Date of Graduation

7-2015

Document Type

Thesis

Degree Name

Master of Science in Mechanical Engineering (MSME)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Spearot, Douglas E.

Committee Member

Nair, Arun K.

Second Committee Member

Dingreville, Remi

Keywords

Applied sciences; Atomistic; Decohesion; Grain boundary; Molecular dynamics; Simulation; Traction-separation

Abstract

The deleterious effects of atomic and molecular hydrogen on the mechanical properties of metals have long been observed. Although several theories exist describing the mechanisms by which hydrogen negatively influences the failure of materials, a consensus has yet to be reached regarding the exact mechanism or combination of mechanisms. Two mechanisms have gained support in explaining hydrogen’s degradative role in non-hydride forming metals: hydrogen-enhanced localized plasticity and hydrogen-enhanced decohesion. Yet, the interplay between these mechanisms and microstructure in metallic materials has not been explained. Accordingly, for this thesis, the three main objectives are: (i) to develop a numerical methodology to extract traction-separation relationships from atomistic simulation data during steady-state crack propagation along a grain boundary, building upon prior work employing atomistic cohesive zone volume elements (CZVEs); (ii) to apply the numerical methodology to specific grain boundary systems with different amounts of hydrogen located at the grain boundary interface; (iii) to further the understanding, based on the traction-separation relationships, of the mechanisms by which hydrogen effects the decohesion of a grain boundary system. A range ofsymmetric tilt grain boundaries in Ni are studied, with hydrogen coverages and favorable sites for hydrogen segregation motivated by Monte Carlo calculations. A sensitivity analysis is performed on the CZVE approach, clarifying the role of CZVE size and numerical parameters necessary to differentiate elastic and decohesion data. Results show that increasing hydrogen coverage can asymmetrically influence crack tip velocity during propagation, leads to a general decrease in the work of separation and promotes a reduction in the peak stress in the extracted traction-separation relationships, though these trends are dependent on the grain boundary structures.

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