Study on FAD curves of steel pipes based on stress-strain curves

The failure assessment diagram (FAD) method is one of the mainstream methods for assessment of defects in steel pipes and girth welds. In this paper, the FAD curves based on Ramberg-Osgood model and tensile test requirements for steel pipelines from X52 to X100 are established with an accuracy of less than 3%. The results show that the generalized FAD curve is more conservative in evaluating high-grade defective pipelines than in evaluating low-grade pipelines. Based on the tensile test data of DIC technology, the FAD evaluation models of the weld, base metal and the HAZ are established, and the critical crack lengths are evaluated respectively combining with CVN impact test data. R6 Option 1 and BS 7910 Option 1 are found to be conservative in the same regional assessment, ranging from 20.83% to 31.09%. This study can provide convenience and reference for defect assessment of pipeline and girth weld.