Development of the crack pattern during stress corrosion in zircaloy-tubes

To investigate SCC of Zircaloy, creep tests with pressurized tubular specimens were performed at temperatures between 350 and 4500C and an iodine loading of 10-5 g/cm2. Inspection of the surface of specimens with different strain revealed a plurality of cracks with a log normal distribution of lengths (and depths). With increasing strain, the mean crack length increases, whereas the scattering factor stays rather constant and the density of 'active' cracks decreases. At lower temperature the same mean crack length requires less strain. Both the individual growth and the coalescence of cracks contribute to the overall growth mechanism. 'Defection' of a specimen occurs when just one crack statistically reaches the depth of the wall thickness. This 'perforation criterion' can be used to predict the lifetime and the maximum hoop strain of a tube of any length. (orig.)