Hydrogen embrittlement (HE) is widely believed to be harmful to engineering structures made of ferritic steel, particularly in the presence of pre-cracks. However, in this study, the ultimate tensile strength (UTS) of shallow pre-cracked cylinder specimens made of interstitial-free (IF) steel, which represents a standard microstructure of ferritic steel, does not always decrease in the hydrogen environment. Namely, the fracture characteristic is sensitive to hydrogen, but UTS is not under specific conditions. This influence of HE contrary to the common-sense understanding is attributed to the following: (1) the crack propagation assisted by hydrogen-enhanced localized plasticity (HELP) is stable before the onset of plastic instability because of exceedingly-high fracture instability toughness; and (2) the plastic strain localization at the pre-crack tip and secondary crack tips resisted the onset of plastic instability. Additionally, this effect calls into question the general applicability of conventional investigation of HE susceptibility that mainly focuses on the variation of fracture characteristic, which is often defaulted to cause changes in mechanical properties. Here, HE susceptibility is deduced to be depended mainly on geometric properties (geometric HE susceptibility) for shallow pre-cracked structures, while that for deep pre-cracked structures depends mainly on material properties (metallurgic HE susceptibility). Subdividing HE susceptibility helps identify conditions under which plastic strain localization caused by HE susceptibility is beneficial for UTS in fail-safe design.
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