Effect of texture on crack initiation toughness and the corresponding anisotropy in ductile fracture resistance of thin Zircaloy-4 sheets of Indian PHWR core component

Abstract

Zirconium alloy core components of nuclear reactors are manufactured following a series of cold-working and heat treatment processes which alter the orientation of grains and hence, their mechanical and fracture properties. Due to the hexagonally close-packed nature of crystal lattice in these alloys, the plastic deformation and the corresponding development of crack-tip constraint is dependent upon the orientation of grains and the major texture. In this work, a thin sheet of Zircaloy-4 has been used to fabricate deeply-cracked compact tension specimens with initial cracks orientated along the rolling and transverse directions respectively. The fracture specimens have been tested at 25 and 300 deg. C, which is the operating temperature of pressurized heavy water type reactors. The crack initiation toughness values have been measured through a modified blunting line expression as well as through measurement of width of stretched zone prior to stable crack growth from scanning electron microscope images. The values of crack initiation toughness as measured through the later method is lower compared to the former signifying physical initiation of crack unlike the approximation associated with 0.2 mm offset line. In addition, the initiation toughness as well as the fracture resistance for specimens with cracks oriented along the rolling direction of the strip are higher when compared to corresponding data for specimens with cracks oriented along the transverse direction. These differences have been explained from point of view of orientation of grains along the rolling direction during the fabrication process of these sheets. The grains plastically deform easily for loading along the transverse direction which is the mode-I loading direction for cracks oriented along rolling direction. The associated loss of crack-tip constraint raises the fracture resistance for rolling direction cracks.