A number of standard methods have been proposed to determine the Mode I fracture toughness of rock. They include those based on short rod specimen, chevron bend specimen and cracked chevon-notched Brazilian disk specimen. The semi-circular bend (SCB) specimen shown in Figure1 has been widely used for fracture toughness determination of geomaterials owing to inherent favourable properties such as its simplicity, minimal requirement for machining and the convenience of testing that can be accomplished by applying 3-point compressive loading using a laboratory load frame. It is made from typical rock cores. Despite the application of compressive loading the stresses at the crack tip are tensile and it causes tensile failure due to the opening mode (i.e. Mode I) of the crack propagation. The critical crack initiation parameter in linear elastic fracture mechanics is defined as the fracture toughness Kic and is considered as a material property. However unless the crack tip process zone where the micro cracks that generate due to the tensile stresses coalesces and form a macro crack is contained within a small volume of area in comparison to the specimen size, the resulting toughness value may not be equal to the fracture toughness. This paper discusses the minimum size requirements as appalicable to the determination of plane strain fracture toughness of rock materials using the SCB specimen.

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