure.
The demand for new tools, which may contribute to a better understanding of failure mechanisms of brittle rocks. initiated the development of RFPA. Mathematically, the RFPA code is a combination of the linear finite clement method (FEM) and continuum mechanics method capable of simulating non-linear and discontinuous mechanical behavior. In all cases. the models simulate plane strain compression of the specimens, which may be applied either uniaxially or biaxially.