Effect of constraint on fracture parameters of piping materials

Abstract

Fracture behaviour under quasi-static tearing load has been studied through fracture toughness tests on SA333 Grade 6 and 20MnMoNi55 steels using single-edge notched bend (SENB) specimens. To understand the response of variation of constraint, J-R curves were obtained from specimens precracked to a/W ratio in the range of 0.25 to 0.75, in steps of approximately 0.1. Stretch zone widths (SZW) were measured on the fractured surfaces of broken specimens. The stretch zone dimensions that were determined have been used in conjunction with the experimentally derived J-R curve to obtain a value of the ductile fracture toughness parameter J SZW. The initiation toughness, J i, obtained at the intersection of the blunting line and the power-law fit to the J-R curve, and the critical toughness, J c, determined following the procedure of the ASTM standard, were estimated. Comparisons of J SZW with J i and J c have been made for 20MnMoNi55 and SA333 steels as a function of the a/W ratio. The J i and J c values are higher at lower a/W, i.e. under low constraint, and fall with increasing a/W. A loss of constraint thus enhances these parameters. It was observed that J SZW does not effect by a/W, and that its value is lower bound to J i and J c values for both the materials investigated.