Evaluation and modeling of fracture toughness based on the mezzo-scopic elastic-plastic fracture mechanics

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Cleavage fracture toughness of steels was mezzo-scopically analyzed on the basis of the statistical local fracture criterion approach. The statistical Weibull stress criterion at the crack tip region suggests that the cleavage fracture toughness of steels can be described as a function of the yield stress, and other mechanical properties of the materials. Formulation of the cleavage fracture toughness was first examined through an investigation on correlation between the cleavage fracture toughness and the cleavage fracture stress obtained in notched round bar specimens in accordance with the theoretical prediction. Then, scatter of the toughness, strain rate and specimen thickness effect on the toughness, deterioration of the toughness due to cold working and irradiation, and improvement of the toughness caused by the Ni addition, were analyzed through the formulation above.The specimen size requirements in toughness qualification are also discussed on the basis of statistical local fracture criterion approach. The Weibull stress criterion for cleavage fracture is applied to the constraint loss field at the crack tip described with the toughness scaling model (TSM). The new model proposed demon strates that the TSM is a particular case for a material with a particular combination of the Weibull shape parameter, m and the strain hardening exponent, n. Analytical prediction indicates that the specimen size re quirements in toughness must be significantly depending on the value of n and m of the material. The 2-and 3- dimensional FE analysis were performed to confirm above analytical prediction.