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Much recent work in the field of elastic-plastic fracture mechanics has been directed to developing a mechanics-based relationship between the onset of cleavage fracture in structural components and that of Charpy specimens. In the case of a commercial nuclear power plant the surveillance specimens are expected to have undergone an irradiation history equivalent to that of the pressure vessel, and hence to have the correct fracture toughness properties. The small size of these specimens, however, makes it difficult to obtain fracture toughness measurements that are transferrable to the much larger and higher constraint pressure vessel.
Application of proposed specimen size requirements to this problem shows that for the typical fracture toughness and strength levels in nuclear pressure vessels, the pre-cracked Charpy specimen could lose constraint well before the onset of cleavage and the results obtained will then provide a very non-conservative estimate of the conditions required for the onset of cleavage in the vessel. Recent computational work by Koppenhoefer and Dodds has strongly suggested, however, that if the specimen is loaded rapidly, additional constraint might be present which would allow the Charpy results to predict correctly the onset of cleavage in the nuclear pressure vessel. These authors predict that the presence of modest viscoplasticity will increase the deformation level at which constraint is lost and this in turn will allow Charpy size specimens to be adequate predictors of the onset of cleavage fracture in structural size elements.
In this paper pre-cracked Charpy specimens of A515 steel are tested at four different loading rates and five different temperatures in the lower ductile-to-brittle transition. The reference temperature, T0, is developed and shown to be very dependent on the loading rate and very independent of the temperature at which the data set was measured.
fracture toughness, A515, rapid fracture toughness, constraint, J integral, master curve, precracked Charpy
Professor of Mechanical Engineering, U.S. Naval Academy, Annapolis, MD