Micro- mechanical analysis of hydrogen assisted damage in metallic microstructures

Abstract

Hydrogen-induced loss in structural integrity of metallic materials is a major obstacle to the of future of sustainable and eco-friendly hydrogen-based energy solutions. The intricate interplay between the small size of hydrogen and its complex interaction with microstructural constituents poses formidable challenges in characterizing the site-specific role hydrogen in the microstructure. Consequently, despite years of research, a common understanding of the underlying mechanism contributing to hydrogen-assisted damage remains elusive. As a major part of this thesis work, both numerical simulations and experimental investigations are used to elucidate the underlying micro-mechanisms responsible for hydrogen embrittlement (HE) in Nickel. As the first objective in the present work, a numerical framework comprising the crystal plasticity model coupled with the hydrogen transport model is used to understand the possible failure scenarios emanating from the hydrogen-enhanced localized plasticity (HELP) mechanism of HE. In this two-way numerical framework, while deformation affects hydrogen redistribution, the effect of hydrogen concentration on dislocation activitics is also considered. This first study helped in understanding the mico-mechanics of HE possible under the proposition of the HELP mechanism. In the second objective, an oligocrystal approach is employed to study the transition in failure mode from ductile transgranular to HE-induced brittle intergranular fracture in nickel. To discern the exact role of hydrogen, at first different types of tensile oligocrystals are tested at different strain rates. Thereafter these experimental results are corroborated with a coupled crystal plasticity and phase field fracture model. This combined experimental and numerical approach rationalized the reduction in grain boundary cohesion energy to promote intergranular fracture in H-charged oligocrystals, particularly along the random high-angle grain boundarics (RHAGBs). Similar to the other metallic materials, the hydrogen-rich service environment results in the HE in pipeline steels. HE However, one of the uncertainties associated with evaluating sensitivity of pipeline steels is their high susceptibility to hydrogen-induced blistering/cracking (HIC). In the third and final objective, the role of hydrogen charging-induced blisters on tensile as well as short fatigue crack growth behavior in pipeline X65 grade steel is investigated experimentally. The results in work highlight the significant contribution of hydrogen-induced blisters (developed during the hydrogen-charging process this prior to mechanical loading) in expediting the loss of structural integrity during the external mechanical loading of hydrogen-charged samples. This thesis work contributes not only to the advancement of the mechanics of materials understanding but also to the development of innovative strategies, ultimately fostering the understanding required for a more sustainable and hydrogen-based energy economy.