2026 Pohl Delayed Hydrogen Cracking Fractography

Delayed hydrogen cracking represents a critical damage phenomenon in metallic materials, characterized by a temporal separation between hydrogen uptake and crack initiation. This time dependency often complicates failure analysis, as cracking may occur hours, days, or even years after manufacturing or commissioning. While hydrogen embrittlement has been extensively studied, the mechanisms governing delayed cracking are frequently discussed in isolation rather than as an integrated process. This paper presents a mechanistic framework for delayed hydrogen cracking by systematically linking hydrogen uptake, hydrogen configurations, stress-assisted hydrogen redistribution, and hydrogen-induced damage mechanisms. It is shown that the mere presence of hydrogen is insufficient to cause delayed cracking. Instead, the phenomenon arises from the ability of hydrogen to remain mobile or to be re-activated from initially benign configurations, followed by time-dependent redistribution toward regions of elevated mechanical stress and thus strain. Only when a critical local hydrogen concentration is reached do hydrogen-enhanced damage mechanisms such as hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity become operative. The resulting fracture morphologies are discussed as the fractographic manifestation of these time-dependent processes rather than as indicators of a single dominant mechanism. Differences in fracture appearance across various material classes are addressed in this context. Finally, selected technical failure cases are presented to illustrate how hydrogen introduced during manufacturing or service can lead to delayed cracking under practical operating conditions. By integrating fundamental hydrogen–material interactions with real-world failure examples, this paper provides a comprehensive understanding of why hydrogen-induced cracking may occur long after hydrogen exposure and offers a structured basis for failure analysis and risk assessment of hydrogen-exposed components.