MEASUREMENT OF THE FRACTURE-TOUGHNESS OF A FIBER-REINFORCED COMPOSITEUSING THE BRAZILIAN DISK GEOMETRY

A technique, using the Brazilian disk specimen, for measuring the frac ture toughness of unidirectional fiber-reinforced composites, over the entire range of crack-tip mode mixities, was developed. The fracture toughness of a graphite/epoxy fiber-reinforced composite was measured, under both mode-I and mode-II loading conditions. We found that for c ertain material orientations the mode-II fracture toughness is substan tially higher than the mode-I toughness. The complete dependence of th e fracture toughness on the crack-tip mixity was determined for partic ular material orientations and the phenomenological fracture toughness curves were constructed. Using the Brazilian disk specimen, together with a hydraulic testing machine, the fracture toughness of the compos ite under moderate loading rates was measured. We observed that the mo de-I fracture toughness was not sensitive to the loading rate at the c rack tip, K, while the mode-II 'dynamic' fracture toughness increased approximately 50 percent over the quasi-static fracture toughness. A q ualitative explanation of the dependency of fracture toughness on crac k-tip loading rate is discussed. Finally, a mechanical fracture criter ion, at the microscopic level, which governs the crack initiation unde r mixed-mode loading conditions is presented; these theoretical predic tions closely follow the trend of experimental measurements.