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J‐Q MODEL FOR PREDICTING FRACTURE IN THE DUCTILE‐BRITTLE TRANSITION
Author(s) -
Landes J. D.
Publication year - 1996
Publication title -
fatigue and fracture of engineering materials and structures
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.887
H-Index - 84
eISSN - 1460-2695
pISSN - 8756-758X
DOI - 10.1111/j.1460-2695.1996.tb01022.x
Subject(s) - fracture toughness , cleavage (geology) , materials science , toughness , brittleness , structural engineering , geometry , fracture (geology) , stress (linguistics) , composite material , metallurgy , mechanics , mathematics , physics , engineering , linguistics , philosophy
— A model is proposed to predict cleavage failure of precracked bodies in the transition region for steels. It is based on the concept that the 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 assumes that the weak link has a given level of stress labeled the cleavage stress needed to cause failure. This weak link is located at some characteristic but variable distance from the crack tip. The variation in distance from the crack tip to the weak link causes variation in the transition fracture toughness. The crack tip stress is given by the J‐Q model of O'Dowd and Shih where Q characterizes the constraint level in the body. Given a set of input data, that is fracture toughness from a given specimen geometry at a known temperature, toughness values 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 DIN 20MnMoNi55 steel and a CrMoV steel. For the first steel fracture toughness values measured on compact specimens are used to predict cleavage fracture throughout the transition for the same geometry. For the second steel the transition trend is predicted for the same compact geometry. The results 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 mechanism may not be operating. SUMMARY A model is proposed to predict the cleavage fracture toughness in the transition for steels. It is based on a weak link failure concept and a crack tip stress distribution that is influenced by the constraint level. The model uses measured fracture toughness data to predict a distance from the crack tip to the weak link. This distance is a variable that is used to account for all of the variability in the fracture toughness. For a new temperature or geometry the toughness can be predicted by assuming that the cleavage stress is fixed and the material has the same range of weak link distances. The magnitude of the crack tip stresses can change due to a change in yield stress or constraint. The model was used to predict the toughness trend in the transition and the end of the transition. It was applied to two steels, a 20MnMoNi55 steel and a CrMoV steel. For the 20MnMoNi55 steel the transition trend and the end of the transition that were predicted by the model compared well with experimental results. For the CrMoV steel the interesting part was that the lower shelf region of the transition was not predicted well. The prediction from the model overestimated the measured fracture toughness values. This suggests, as was previously believed, that a weak link mechanism may not operate near the lower shelf part of the transition. The model could also be used to predict the toughness for a new geometry such as a component model geometry. That will be reported in a later work.