Thesis (M. Sc.)--University of Hong Kong, 1998. / Includes bibliographical references (leaf 107-110).

The Portevin-le Chatelier effect in a copper-3.2 at.% tin alloy has been used to investigate the rate of vacancy production during reversed plastic flow. The production rate per unit strain has been shown to be approximately half the value found for straight tensile deformation. It was inferred from this that fatigue is not a highly efficient method for producing vacancies. The relationship between the dislocation density and the tensile plastic strain for this alloy has been determined by transmission electron microscopy. It was found to obey the law, p = const. E^1.17±0.13. It was also found that during reversed plastic flow this law was no longer valid. / Thesis / Master of Science (MSc)

The purpose of this thesis was to evaluate orthogonal cutting as a method to determine the fracture toughness of low and high density polyethylene and wood plastic composites. A test fixture was developed to capture normal and tangential cutting forces at variable depth and rake angle. A tool interface method (TIM) is proposed to separate forces on both sides of the tool from the energy needed to propagate the crack. Also investigated were shear plane models of chip failure, which seek to measure toughness by modeling internal stresses in the chip. These results are compared to current methods of determining essential work of fracture (EWF). It is found that cutting requires much less energy than current methods of EWF. Further work is suggested to better parameterize failure. / Graduation date: 2013

Material failure plays an important role in human life. By investigating the failure mechanisms, people can more precisely predict the failure conditions to develop new products, to enhance product performances, and most importantly, to save lives. This work consists of three parts corresponding to three different failure mechanisms in metals, i.e., the localized necking in sheet metals, the bifurcation in bulk and sheet metals, and the ductile fracture induced by the void nucleation, growth, and coalescence. The objective of the first part is to model the localized necking in anisotropic sheet metals to demonstrate that localized geometric softening at a certain stage of deformation rather than the initial defects is the main cause of localized necking. The sheet is assumed to have no initial geometric defects. The deformation process is divided into two stages. The critical strains for a neck to form are obtained from a Considère-type criterion. The defect ratio at the neck formation is obtained using an energy-based approach. The neck evolution is considered. A novel failure criterion is proposed. Two types of necks are fond to be most competitive to cause material failure during continued deformation. The forming limit curves are hereby found to exhibit different characteristics in different region. The predicted forming limit curve for 2036-T4 aluminum is found to fit with the experimental results well. The sheet thickness, the strain hardening behavior, and plastic anisotropy are found to affect the sheet metal formability. More realistic yield criterions and strain hardening behaviors can be implemented into the proposed model. This part provides an alternative approach to modeling the localized necking in anisotropic sheet metals. The objective of the second part is to model the bifurcation in anisotropic bulk and sheet metals to couple plastic anisotropy and the strain hardening/softening behavior and also to identify different bifurcation modes in sheet metals. The material is assumed to exhibit a non-linear strain hardening/softening behavior and to obey the Hill-type Drucker-Prager yield criterion along with a non associated flow rule. The constitutive relations and the conditions for bifurcation in bulk and sheet metals are derived. The internal friction coefficient, plastic anisotropy, the terms introduced by the co-rotational stress rates, and the terms introduced by the stress resultant equilibrium are found to affect the onset of bifurcation. Two bifurcation modes are found to exist in sheet metals. More realistic material properties can be implemented into the proposed model. This part provides an applicable approach to modeling the bifurcation in anisotropic bulk and sheet metals. The objective of the third part is to derive the constitutive relations for porous metals using generalized Green’s functions to better understand the micromechanism of the ductile fracture in metals. The porous metals are assumed to consist of an isotropic, rigid-perfectly plastic matrix and numerous periodically distributed voids and to be subject to non-equal biaxial or triaxial extension. Two types of hollow cuboid RVEs are employed represent the typical properties of porous metals with cylindrical and spherical voids. The microscopic velocity fields are obtained using generalized Green’s functions. The constitutive relations are derived using the kinematic approach of the Hill-Mandel homogenization theory and the limit analysis theory. The macroscopic mean stress, the porosity, the unperturbed velocity field, and the void distribution anisotropy are found to affect the macroscopic effective stress and the microscopic effective rate of deformation field. The proposed model is found to provide a rigorous upper bound. More complicated matrix properties (e.g., plastic anisotropy) and void shapes can be implemented into the proposed model. This part provides an alternative approach to deriving the constitutive relations for porous metals.

