J–R curves corrected by load-independent constraint parameter in ductile crack growth

Abstract

The constraint effect on J–resistance curves of ductile crack growth is considered under the condition of two-parameter J–Q* controlled crack growth, where Q* is a modified parameter of Q in the J–Q theory. Both J and Q* are used to characterize the J–R curves with J as the loading level and Q* as a constraint parameter. It is shown that Q* is independent of applied loading under large-scale yielding or fully plastic deformation, and so Q* is a proper constraint parameter during crack growth. An approach to correct constraint effects on the J–R curve is developed, and a procedure of transferring the J–R curves determined from standard ASTM procedure to nonstandard specimens or real cracked structures is outlined.The test data of fracture toughness, JIC, and tearing modulus, TR, by Joyce and Link (Engng. Fract. Mech. 57(4) (1997) 431) for a single-edge notched bend specimen with various depth cracks are employed to demonstrate the efficiency of the present approach. The variation of JIC and TR with the constraint parameter Q* is obtained, and then a constraint-corrected J–R curve is constructed for the test material of HY80 steel. Comparisons show that the predicted J–R curves can match well with the experimental data for both deep and shallow cracked specimens over a reasonably large amount of crack extension.Finally, the present approach is applied to predict the J–R curves of ductile crack growth for five conventional fracture specimens. The results show that the effect of specimen geometry on the J–R curves is generally much larger than the effect of specimen sizes, and larger specimens tend to have lower crack growth resistance curves.