Fracture behaviour of finite length flaws in pressure tubes

Flaws encountered in nuclear pressure tubes must be evaluated to ensure that a hydride induced cracking mechanism, called delayed hydride cracking (DHC), is not initiated. The stress concentration at a flaw tip causes diffusion of hydrogen and precipitation of zirconium hydride at the flaw tip. Typically, assessment is done based on experimental data obtained from two-dimensional flaws. However, realistic lengths of flaws make the two-dimensional approach overly conservative in many cases, and costly remedial action may be prescribed unnecessarily. A fracture initiation model for DHC involves a type of process zone description to account for the interaction of hydride precipitation with the flaw tip stress distribution. Analytical techniques for this model based on weight functions are practical and accurate for two-dimensional geometry, but cannot be easily applied to the three-dimensional features of finite length flaws. Recently, a numerical rendition of the model has been incorporated into a finite element program so that arbitrary geometry and material properties can be managed. The process zone is automatically generated as hydride formation progresses, and a displacement parameter derived from the finite element distributions quantifies the response relative to an experimentally established fracture initiation threshold. The three-dimensional finite length model is applied to specific flaw geometries used in an experimental program. Comparison with corresponding two-dimensional tests demonstrates that the finite length flaw has a significantly higher threshold load than that predicted on the basis of a two-dimensional model. (author)