Stress Intensity Factor Predictions at Clad Interfaces

Abstract

Where a crack has grown such that it crosses a material interface, such as in a clad pressure vessel, there is an additional assessment requirement, above the surface and depth points to evaluate the material interface region. This is typically in terms of fatigue crack growth behaviour and crack extension by ductile tearing. Generally, elastic stress intensity factor (KI) solutions, such as those in R6, only consider the surface and deepest point locations, and therefore do not provide a methodology to account for any interactions at material interfaces. Therefore, for a material interface, an assessment may look to describe the region by the maximum of the surface or deepest point values, or it may be argued to be represented by the value from either the surface or deepest point, depending on which is geometrically closer. However, this has the potential to lead to overly conservative interpretations in terms of KI and the elastic-plastic equivalent, KJ, at the material interfaces, depending on the fracture toughness and growth laws used. The work presented here investigates the influence of the clad to plate interface on stress intensity factor under elastic (KI) and elastic-plastic (KJ) conditions for a number of defect geometries, loading conditions and material properties. The results presented in this paper seek to provide guidance on the suitability of KI and KJ for the surface and deepest points, extracted from models containing only plate material properties, to describe the behavior of a crack which crosses a material interface region in a clad pressure vessel wall.