A study of crack depth effect and scattering of fracture toughness in three point bend COD specimens

Fracture toughness obtained under the same experimental conditions has the following two characteristic features: fracture toughness has the dependency of crack depth and shows a large scatter even though the same geometrical shaped specimens were applied to the tests, which derives from the sensitivity of micro structures of steel. Quantificational evaluation of these phenomena is performed by applying strain rate-temperature parameter in the fracture process zone (Rγ), which is the function of strain rate and temperature, as the evaluation parameter. Postulating that fracture toughness is a function of Rγ , it makes clear that there is no crack depth effect, namely the plastic constraint effect, on fracture toughness and that the scattering on fracture toughness decreases considerably. Moreover, the possibility which the dimensionless parameter derived from Rγ may be the universal parameter to characterize fracture toughness is indicated.