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Simulation of Hydride-Induced crack growth in hydride forming metals under loading and temperature transients

Final Activity Report Summary - HYDEMBMETAL (Simulation of Hydride-Induced Crack Growth in Hydride Forming Metals under Loading and Temperature Transients)

For metals such as zirconium, titanium, niobium, hafnium, uranium, magnesium etc. hydrogen in excess of solid solubility precipitates out as solid hydride phase. These hydride phases are usually brittle and their presence renders the metal weaker. This phenomenon is called Hydride embrittlement (HE) and is of concern for nuclear, aerospace and automobile industry.

Dilute zirconium alloys are used as structural materials in water-cooled nuclear reactors. These materials are subjected to corrosion during service and one of the consequences of the corrosion reaction is the evolution of hydrogen, part of which enters the components made from these materials. HE has been identified as one of the primary failure mechanism for these components. Nevertheless, many features of HE are not well understood. It is therefore of vital importance that the formation of hydrides and various degradation mechanisms are fully understood in order to avoid component failure and its consequences.

Extensive work covering the entire spectrum of hydrogen-induced degradation of mechanical and physical properties of hydride forming metals was performed in the present project. The hydride-induced problem was multi-disciplinary in nature involving thermodynamics, diffusion, phase transformation, solid mechanics, fracture mechanics and computational materials' science. Hence, experimental work was complimented by carrying out simulation of the aforementioned problem using finite element analysis. Issues such as the influence of hydrogen content on fracture toughness (Singh et. al., proceedings of the international congress on advances in nuclear power plants ICAPP06), threshold stress for hydride reorientation (Singh et. al., journal of nuclear materials JNM2006), residual stress variation across component thickness (Singh et. al. JNM2006), computation of temperature and composition dependence of stress free transformation strains of hydrides (Singh et. al., joint analysis centre JAC2007), habit plane of hydride using strain energy minimisation technique, modelling of the stress reorientation behaviour of hydrides, computation of stress field of hydride blister and determination of threshold stress intensity factor for Delayed hydride cracking (DHC) initiation were carried out in the main phase of the project.

In the return phase of the project, the computation of habit plane of hydride using strain energy minimisation technique (Singh et. al. defect and diffusion forum 2008 and water reactor fuel performance meeting WRFPM2008), modelling of the stress reorientation behaviour of hydrides (Singh et. al. WRFPM2008) and computation of stress filed of a hydride blister in zirconium (Zr) alloy (Singh et. al. WRFPM2008) were further refined. The work on understanding the hysteresis associated with Terminal solid solubility (TSS), as presented by Singh in a Board of research in nuclear science (BRNS) theme meeting in 2007, modelling of threshold stress intensity factor associated with DHC (Singh et. al. MSEA2009), computation of stress field of blister forming in a metallic fuel and its interaction with clad (Singh et. al., conference on characterisation and quality control of nuclear fuels CQCNF2009) and modelling of spontaneous fracture of growing precipitates with large misfit strain (Stahle et. al. IJF2009) were carried out as part of the project return phase.

These studies aimed at generating data and improve understanding of the hydride embrittlement problem required for safety analysis, residual life assessment and life extension of components. The results of these studies were compiled in publications of various forms, such as reports and articles in peer-reviewed journals, and were presented in national and international conferences, meetings and workshops.