2008 Volume 94 Issue 2 Pages 57-65
The effort of this study is to reveal the material properties controlling the resistance to ductile crack initiation and growth (CTOD-R curve) on the basis of the microscopic observation of crack growth, so that the CTOD-R curve can be numerically predicted only from those properties. The crack growth resistance tests using 3-point bend specimens with fatigue pre-crack for two steels provide different ductile crack growth resistance, CTOD-R curve, whereas both steels have the same “mechanical properties” in terms of strength and work hardening. The crack-tip constraint effect, in this case the pre-crack length effect, on the CTOD-R curve is also different between both steels. The observation of crack growth behaviors reveals that micro-mechanisms of ductile crack initiation from fatigue pre-crack and subsequent extension can be different. It is shown that two “ductile properties” of steel associated with ductile damage can control the CTOD-R curve. The one is “critical local strain” evaluated with the notch-tip strain for ductile cracking obtained with bending test of notched specimen, which controls ductile crack initiation of pre-cracked specimen. The other one is “stress triaxiality dependent ductility” obtained with circumferentially notched round-bar specimens subjected to tension, which controls the ductile crack extension for pre-cracked specimen.
