Evaluation of Fracture Toughness Test Methods for Linepipe Steels
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abstract
Reliable evaluation of fracture toughness resistance curves, including both J–resistance (J-R) and crack tip opening displacement–resistance (CTOD-R) curves, is becoming an integral part of pipeline integrity assessment for demanding applications. Using MTS TestSuite Software with Multipurpose Elite, we have successfully realized the automation of fracture toughness test procedures for single-edge bend [SE(B)] test specimens based on ASTM E1820-11 and BSI 7448. The software offers the option to use either load-line displacement (LLD) or crack mouth opening displacement (CMOD) as the control parameter for both shallow- and deep-cracked SE(B) specimens and exports both the J-R and the CTOD-R curves simultaneously. In order to validate the developed software, we have tested two different pipe steels, namely, X60 and X100 steels. The nominal initial crack lengths a/W after precracking were equal to 0.2 and 0.5 for the shallow- and deep-cracked specimens, respectively. A 10% deep side groove was machined after the precracking on each side of the specimen. The results show that for J-R curves, the differences between the LLD and CMOD methods are negligible within the Jmax criteria range. Beyond this range, however, some differences were observed. There was no significant effect of work hardening rate on the J-R curves obtained using either LLD or CMOD for the two pipe steel materials investigated. However, it is worth noting that there were large discrepancies between the ASTM and BSI methods in terms of CTOD-R curve measurements, especially for the shallow-cracked specimens. This was attributed to the fact that the standards use different definitions of CTOD.
