Constraint effects on fracture toughness for circumferentially oriented cracks in reactor pressure vessels

Pressurized-thermal-shock (PTS) loading produces biaxial stress fields in a reactor pressure vessel (RPV) wall with one of the principal stresses aligned parallel to postulated surface cracks in either longitudinal or circumferential welds. The limited quantity of existing biaxial test data suggests a significant decrease of fracture toughness under out-of-plane (i.e., parallel to the crack front) biaxial loading conditions when compared with toughness values obtained under uniaxial conditions. Any increase in crack-tip constraint resulting from these out-of-plane biaxial stresses would act in opposition to the in-plane constraint relaxation that has been previously demonstrated for shallow cracks. Proposed in this report are criteria for a biaxial specimen that would form the basis of a testing program designed to provide data to explain differences between theoretical predictions and measured material behavior. Results of design studies on the biaxial specimen will be presented in a future report from the Heavy-Section Steel Technology Program.