The digital image correlation method has been applied to directly observe the local strain and crack formation in mooring chain steel during tensile tests in air or when simultaneously charging hydrogen in different ways, viz. including/excluding the notch root. Interestingly, hydrogen accumulation promoted crack initiation on the surface when the local strain reached approximately 0.9%, while strain of 19% to 20% was reached in the hydrogen-free specimens before visible crack formation. Even through the stress-strain curves prior to the sudden drop indicate a negligible effect of hydrogen, its presence can greatly reduce the stress-induced crack initiation. In addition, hydrogen introduced while avoiding the notch root can disorganize the regular distribution of strain produced by the notch before crack initiation. Therefore, hydrogen-induced embrittlement below the critical stress criterion can be explained by hydrogen-enhanced localized plasticity, while above the critical stress criterion, hydrogen-enhanced decohesion provides a better explanation.
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