Effect of Applied K Level on the Crack-Arrest Threshold in Hydrogen Environments: Mechanics-Based Interpretation

The objective of this study was to measure the threshold stress-intensity factor (K_TH) at crack arrest for commercial Cr-Mo and Ni-Cr-Mo pressure vessel steels exposed to 100 MPa hydrogen gas using bolt-loaded, wedge-opening-load (WOL) specimens. The primary aim was to evaluate the effects of material variables, such as yield strength, on the measured arrest threshold (K_THa). Another goal was to assess testing procedures to identify any variables that may bias the measurements. The measured K_THa decreased as a function of increasing yield strength, consistent with the well-known effect of strength on hydrogen-assisted fracture. Unexpectedly, the measured K_THa increased as the initial applied stress-intensity factor (K_app) increased. This trend did not result from any compromise in the crack-tip mechanics associated with the dimensions and mechanical loading conditions of the WOL specimens. Rather, it is hypothesized that the K_THa vs. K_app relationship is linked to evolution of the crack-tip strain field from the stationary-crack limit toward the propagating-crack limit as subcritical cracking proceeds. Such strain field evolution is the basis of the conventional crack-growth resistance curve (K vs. crack-growth increment, Δa), which can be viewed as a locus of crack-driving forces that satisfy the strain-based micromechanics criterion for crack extension. It is surmised that an analogous K_TH vs. Δa locus exists for strain-controlled, hydrogen-assisted cracking. Graphical representation of this notional K_TH vs. Δa locus coupled with K vs. Δa trajectories for the WOL specimens can reconcile the K_THa vs. K_app relationships.