Role of stress triaxiality in assessing the fracture behaviour of cracked components

Transferability of the specimen J-R/J-T curve to the component level is an important issue in the field of fracture mechanics. Towards this goal, fracture experiments have been carried out on three-point bend bar (TPBB), compact tension (CT) specimens and real life piping components with through-wall flaws. The pipe material is SA 333 Gr 6 steel (low strength and high tough material) and specimens are machined from the pipes. Subsequently, 3D elastic-plastic finite element analyses have been carried out using WARP3D on these cracked components/ specimens in order to evaluate the stress triaxiality conditions ahead of crack tip. It is found that the triaxial conditions ahead of crack tip for these cracked components are similar. Hence, similar fracture behavior is expected for these components. A novel numerical methodology is proposed to consider crack growth in prediction of load-deflection behavior of specimen/components of similar stress triaxialities. In addition to NUREG-1061 J-R curve extrapolation method beyond test range, a new three parameter power law fit equation is also proposed in this paper. Consequently, the load-deformation behavior of piping components is predicted using extrapolated specimen J-R curve that exhibits the similar triaxiality level as that of the cracked components. On the other hand, one of the pipe J-R curves is also used as a reference J-R curve to consider the crack growth in the analysis and the load-deformation behavior of other pipes/elbows is predicted. The predicted results are in good agreement with the experiments. (orig.)