Hybrid approach for calculation of J-R curve using R6

Abstract

The fracture property, J-R curve is an essential material property for assessing the cracked structures for unstable ductile tearing. For obtaining this material property, fracture test is conducted and resultant test data namely load, load line displacement (LLD) and relevant crack extension data are post processed to obtain J-R curve. However, in this method, geometry functions η and γ are essential which are available for only limited geometries. To avoid this limitation, recently two approaches based on R6 failure assessment diagram were proposed. First was the load based R6 approach where load vs. crack growth data is converted to fracture property J-R curve using R6 Failure Assessment Diagram (FAD). This method was further modified to displacement based R6 approach where LLD vs. crack extension data is converted to J-R curve using R6 FAD. The predicted J-R curves were in good agreement with conventionally calculated values especially for displacement based approach. However, when these load and displacement based approaches are attempted to be used in case of throughwall cracked elbows, the predicted J-R curve deviated significantly with respect to those obtained using conventional method. On further investigation, it is found that the assumption involved in the displacement based approach is violated for cracked elbows because of severe ovalization of cross section. Hence, a modified approach is proposed in this work where both load and displacement parameter are used for calculation of J-R curves and named as hybrid approach. Further, this approach is adopted for the six through wall cracked pipes which are already investigated by load and displacement based R6 approaches for J-R curves. The predicted J-R curves are found to be in good agreement for all the investigated pipes and elbows. Hence, a simpler yet more accurate hybrid approach based on R6 method is proposed in this paper for post processing the test data to obtain, the material J-R curve.