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Equal channel angular pressing ECAP is a process whereby simple shear is applied to a billet during multiple passages through an angled channel of constant cross section. The process is capable of generating very large plastic strains that significantly refines the microstructure without altering the external dimensions of the billet. A number of properties are influenced by grain refinement with the generation of a submicron grain structure SMG by ECAP resulting in improved strength and hardness and enhanced superplasticity. In this thesis, both an AA7075 alloy and AA7075 Al-base metal matrix composite MMC reinforced with 5 wt. percent of 50 nm diameter SiC particles was produced by a powder metallurgy route followed by hot extrusion. The materials were subsequently deformed by ECAP at 350 C to a true effective strain of 4.6 in an attempt to refine the microstructure and further distribute the SiC reinforcement phase in the composite. The high temperature microstructural stability of both the as-deformed alloy and composite was investigated to elucidate the effect of the reinforcement phase on continuous and discontinuous grain coarsening. It was found that ECAP generated a fine equiaxed grain size of ~ 2.3 !m and ~1.8 !m in the alloy and composite, respectively. The composite was more refined after ECAP since the SiC particles allow the matrix to undergo more grain refinement during deformation. ECAP was found to be a reasonable method for further distributing SiC clusters in this composite which is important for optimizing the reinforcement phase in terms of ambient temperature strengthening and enhanced grain stability at elevated temperature. Both the alloy and composite were annealed at times up to 5h at 500 C to assess grain stability. During annealing, the grain structure of both materials evolved in a continuous manner unlike the discontinuous process of recrystallization. Such a process is similar to continuous recrystallization observed in a range of heavily deformed Al alloys. Substantial grain boundary interactions with MgZn2 precipitates and oxide particles were found in the alloy, with precipitate, oxide and SiC particles found in the composite. The strong pinning force exerted by these particles minimised grain growth in both materials with the composite exhibiting a finer less than 2.5 !m grain size than the alloy less than 3.5 !m after extended annealing. This enhanced grain stability was attributed to the high volume fraction SiC particles which resulted in a large value of the dispersion parameter f/d which results in significant boundary pinning during annealing. Grain stability was also analysed in terms of a recently-proposed mean field model of annealing where it was predicted that the composite should not undergo discontinuous coarsening, as observed experimentally.

The problem of cavity expansion in unsaturated soils is investigated. A unified constitutive model for unsaturated soils is presented in a critical state framework using the concepts of effective stress and bounding surface plasticity theory. Consideration is given to the effects of suction and particle crushing in the definition of the critical state. A simple isotropic elastic rule is adopted. A loading surface and bounding surface of the same shape are defined using simple and versatile functions. A limiting isotropic compression line exists, towards which the stress trajectories of all isotropic compression load paths approach. A non-associated flow rule is assumed for all soil types. Isotropic hardening/softening occurs due to changes in plastic volumetric strains as well as suction for some unsaturated soils, enabling account of the phenomenon of volumetric collapse upon wetting. Results of isotropic compression tests, oedometric compression tests and drained and undrained triaxial compression tests performed on Kurnell (quartz) sand in saturated and unsaturated states and subjected to stresses sufficient to cause particle crushing are presented and used to calibrate the model. The model is also calibrated using results reported in the literature for triaxial tests performed on saturated and unsaturated speswhite kaolin and three load paths. For both soils the model leads to a much improved fit between simulation and experiment compared to that for models based on conventional plasticity theory. The model is implemented into a cavity expansion analysis using the similarity technique, extended for application to unsaturated soils. Cylindrical and spherical cavities are considered, as are drained and undrained conditions. Cavity expansion results for the bounding surface model and conventional plasticity models are compared for saturated conditions. Substantial differences highlight the importance of adopting a model that accurately describes stress-strain behaviour. Cavity expansion results for the bounding surface model and saturated and unsaturated conditions are also compared. Substantial differences, particularly in the limit pressure, highlight the major influence of suction and the importance of accounting for this when using cavity expansion theory to interpret results of the cone penetration and pressuremeter tests.

