A PREDICTIVE APPROACH TO CLEAVAGE FRACTURE EVENTS IN THE TRANSITION REGION

Although a number of theoretical approaches have been proposed to mode l fracture behaviour of steels in the brittle to ductile transition re gion, where a limited amount of ductile tearing is followed by cleavag e instability, predictive methods capable of estimating average values and variabilities of fracture toughness for various temperatures and specimen or component sizes have never been developed. In this paper, based on results obtained in tests on compact tension specimens taken from 1CrMoV steel, a statistical approach based on weakest link concep ts, including effects of plasticity and of subcritical cracking, is co nsidered. By coupling this model, calibrated using data obtained from small specimens tested at various temperatures in the transition regio n, with a J-R curve equation accurately determined by fractography of the same specimens, accurate predictions are obtained for specimens tw o and a half times larger and specimens 10 times larger than those use d for calibrating the model. The proposed model is shown to provide a rationale for fracture behaviour over the whole range of temperatures comprising the transition region, i.e. starting from cleavage under li near elastic fracture mechanics conditions (K(ic)) at low temperature, followed by fracture after limited plasticity, then by cleavage after some tearing (transition behaviour), up to full J-R behaviour at high er temperatures where cleavage probability drops to zero in the range in which J values are actually obtainable.