How Residual Stresses Affect Prediction of Brittle Fracture

It is generally accepted that residual stresses present in welded joints alter the conditions for fracture initiation by contributing to the crack-driving force. In this study, it is shown that weld residual stresses also alter crack-tip constraint, affecting fracture in an additional way. The fracture behavior of two crack geometries, a large pressure shell and a small fracture specimen, both containing an idealized welding residual stress state, is examined computationally. Results are discussed in terms of the effect of the residual stresses on fracture prediction. NOMENCLATURE J J-integral K I Mode-I stress intensity factor Q Constraint parameter σ o Uniaxial yield strength t Pipe wall thickness a Defect size b Ligament size (b = t-a) R Pipe radial distance Stress intensity factor including residual stress Stress intensity factor due to applied load Stress intensity factor due to residual stress J-integral due to applied load and residual stress Elastic part of the J-integral due to applied load Plastic part of the J-integral due to applied load J-integral due to residual stress J c J-integral at fracture initiation r, θ Polar coordinates with origin at the crack-tip Crack opening stress l * Characteristic length scale in the RKR failure model σ f * Critical stress in the RKR failure model P c Applied load that causes INTRODUCTION Prediction of brittle fracture is often performed by assuming that global fracture parameters such as the J-integral, J , or the Mode-I stress intensity factor, K I , control the fracture event. The usual method in the presence of a known residual stress field is to use superposition. However, this approach assumes J or K I controlled fracture and therefore ignores the influence of constraint which affects the magnitude of the crack-tip stress fields. Two bodies loaded to the same value of the global fracture parameter, but under differing levels of constraint, will contain different crack-tip stresses. Because crack-tip stresses are altered, constraint effects have been shown to cause large changes in brittle fracture toughness. It is well known that different levels of constraint can exist in structures due to differences in geometry (e.g., thick versus thin) or applied loading (e.g, tension versus bending). Less understood is that constraint changes can also occur due to the presence of residual stress. Previous computational studies by the authors (Panontin and Hill, 1996) (Hill and Panontin, 1997) have demonstrated through the use of J-Q analysis that triaxial residual …