Effect of crack depth on fracture toughness of 20MnMoNi55 pressure vessel steel

Ductile fracture behaviour of 20MnMoNi55 steel under quasi-static loading has been studied using single-edge notched bend specimens. To understand the influence of crack depth on fracture behaviour, J–R curves were obtained from specimens pre-cracked to various a/W in the range of 0.25–0.75, in steps of approximately 0.1. Stretch zone width dimensions were measured on the fractured surfaces of the broken specimens. The stretch zone dimensions that were determined have been used in conjunction with the experimentally derived J–R curves to obtain a value of the ductile fracture toughness parameter JSZW. The initiation toughness, Ji, at the intersection of the blunting line and the power-law fit to the J–R curve, and the critical toughness, Jc, following the ASTM standard, were estimated. Comparisons of JSZW with Ji and Jc have been made for 20MnMoNi55 steel as a function of the a/W ratio. The Ji and Jc values are found to be higher at lower a/W, i.e. under low constraint, and decrease with increasing a/W. It is observed that JSZW is not affected by a/W, and that its value is lower bound to Ji and Jc values of the material investigated.