Thesis (Ph. D.)--School of Chemical Engineering, Georgia Institute of Technology, 2004. Directed by Sue Ann Bidstrup. / Includes bibliographical references (leaves 165-168).

The main objective of this thesis is to investigate and quantify the influence of parent-twin and twin-twin interactions on the mechanical response of hexagonal close-packed metals. To study parent-twin interactions, a mean-field continuum mechanics approach has been developed based on a new twinning topology in which twins are embedded in twinned grains. A first model generalizing the Tanaka-Mori scheme to heterogeneous elastic media is applied to first and second generation twinning in magnesium. In the case of first generation twinning, the model is capable of reproducing the trends in the development of backstresses within the twin domain as observed experimentally. Applying the methodology to the case of second-generation twinning allows the identification, in exact agreement with experimental observations, of the most likely second-generation twin variants to grow in a primary twin domain. Because the elastic behavior assumption causes internal stress level magnitudes to be excessively high, the first model is extended to the case of elasto-plasticity. Using a self-consistent approximation, the model, referred to as the double inclusion elasto-plastic self-consistent (DI-EPSC) scheme, is applied to Mg alloy polycrystals. The comparison of results obtained from the DI-EPSC and EPSC schemes reveals that deformation system activities and plastic strain distributions within twins drastically depend on the interaction with parent domains. The influence of twin-twin interactions on nucleation and growth of twins is being statistically studied from zirconium and magnesium electron back-scattered diffraction scans. A new twin recognition software relying on graph theory analysis has been developed to extract all microstructural and crystallographical data. It is capable of identifying all twinning modes and all twin-twin interaction types occurring in hexagonal close-packed materials. The first results obtained from high purity Zr electron back-scattered diffraction maps reveal that twin-twin interactions hinder subsequent twin nucleation. They also show that mechanisms involved in twin growth may differ significantly for each twinning mode. A second study performed on AZ31 Mg presents statistics about low Schmid factor {10-12} tensile twins and about {10-12}-{10-12} sequential double twins coupled with a simplified version of the Tanaka-Mori scheme generalized to heterogeneous elasticity with plastic incompatibilities.

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The demand for renewable load-carrying materials is increasing with increasing environmental awareness. Alternative sources for materials manufacturing and design have to be investigated in order to replace the non-biodegradable materials. The work presented in this thesis investigates structure-property relations of such renewable materials based on cellulose nanofibrils. Cellulose is the most abundant polymer on earth and exists in both ordered and disordered phases, where the ordered crystalline cellulose shows excellent mechanical properties. The celluloses nanofibril is composed of partly crystalline cellulose where the stiff crystal regions, or crystallites, are orientated in the axial direction of the fibrils. The cellulose nanofibrils have a high aspect ratio, i.e. length to diameter ratio, with a diameter of less than 100 nm and a length of some micrometres. In the presented work, different properties of the cellulose nanofibril were studied, e.g. elastic properties, structure, and its potential as a reinforcement constituent. The properties and behaviour of the fibrils were studied with respect to different length scales, from the internal structure of the cellulose nanofibril, based on molecular dynamic simulations, to the macroscopic properties of cellulose nanofibril based materials. Films and composite materials with in-plane randomly oriented fibrils were produced. Properties of the cellulose nanofibril based materials, such as stiffness, thickness variation, and fibril orientation distribution, were investigated, from which the effective elastic properties of the fibrils were determined. The studies showed that a typical softwood based cellulose nanofibril has an axial stiffness of around 65 GPa. The properties of the cellulose nanofibril based materials are highly affected by the dispersion and orientation of the fibrils. To use the full potential of the stiff fibrils, well dispersed and oriented fibrils are essential. The orientation distribution of fibrils in hydrogels subjected to a strain was therefore investigated. The study showed that the cellulose nanofibrils have high ability to align, where the alignment increased with increased applied strain.

Thesis (Ph. D.)--Ohio State University, 2008. / Title from first page of PDF file. Includes bibliographical references (p. 132-138).

Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Dr. Arun Gokhale; Committee Member: Dr. Hamid Garmestani; Committee Member: Dr. Karl Jacob; Committee Member: Dr. Meilin Liu; Committee Member: Dr. Steve Johnson.

Thesis (Ph.D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Gokhale, Arun; Committee Member: Gall, Kenneth; Committee Member: Garmestani, Hamid; Committee Member: Kurtis, Kimberly; Committee Member: Thadhani, Naresh

Thesis (M.S.)--Oregon State University, 2004. / Typescript (photocopy). Includes bibliographical references (leaves 85-88). Also available via the World Wide Web.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 191-204). Also available in electronic version.

A theoretical model is derived to study the point-wise stress variations which occur in the constituents of a hybrid 3-D short fiber composite subjected to arbitrary homogeneous loading conditions. The model includes the capability to analyze composites containing different types of fibers, different aspect ratios of fibers (as well as continuous fibers), and different fiber orientations. The composite’s stiffness tensor is developed by volume averaging the point-wise stress field in each constituent present in the material system. Validation of the model is accomplished by comparing predicted stiffness properties to experimental data and other accepted models presently available in the literature for PMC’s, MMC’s, and BMC’s. A derivation of a theoretical model describing the resulting point-wise stress redistribution which occurs in the matrix and the fibrous regions caused by fiber-fiber interaction at the ends of finite length fibers or fractured fibers is also presented. This theoretical development includes the significant dependence of stress redistribution on fiber volume fraction, constituent properties, and crack size. Therefore, its use is not limited to polymeric composites but is also applicable to metal matrix and ceramic matrix systems. The model is extended to include one of the first quantitative analyses of variable fiber spacing which occurs in virtually every composite manufactured. A novel fiber discount method is proposed to study multiple fiber fractures which are of extreme importance when attempting to predict tensile strength of fiber dominated composite laminates. A test methodology employing a macro-model composite with embedded strain gauges is presented which can be used to validate (or invalidate) micro-mechanical models currently being developed and used by the scientific community. Results obtained with the embedded resistance gauges and the embedded fiber optic strain sensors (FP-FOSS) are validated with classical test and analytical techniques. These techniques include model composites subjected to thermal effects and mechanical loading sequences. The ability to vary specific physical parameters in the experimental model, such as fiber aspect ratio, fiber volume fraction, interphase/interface, and constituent properties (i.e. model PMC’s and MMC’s), in a systematic fashion enables this technique to study various physical aspects present in actual composite systems. The capability to initiate a fiber fracture at a specific location and load level is demonstrated. It is revealed that significantly different strain concentration exists in PMC composites which contain different fiber volume fractions and crack sizes. By varying fiber spacing between neighbors, a study is initiated on composites containing eccentrically located fibers. These results demonstrate that an asymmetric stress state exists in composites containing variable fiber spacing and fiber fractures. The fact that multiple fiber fracture is achieved in a methodical fashion demonstrates the versatility of the model. These studies show that this experimental technique can model various physical phenomena which occur in actual composite systems. / Ph. D.