The impact of hydrogen on crack-tip plastic-zone development was revisited via a novel approach, utilizing the measurement of fatigue crack-growth retardation in a medium-strength martensitic steel after a single overloading in laboratory air and in 90-MPa-hydrogen gas. The plastic zone can be characterized according to the crack-propagation length for reverting from the retardation caused by plasticity-induced crack-closure ascribed to overloading (overloading-affected, crack-growth distance). Hydrogen sharpened the shape of overloaded crack-tip and suppressed the extension of the severely-deformed zone in the crack proximity. Besides, it enhanced frequent crack-tip branching, giving rise to a slower crack growth rate than the in-air situation at the initial stage of retardation. However, no change in the overloading-affected, crack-growth distance was detected between the in-air and hydrogen-gas conditions. Ultimately, hydrogen barely altered the overall plastic-zone size.
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