Two empirical relations by Barsom and Hagiwara between fracture toughness and impact toughness were examined, based on the assumption for the fracture occurrence condition that the local flow stress at crack tip attains to the microscopicalcleavage fracture stressρF. It was shown that(1) The stain rate sensitivity of fracture toughness is controlled not only by that of the yield strength, but also by the fracture mode characteristic to the microstructure.(2) It was suggested that ρF depends on temperature in such a high temperature range that cleavage fracture is possible only in the dynamic loading condition.
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