KYUDAI NEWS KYUSHU UNIVERSITY CAMPUS MAGAZINE Spring 2014 No.25

14/28

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Although the subject has been investigated over several decades, we still lack complete understanding of how the deformation and failure mechanisms acting at the microscale induce macroscopic failure. The fatigue problem becomes even more severe in the presence of gaseous hydrogen which reduces the load intensity for the onset of crack propagation and accelerates the rate at which the failure of materials take place (Fig.1). In addition, there are no models for the design of material components that can operate in a predictive way when exposed to hydrogen. To understand hydrogen induced degradation our team uses methodologies from the disciplines of materials physics, mechanical testing, microstructural characterization, solid mechanics, and computational materials science. Such fusion of disciplines is required because the problem is complex and involves multiple steps, the study and analysis of which requires different scientic principles and tools: dissociation of gaseous hydrogen, surface adsorption, transport within the bulk material, and interaction of the dissolved hydrogen with the deforming material both at the atomic and microstructural level. In the present work, aiming to develop a mitigating strategy for the X52 line pipe steel, our international team found out that a few molecules of oxygen in the hydrogen gas in concentrations of a few atomic ppm can slow down the hydrogen degradation effect and in some case suppress the effect altogether. More specically, the presence of oxygen pushes the onset of hydrogen-accelerated fatigue to higher stress intensity ranges (Fig.1), which means that the component will not sense the hydrogen effect up until a higher measure of loading is applied. The reason for this mitigation effect is that oxygen is preferentially adsorbed on the surface of a microstructural defect (crack) and as a result, the created oxide prevents hydrogen from entering the material and in turn degrade the material resistance to cyclic loading. Based on this fundamental principle, we developed a model that quanties this conditions under which hydrogen acceleration of fatigue takes place. The model is summarized by a compact formula that denes the critical inert environment crack growth rate at which acceleration takes place in terms of the loading frequency, the oxygen partial pressure in the gas mixture, the ratio of the minimum to the maximum load, and the oxide and material properties. Amongst the interesting conclusions of our work is that the presence of oxygen reverses the dependence of hydrogen-induced acceleration of fatigue on the loading ratio and the cyclic frequency: whereas in pure hydrogen, lower loading ratios and higher cyclic frequencies retard hydrogen-induced crack growth rates, higher loading ratios and lower cyclic frequencies are required when oxygen is present in the gas. In addition, an interesting aspect of the proposed model is that it involves the size of the fatigue crack, a fact which can be used to advance our understanding of the notion of environmentally short cracks. The transition to a hydrogen-based economy faces a number of challenges in terms of hydrogen production, transport and delivery to end-user stations, and storage. In particular, hydrogen transport and containment is affected by hydrogen embrittlement which is a severe environmental type of failure that can cause a sudden and catastrophic failure of materials in contact with hydrogen under normally safe working loads. Highlight of Recent ResearchKyudai News No.2513Understanding hydrogen-induced of fatigue crack growth in steelsUsing a few ppm of oxygen to mit Fig.1 Hydrogen-induced fatigue crack growth acceleration: fatigue crack growth rate (da/dN) vs. stress intensity factor range (ΔK) relationships for X52 line pipe steel in mixed H2 + O2 gases (R = 0.1 or 0.5), high-purity hydrogen gas (R = 0.1), and ambient air (R = 0.1 and 0.5).Research SummaryBackground

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