J-Q MODEL FOR PREDICTING FRACTURE IN THE DUCTILE-BRITTLE TRANSITION

A model is proposed to predict cleavage failure of precracked bodies i n the transition region for steels. It is based on the concept that th e failure of a weak link triggers the failure of the entire body. The model is similar to that originally proposed by Heerens et al. which a ssumes that the weak link has a given level of stress labeled the clea vage stress needed to cause failure. This weak link is located at some characteristic but variable distance from the crack tip. The variatio n in distance from the crack tip to the weak link causes variation in the transition fracture toughness. The crack tip stress is given by th e J-Q model of O'Dowd and Shih where Q characterizes the constraint le vel in the body. Given a set of input data, that is fracture toughness from a given specimen geometry at a known temperature, toughness valu es can be predicted from these inputs at a different temperature where the yield stress is known or for a different size or geometry where Q is known. The model is applied here for two steels; a DM 20MnMoNi55 s teel and a CrMoV steel. For the first steel fracture toughness values measured on compact specimens are used to predict cleavage fracture th roughout the transition for the same geometry. For the second steel th e transition trend is predicted for the same compact geometry. The res ults show that the model is good for predicting the sudden end of the transition that is often observed but does not predict the lower shelf or early transition very well. This is a region where a weak link mec hanism may not be operating.