Fracture toughness determination of layered materials

Layered materials, such as sedimentary rocks, are ubiquitous. A newly developed fracture toughness test specimen, which is semi-circular in shape and contains an edge-crack, is subjected to three-point-bend loading. The fracture load and the fracture energy of a layered rock are then measured with static tests, and the fracture toughness is determined using a stress intensity factor method, a compliance method and a J-integral based method. The results of all three methods agree satisfactorily showing that the fracture toughness determination using linear elastic fracture mechanics is valid for anisotropic rock materials. Specifically, the static fracture toughness of oil shale, which is a typical layered rock containing fossil fuel, was measured. The static fracture toughness decreases with the specific gravity (which is related to oil yield). No specimen size dependence was observed for the range of specimen sizes used in the experimental program. A minimum dimensional requirement is proposed for plane strain fracture toughness. The proposed specimen is applicable for both layered and non-layered materials. It is especially useful for core-based specimens.