Ductility of Zircaloy Canning Tubes in Relation to Stress Ratio in Biaxial Testing

Abstract

A biaxial-burst testing machine has been designed in which biaxial plane stress tests can be carried out on tubular material with any stress ratio in the range σθ:σz 4:5 to 2:1, where 2:1 implies the closed-end burst test. Unirradiated Zircaloy tubes cold worked 80 percent, recrystallized at 575°C, and characterized by a texture having a basal pole tilt angle of 33 deg were tested at room temperature with various stress ratios, namely, 4:5, 1:1, 5:4, 3:2, and 2:1. In addition, tension and open-end burst tests were carried out in conventional machines. The uniform ductility measured as uniform effective strain displays a minimum at the stress ratio 3:2. It has been found experimentally that the stress ratio for minimum uniform ductility corresponds to deformation under a plane strain loading condition, that is, the stress ratio which gives no change in the tube length. The fracture ductility defined as the effective strain at fracture exhibits a minimum at the stress ratio 5:4. When bulging begins, the applied stress ratio cannot be kept constant, but the stress ratio decreases due to an increased axial stress. This behavior probably explains the observed shift in minimum ductility from the uniform value 3:2 to the fracture value 5:4.