Transferability of specimen J-R curve to straight pipe with circumferential surface flaw

摘要

Specimen J-R curve is extensively used for structural integrity of large components. It is well known that J-R curve heavily depends on constraint level ahead of crack tip in remaining ligament. In earlier work, it was demonstrated that J-R curve from Three Point Bending (TPB) specimen is transferable to straight pipe with circumferential through wall crack. In this paper, the transferability of J-R curve is investigated from TPB specimen to pipe with circumferential surface crack. A 16 in. diameter pipe with circumferential surface crack and TPB specimen machined from same piping material (SA333Gr6 Steel) are tested. Consequently, 3D finite element analysis (FEA) has been performed on surface cracked pipe and TPB specimen. Crack-initiation load is also predicted for surface cracked pipe by FEA and compared with experimental result. J-R curve is calculated for the pipe using experimental data, that is, load, load line displacement and crack growth. J-R curve of pipe is compared with TPB specimen and it is found that the pipe is predicting much higher J-R curve than TPB. This difference of J-R curve is investigated by evaluating stress triaxiality in remaining ligament for both cases. Stress triaxiality is quantified using triaxiality factor (h) ahead of crack tip for pipe and TPB specimen. It is found that the TPB specimen has considerably higher constraint level than pipe with surface crack, which is well supported by trend of J-R curves for specimen and pipe. A study has also been carried out to investigate the effect of internal pressure on the stress triaxiality. It is found that there is negligible difference in stress triaxiality because of internal pressure. The stress triaxiality is re-established as a qualitative parameter to assess the transferability of J-R curve from specimen to component.