Why is the modified wedge-opening-loaded test inadequate for characterizing gaseous hydrogen embrittlement in pipeline steels? A review

Abstract

International hydrogen pipeline code ASME B31.12 requires the measurement of hydrogen affected fracture toughness using constant load or displacement tests standardized in ASTM E1681. The modified wedge-opening-loaded (WOL) specimen is loaded with a bolt that sets a constant crack mouth opening displacement and the initial stress intensity factor (KIapp). In this review paper, the sizing restrictions imposed by the testing procedures are analyzed considering the strength, wall thickness, diameter and toughness of existing and commercially available pipelines. The conclusion is that specimens with the standard geometry for constant displacement test in ASTM E1681 cannot be extracted from most used pipelines. The equations proposed in standards to calculate KIapp as a function of crack mouth opening displacement cannot be applied and particular expressions must be calculated case by case. Furthermore, loading the specimen to the required KIapp results in exceedingly large normal stresses in the bolt, increasing the risk of fracture of the bolt during loading of the specimen. Additionally, literature results of pipeline stees tested with the constant displacement in gaseous hydrogen are discussed and their significance is analyzed considering the material performance measured under rising displacement tests in gaseous hydrogen.