Size-effect on the fracture toughness and the R-curve of carbon materials

A theoretical extension of the Bazant size-effect law to crack propagation problems is made in order to analyze the R-curve-related microfracture mechanisms of brittle materials. The specimen's size-effect on the fracture toughness and crack growth resistance curve (R-curve) is formulated in a generalized fashion without assuming the similitude restriction of test specimen geometry, being more convenient for the study of R-curve behavior. Using the size-effect law proposed in this work, the dimensions of the crack-tip process zone as well as the crack-face bridging region are estimated in the fracture mechanics analysis of carbon materials (a polycrystalline graphite and a carbon-matrix/carbon-fiber composite). Microstructural observations confirm these fracture mechanics estimates.