The relationship between crack-tip strain and subcritical cracking thresholds for steels in high-pressure hydrogen gas

Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. Thresholds for crack extension under rising displacement, K _THi, for crack extension under constant displacement, K_THi*, and for crack arrest under constant displacement K _THa, were identified. These values were not found to be equivalent, i.e. K _THi < K _THa < K _THi*. The hydrogen assisted fracture mechanism was determined to be strain controlled for all of the alloys in this study, and the micromechanics of strain controlled fracture are used to explain the observed disparities between the different threshold measurements. K _THa and K _THi differ because the strain singularity of a stationary crack is stronger than that of a propagating crack; K _THa must be larger than K _THi to achieve equivalent crack tip strain at the same distance from the crack tip. Hydrogen interacts with deformation mechanisms, enhancing strain localization and consequently altering both the nucleation and growth stages of strain controlled fracture mechanisms. The timing of load application and hydrogen exposure, i.e., sequential for constant displacement tests and concurrent for rising displacement tests, leads to differences in the strain history relative to the environmental exposure history and promotes the disparity between K _THi* and K _THi. K _THi is the only conservative measurement of fracture threshold among the methods presented here.