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Title A Framework for Modeling Discrete Deformation Twinning in Hexagonal Crystals
Publication Date
Date Accessioned
Degree PhD
Discipline/Department Mechanical Engineering
Degree Level doctoral
University/Publisher Cornell University
Abstract Modeling the plastic deformation of metals has historically been achieved by considering only crystallographic slip, the dominant mode of plastic deformation. While sufficient for materials that deform primarily by means of slip, many metals may exhibit other modes of plastic deformation, and thus may not be accurately modeled by slip alone. Deformation twinning is another mode of plastic deformation, characterized by a rapid, large uniform shear of a discrete region of material, coupled with a reorientation of the crystal lattice within said region. While witnessed in metals of various crystal symmetries, metals comprised of hexagonal crystals are especially prone to exhibit twinning, as they may require twinning to accommodate generalized plasticity. Due in large part to limitations in computational capabilities, models have often ignored deformation twinning. Existing models rely on the homogenization of the responses due to both slip and twinning via a modified Taylor hypothesis, and thus fail to predict accurate local states. Additionally, these models consider twin systems as modified slip systems, obscuring the discrete nature of deformation twinning, as well as the disparity in relative speeds at which each deformation mode propagates. Advances in computational capabilities and model frameworks have allowed for the possibility to study this deformation mode in more detail. A parallelized finite element framework is uniquely suited to approach this problem, as a proven platform for modeling high fidelity, finely discretized representations of polycrystalline aggregates. A framework is presented, in which grains within a microstructure are pre-discretized - based on their crystallographic orientation - into discrete regions that may deform by deformation twinning. A boundary value problem is solved, in which the displacement of the nodes within a twin region are rapidly mapped to their twinned location, the region's crystal lattice is reoriented via three separate schemes, and the remainder of the body deforms by means of crystallographic slip to accommodate this deformation. In this way, the extended framework retains the characteristic differences between crystallographic slip and deformation twinning in a way that existing models do not. Work is calculated due to the changes in local environments due to twinning. Changes in local stress states are discussed in light of global and local work measures and various parameters, including twin size and reorientation schemes.
Subjects/Keywords Crystal Plasticity; Mechanical engineering; Finite element method; Hexagonal symmetry; Solid mechanics; Twinning
Contributors Dawson, Paul Richard (chair); Baker, Shefford P. (committee member); Miller, Matthew Peter (committee member)
Language en
Country of Publication us
Record ID handle:1813/59581
Repository cornell
Date Retrieved
Date Indexed 2020-09-09
Grantor Cornell University
Issued Date 2018-08-30 00:00:00

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