Effects of residual stresses on the fracture toughness of Zircaloy-2 tubes

Data applicable to evaluating the fracture toughness of Zircaloy-2 pressure tubes were evaluated according to the criteria in ASTM Standards E399 and E813. It was found that the data did not meet the criteria specified in the standards, thus making it necessary to account for residual stress in determining the fracture toughness of the tubes. Therefore, residual stress in pressure tube specimens was experimentally determined to allow the incorporation of residual stresses in the calculation of fracture toughness. Sections of as fabricated 82.5 mm OD Zircaloy-2 tubes in the 30% cold worked condition were used in testing. Electrochemical Machining (ECM) was used to remove material from the interior of the tube, while measuring the resulting change in strain on the exterior of the tube with delta rosette strain gages. Utilization of this technique to reduce the cross sectional area of a tube without introducing additional stresses offers an accurate procedure for obtaining data to calculate the residual stress distribution. Assuming elastic anisotropy, the stress equations developed by Voyiadjis, Kiousis, and Hartley were used to determine the residual tangential, radial, and longitudinal stresses as a function of wall thickness. Additionally, these equations allow the determination of the shear stress and subsequently the principal stress profiles. It was found that the measured residual stress level was high enough that it must be accounted for when determining the fracture toughness of the subject Zircaloy-2 tubes. 7 refs., 7 figs