A novel semi-empirical technique for improving API X70 pipeline steel fracture toughness test data

Accurate measurement of K IC values for gas pipeline steels is important for assessing pipe safety using failure assessment diagrams. As direct measurement of K IC was impossible for the API X70 pipeline steel, multi -specimen fracture tests were conducted to measure J IC using three-point bend geometry. The J values were calculated from load -displacement (F-delta) plots, and the associated crack extensions were measured from the fracture surface of test specimens. Valid data points were found for the constructed J-Delta a plot resulting in J IC =356kN/m. More data points were added analytically to the J-Delta a plot to increase the number of data points without performing additional experiments for different J-Delta a zones where test data was unavailable. Consequently, displacement (delta) and crack -growth (Delta a) from multi -specimen tests (with small displacements) were used simultaneously, resulting in the variation of Delta a-delta (crack growth law) and delta-Delta a obtained for this steel. For new Delta a values, corresponding delta values were first calculated from delta-Delta a. Then, corresponding J values for the obtained delta values were calculated from the area under the F-delta record of a full -fractured specimen (with large displacement). Given Delta a and J values for new data points, the developed J-Delta a plot with extra data points yielded a satisfactory estimation of J IC =345kN/m with only a -3.1% error. This is promising and showed that the developed technique could ease the estimation of J IC significantly and reduce the time and cost of expensive extra fracture toughness tests.