Determination of fracture toughness using the semi-circular specimen, the chevron edge notch round specimen, the short rod specimen and the cylindrical burst specimen, are critically reviewed. These methods, among all, are the most applicable for fracture toughness determination of rocks. They require minimum machining. Samples are small, and testing procedures are simple. They have a sound analytical basis and allow the fracture toughness to be determined without measurement of fracture length.
An understanding of the mechanics and mechanism of rock fracture is a key element in solving a great many engineering problems that involve geotechnical structures. With the exception of a few early investigations, rock fracture mechanics is a relatively new field of study. The application of fracture mechanics to rocks requires the understanding that linear elastic fracture mechanics (LEFM) principles were not developed with rock materials and geological structures in mind. While certain basic theories will apply, large differences in basic material response and engineering application between rock and metallic materials must be considered when adopting these principles, practice, sample preparation, and test methods.
One concept commonly used to predict the onset of unstable crack propagation in isotropic materials is the plane strain fracture toughness. When measured under certain conditions, the fracture toughness has been shown to be a true material constant for many materials, and thus is independent of specimen size and crack length. This concept has also been shown to be applicable to anisotropic materials such as rocks. A two-dimensional (2D) state of stress is assumed to exist along the crack front in plane strain fracture toughness testing. However, the necessary requirements to achieve that state, such as the minimum specimen dimensional requirement, may vary from those of metallic materials.
The standardized test methods of LEFM, such as the E399 of ASTM, are based on conservative design criteria, since it is the prevention of failure that is usually desired in man-made structures. However, it is the creation and propagation of fracture that are desired in many rock fracture applications such as blasting (Rossmanith, 1983; Ouchterlony, 1974), drilling (Ouchterlony, 1977), massive hydrofracturing (Simonson et a1.,1976; Abe et al., 1976) and hydraulic fracturing stress measurements (Abu- Sayed and Brechtel, 1977; Rummel and Winter, 1983; Karfakis, 1986). This difference may not impact the physics and mechanics of the fracture process itself, but the engineering application should dictate how the important parameters are measured and utilized. Furthermore, since the standard tests were designed with metal in mind, the specimen configurations require extensive machining which, in most cases, is impossible or impractical to perform on rocks. Core specimens are more suitable for rock testing as they require minimum preparation effort. Moreover, there is a definite need for the establishment of a standard test procedure for the determination of fracture toughness of rocks. Such an endeavor must be backed by developments of appropriate theoretical and numerical methods.
In the following sections the newly developed semi-circular specimen (SCB), two chevron-notched specimens namely the Short Rod and the chevron edge notch round bar (CENRBB) and the cylindrical burst specimen are described and reviewed.