Even though the elastic deformations that occur during the impact of colliding bodies may be small in comparison to their actual dimensions, they play an important role in mechanical collisions. During the time the bodies are in contact, elastic, friction, and inertia properties combine to produce a complex variation of sliding and sticking throughout the contact surface. Detailed analysis of this interaction is quite tedious, but would seem to be necessary for accurately predicting the impulse and velocity changes that occur during contact. However, a considerably-simplified model captures the essential characteristics of the elastic-friction interaction during contact, leading to predictions of impulse and velocity changes that agree well with those of more detailed analyses of a number of different collisions. The model's simplicity enables an examination of parameters that affect a general class of collisions. For planar collisions, the model contains five dimensionless parameters; the effects of four of these on the rebound velocity are examined here. In addition, comparisons are made with a previously-used, somewhat simpler model, which neglects the tangential compliance in the region of contact. / Graduation date: 1993

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 109-116).

Thesis (Ph. D.)--University of Rhode Island, 2005. / Typescript. Includes bibliographical references (leaves 126-134).

Thesis--M. Sc.(Eng.), University of Hong Kong. / Errata slip inserted. Mimeographed.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Nanostructured materials have attracted intensive scientific interests during the past two decades due to their outstanding physical and mechanical properties. However, the brittleness of nanostructured materials posed a great challenge for their engineering applications. Recently, several strategies were successfully adopted to produce nanostructured materials with both high strength and ductility such as surface-nanocrystallized (SNC) materials, nanocrystalline materials with stress-induced nanograin growth and nanotwinned metals. A lot of molecular dynamics (MD) simulations, modelling and experiments have been conducted to investigate the deformation mechanisms and the correlated exceptional mechanical properties and considerable progress has been made. However, some problems remain unsolved. For example, the complicated structure of SNC materials due to its grain size gradient (GSG) surface layer makes it difficult to establish a quantitative model for prediction of their strength and ductility; the main mode of nanograin growth in nanostructured materials, i.e., shear-coupled migration of grain boundaries (GBs), was experimentally observed as contributing to their enhanced ductility, but the mechanism of the enhancement remains unclear. In addition, there exist contradictory results for the grain size dependence of transitional twin thickness that corresponds to the maximum strength of nanotwinned metals. All these issues should be addressed to gain a better understanding of the mechanism-ductility correlation in order to provide some guidelines for designing lighter, stronger and ductile nanostructured materials. Therefore, an attempt was made to study the plastic deformation of nanostructured materials with high strength and ductility by theoretical modelling and numerical simulations. Firstly, the enhanced balance of strength and ductility of SNC materials was studied using a combination of theoretical analysis and finite element simulation. A criterion was established for determining the ductility of SNC materials. The results obtained showed that the ductility of a SNC sample could be comparable to that of its coarse-grained counterpart, while it simultaneously possessed a much higher strength than that of the latter if optimal GSG thickness and topmost phase grain size were adopted. Then a dislocation-density-based model was proposed to quantitatively predict the plastic deformation of SNC materials; the stress-driven nanograin growth was also incorporated in the said model. The capability of the model in predicting the strength and work hardening of SNC materials was validated by the existing experimental results. Thirdly, physical models for shear-coupled migration of GBs in nanostructured materials were developed to explain the general coupling between the shear and the normal migration of GBs observed in MD simulations and experiments. The coupled migration process was found to be a general and effective toughening mechanism in nanostructured materials. Moreover, our study showed that the shear-coupled migration is able to enhance the intrinsic ductility considerably when it cooperates with GB sliding. Finally, an elastic-viscoplastic constitutive model based on the competition of intra-twin and twin-boundary-mediated deformation mechanisms was proposed to predict the grain size dependent transitional twin thickness of nanotwinned metals. A linear relation between the transitional twin thickness and the grain size was predicted, which was in excellent agreement with the results obtained from MD simulations and experiments available in the literatures. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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