Kinetics of crack growth in zirconium alloys (II): Stress dependence of the crack growth rate

Abstract

Delayed hydride cracking tests were conducted on a cold-worked Zr-2.5Nb tube with hydrogen at different test temperatures ranging from 160 to 280 °C in the load increasing mode where an applied KI was increased step-wise by 0.5 MPa m1/2 from 4.5 MPa m1/2. In Stage I with a low KI being close to the threshold stress intensity factor KIH, the striation spacing or the critical hydride length lc that was larger at a lower KI sharply decreased with increasing KI and the crack growth rate (CGR) showed a rapid increase with KI, arising from a reduced hydride cracking rate by creep at a lower KI. In Stage II with KI being much larger than KIH, however, the lc was much smaller and the CGR became independent of KI, demonstrating that the hydride cracking rate becomes so high as to be unaffected by the applied KI, and diffusion-controlled hydride growth becomes a rate-controlling factor for the CGR in Stage II. This explains the constant CGR independent of KI in Stage II. It is demonstrated that the CGR of zirconium alloys is determined by the rate of a slower process between two processes of growth and cracking of hydrides.