The displacement ( o) of the specimen (referred to as "CMOD", crack mouth opening displacement) is measured at each end of the groove using a clip Gage. The two measurements of o serve two purposes: to reduce uncertainty through the use of mean values, and to confirm that the crack propagated parallel to the starter notch in the direction of the linear support. If these values were found to differ by more than 10 % , then the results of a particular specimen test were considered invalid. Force (Fm) and the two end displacements ( o 1 and o 2) were recorded by an electronic data logger. This data was then analyzed to produce a load-displacement curve (Fh- o curve). This load-displacement graph characterizes the fracture properties of the specimen. The area under the Fh-o curve represents the energy needed to fracture the specimen (fracture energy). The specific fracture energy (Gt) is a measure of the crack growth resistance of the material and is defined as the ratio of fracture energy to fracture area, normal to the wedge penetration direction. The
maximum load in the load-displacement diagram (Flnnax) may be used to calculate a conventional notch tensile strength. Details on the evaluation and handling of the wedge splitting test data have been described by
Tschegg (1991).