THERMAL-CREEP INDUCED STRESS RELAXATION NEAR A NOTCH IN Zr-2.5Nb

In CANDU nuclear reactors, flaws on the surface of Zr-2.5Nb pressure tubes naturally cause stress concentration, and subsequently potentially lead to crack initiation through delayed hydride cracking. However, the concentrated stresses tend to relax due to creep, which is suggested to reduce the occurrences of cracks. In this study, C-shaped specimens with a small radius notch (15 mu m or 30 mu m) manufactured from Zr-2.5Nb pressure tubes were first preconditioned through an ex-situ creep process (K-eff=6MPa root m, 583K) for a series of hold times. Elastic lattice strains around the notch were measured using high energy X-ray diffraction at the Advanced Photon source. Results show that strains along the loading direction are tensile and concentrated in front of the notch tip, while those perpendicular to the loading direction are compressive and distributed symmetrically about the notch plane. The concentrated strains relax with creep time, and the larger-size notch has a higher rate of relaxation.