1. INTRODUCTION
Understanding the fracture mechanism of brittle rock for shear rupture is essential to the study of the mechanisms responsible for rock fracturing. To analyze these types of failure mechanisms, the stress intensity factor of the fracture has to be defined. Research involving shear fracture has played a very important role in the areas of mining, civil engineering, geology and geological engineering. In this study, a shear fracture device has been constructed, analyzed and used to create the shear fracture of brittle rock.
Many researchers have investigated application of fracture mechanics concept to the problem of shear fracture. The mechanism of shear fracture is still the subject of considerable confusion for many researchers. To improve understanding of the shear mechanism, a number of studies have been conducted to develop a suitable test sample configuration and an accurate loading arrangement in order to establish some correlations between theoretical and experimental investigations. Kaplen (1961) was the first to apply the concept of fracture mechanics to investigation of the failure mechanism of a concrete beam under three and four point loading arrangements. Iosipescu (1967) has investigated the in-plane shear properties of composite materials. The idea behind the Iosipescu shear test is to produce a state of pure shear loading between the two notches in the test specimen by application of two counter-acting moments produced by two force couples. Ingraffea and Panthaki (1981, 1985), Bazant and Pfeifer (1985, 1986), Huang and Wang (1985), Davies and So (1986), and Jung et. al, (1992) used similar methods to prove the existence of shear failure with a symmetrically notched specimen. The objective of this study is to enhance the understanding of the shear mechanism.
The shear fracture device was constructed during a cooperative research effort between the University of Idaho and the U.S. Bureau of Mines (Enbaya, 1991, Jung, 1991). The device was designed to induce shear fractures in laboratory specimens of brittle materials under shear loading conditions. Rock samples were cut into 10cm (4 inch)cubes with a diamond rock saw. Samples were then ground to insure parallel surfaces and to avoid any local crushing. Vertical and inclined notches were cut to a depth of a few inches from the top and bottom, leaving a few inches of intact area (Fig. 1 & 2). In preparing specimens, six different types of angled notches were cut at O, 10, 20, 30, 35, and 40 degree from the loading direction. In figure 3a, the failure pattern indicates that the cracks propagated approximately vertically. In figure 3b, the pattern illustrates that cracks tend to develop with angles (about 63.5°) at the tip of the notch and propagate with a curved shape. Two cracks from the tip of each notch merged at the middle of the center line. However, a higher angle of notch illustrates that local failure had occurred along the angled notch. In this loading arrangement, stress tends to build up near the location of loading and along the angled notch due to loading alignment.
