Fracture assessment of shallow-flaw cruciform beams tested under uniaxial and biaxial loading conditions

A technology to determine shallow-flaw fracture toughness of reactor pressu re vessel (RPV) steels is being developed for application to the safety ass essment of RPVs containing postulated shallow surface flaws. Matrices of cr uciform beam tests were developed to investigate and quantify the effects o f temperature, biaxial loading, and specimen size on fracture initiation to ughness of two-dimensional (constant depth), shallow, surface flaws. The cr uciform beam specimens were developed at Oak Ridge National Laboratory (ORN L) to introduce a far-field, out-of-plane biaxial stress component in the t est section that approximates the nonlinear stresses resulting from pressur ized-thermal-shock or pressure-temperature loading of an RPV. Tests were co nducted under biaxial load ratios ranging from uniaxial to equibiaxial. The se tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for an RPV material. The cruciform fracture toughness data were used to eva luate fracture methodologies for predicting the observed effects of biaxial loading on shallow-flaw fracture toughness. Initial emphasis was placed on assessment of stress-based methodologies, namely, the J-Q formulation, the Dodds-Anderson toughness scaling model, and the Weibull approach. Applicat ions of these methodologies based on the hydrostatic stress fracture criter ion indicated an effect of loading-biaxiality on fracture toughness; the co nventional maximum principal stress criterion indicated no effect. A three- parameter Weibull model based on the hydrostatic stress criterion is shown to correlate with the experimentally observed biaxial effect on cleavage fr acture toughness by providing a scaling mechanism between uniaxial and biax ial loading states. (C) 1999 Elsevier Science S.A. All rights reserved.