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Fracture in Single and Bicrystals of Zinc: Experiments and Computational Modeling

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Abstract:
In polycrystalline materials where transgranular cleavage is the preferred fracture mode, the crystallographic misorientation of fracture planes across grain boundaries can provide resistance to crack growth, though the details of its contribution to fracture resistance is not fully understood. Recent studies on diverse structural materials such as steels, aluminum alloys and intermetallics show a correlation between an increased fracture resistance and the twist component of grain misorientation; however the lack of control over the degree and type of misorientation has precluded a systematic analysis of the problem. In this dessertation, this phenomenon is investigated through in situ crack propagation experiments across grain boundaries of controlled twist misorientation in bicrystals of zinc. The lack of comprehensive understanding of the micromechanisms of fracture in single crystals of zinc required in situ experiments to be initially conducted to investigate crack propagation on the basal planes. In single crystals, quasistatic loading caused crack propagation in short bursts of dynamic crack extension followed by periods of arrests. In situ observations confirmed re-nucleation of micro-cracks on parallel basal planes and failure of the linking ligaments and crack-growth resistance due to pre-existing twins in the crack path. The crack growth response, load-displacement behavior and fracture surface topography were all found to be dependent on the crack propagation direction on the basal plane. Significant resistance to crack propagation was observed in bicrystals at the grain boundaries through extrinsic toughening mechanisms that come into play upon crack stagnation at the boundary. Strong dependence of the load displacement behavior and crack propagation resistance on the twist angle was observed. Several accommodation mechanisms such as twinning in the crack-wake, strain localization and slip band blocking contribute to fracture resistance and suppress crack propagation across grain boundary at higher twist angles. Three-dimensional finite element models incorporating crystal plasticity are used to explain the micromechanisms of crack propagation, orientation-dependent crack growth response and fracture surface topography in single crystals, and qualitatively capture several features of crack-grain boundary interaction in bicrystals.
Notes:
Thesis (Ph.D.) -- Brown University (2009)

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Citation

Catoor, Dhiraj, "Fracture in Single and Bicrystals of Zinc: Experiments and Computational Modeling" (2008). Engineering Theses and Dissertations, Materials Science Engineering Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.7301/Z0222S6B

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