Nonlinear Fracture of a Polycrystalline Graphite — Size-Effect Law and Irwin’s Similarity

Abstract

Nonlinear fracture of a polycrystalline graphite is examined through the studies for the test specimen size-effect on fracture toughness parameters. The fracture toughness of polycrystalline graphite significantly depends on the dimensions and the geometries of test specimens. A finite non-negligible frontal process zone dictates this size-effect law. The Bažant’s theoretical approach modified by the present authors is successfully applied to the nonlinear fracture of polycrystalline graphite, allowing an estimate of the size of frontal process zone, as well as the theoretical prediction for the size-effect law of fracture toughness. The Irwin’s similarity relationship (the equivalence between the stress-intensity derived toughness, K c 2/E′ and the potential energy-derived toughness R c ; K c 2/E′=R c ) is also studied in the present nonlinear fracture mechanics regime; R c is always larger than K c 2/E′, and the excess energy (R c -K c 2/E′) becomes progressively significant with the increase in the critical load P c for the onset of main crack extension, independent of the notch/crack length of test specimen. This fact suggests that the excess energy, R c -K c 2/E′, is consumed in the region away of the localized area including the main crack and its adjoint frontal process-zone. The microscopic considerations are made of the characteristic processes and mechanisms for the excess energy consumption in polycrystalline graphite materials.