Damage Model for Predicting the Effect of Steel Properties on Ductile Crack Growth Resistance

This study pays attention to reveal material properties that control a resistance curve for ductile crack growth (CTOD-R curve) on the basis of the mechanism for ductile crack growth, so that the R-curve could be numerically predicted only from those properties. Crack growth tests using 3-point bend specimens with a fatigue pre-crack are conducted for two steels that have different ductile crack growth resistance, whereas both steels have the same mechanical properties in terms of strength and work hardening. Observation of crack growth behaviors provides that different mechanisms between ductile crack initiation from fatigue pre-crack and subsequent growth process can be applied. It is shown that two types of ductile properties of steel associated with ductile damage can mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from a surface of notch root, and the other one is a stress triaxiality dependent ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve is proposed taking the above two ductile properties into account, where the ductile crack initiation from crack-tip is in accordance with local strain criterion, and the subsequent crack growth triaxiality dependent damage criterion. The proposed model accurately predicts the measured different R-curves between two steels used that have the same 'strength properties', and also the stress triaxiality dependence of R-curve.