Threshold stress intensity factor, K-IH for delayed hydride cracking of a Zr-2.5Nb tube with loading mode

Abstract

The threshold stress intensity factors, K-IH'S, for delayed hydride cracking of a Zr-2.5Nb tube were determined using either load-increasing or decreasing modes over a temperature range of 160-280 degrees C with a step-wise load increase or decrease by 0.5 MPa root m, respectively. K-IH's in the load-increasing mode were found to be higher than those in the load-decreasing mode. The striation spacing corresponding to the critical hydride length or I-c showed a sharp drop between the lst striation line to the 9th line and then leveled off to a constant value in the load-increasing mode. In the load-decreasing mode, it was rather constant from the 1st line to the 14th line due to a higher stress intensity factor K-I with its gradual increase at a reduced K-I closed to K-IH. Supplementary tests demonstrated that a higher K-IH in the load-increasing mode is due to the combined effects of a larger tip radius of the fatigue crack and creep working in the load-increasing mode. K-IH had a negative temperature dependency in the load-increasing mode and it had no temperature dependency in the load-decreasing mode. To correlate K-IH with twins, tensile tests were conducted on the Zr-2.5Nb tube and its textural changes during the tensile tests were investigated using an X-ray diffractometer. The number of twins increased over a temperature range of 150-300 degrees C and then decreased rapidly at over 300 degrees C with no twins at 350 degrees C. K-IH of the Zr-2.5Nb tube with the loading modes was discussed in view of cracking of the hydrides by twining. (C) 2007 Elsevier B.V. All rights reserved.