Erneuerbare Werkstoffe (z.B. WPC) können metallische substituieren, indem eine Kombination aus technischen, wirtschaftlichen und ökologischen Vorteilen in Bauweise und Bauteilen in der Intralogistik Anwendung finden. Vorgestellt werden die wichtigsten Schritte zur Entwicklung eines Hängefördersystems mit einem Trag- und Gleitprofil aus WPC. Schwerpunkte sind die Verbindungsstellen, die Kurvensegmente und die Weiche. Es erfolgt eine Beurteilung der Gebrauchstauglichkeit des Gesamtsystems.

The main aim of this master's thesis is the world ocean pollution by plastic waste. The introductory part of the thesis contains general information about the production of plastics and plastic waste, followed by data on ocean pollution by plastic waste including the estimated amount, pollution sources, and composition. Part of the thesis consists of the introduction of known concepts that are already actively looking for a solution to marine waste and of their mutual comparison. The following part deals with the plastic waste management, the possibilities of its treatment and utilization. The last area is the draft of an alternative system that is focused on an effective solution in accordance with environmental aspects. Within this draft, a material and economic balance is made.

The project developed in terms of engineering study of field N2307- 00 Engineering Technology submits a design of components from plastic production technology- cover on the table. The introductory part of this study describes characteristics, division and possible plastics processing. The work consists of production technology proposal (injection into metal mold) and its construction solution. The another part proposes suitable injection material and verification of injection machine. The final part of work consists of financial evaluation of injection process, appropriate blue prints and appendices.

This thesis is concerned with the appropriate design of manufacturing technology for plastic program disc including the design of plastic printing technology. At the beginning of this thesis was made an analysis of requirements for manufactured device including selection of the most appropriate technology for its manufacturing – plastic injection. In the theoretical part are described all skills needed for designing and construction of the injection mould. Partial theoretical parts are concerned with the issue of plastic materials, tools and engines for injection, and of plastic printing technology. Practical part is concerned with description of design of each single system inside the injection mould including calculation. On the basis of these calculations was selected the appropriate injection engine. The end of the practical part is concerned with the selection of the appropriate engine for printing on the plastic device which is manufactured by the technology of plastic injection. For construction of the injection mould were applied normalities of the Hasco Company and the 3D CAD system Solidworks. The conclusion of this thesis achieves the technical- economical evaluation for manufacturing of designed plastic device.

In recent years there has been growing environmental concern regarding ‘microplastics’: microscopic plastic granules, fibres and fragments, categorised as <1 or <5 mm diameter. Microplastics are manufactured to be of a microscopic size, or derive from the photo- and mechanical degradation and subsequent fragmentation of larger plastic litter. Microplastics debris has been identified in the water column and sediments of marine and freshwater ecosystems across the globe, although difficulties in sampling and isolating smaller particulates has resulted in the abundance of <333 µm microplastics being under-reported. Microplastics are bioavailable to a range of aquatic organisms, including fish, seabirds and benthic invertebrates, and can be trophically transferred. The consumption of plastic debris can result in gut blockages, heightened immune response and a loss of lipid reserves. The potential risk to food security, and thereby human health, has led regulators to call for better understanding of the fate and effects of microplastic debris on marine life. Here I tested the hypothesis that microplastics can be ingested by and may negatively impact upon zooplankton. Zooplankton encompass a range of aquatic animals that form a key trophic link between primary producers and the rest of the marine food web. I used a suite of feeding experiments, bio-imaging techniques and ecotoxicological studies to explore the interactions and impacts of polystyrene microplastics on marine zooplankton. My results demonstrate that a range of filter-feeding zooplankton taxa, including copepods and bivalve and decapod larvae, have the capacity to ingest microplastics. Microplastics significantly reduced algal feeding in the copepods Centropages typicus and Calanus helgolandicus. With prolonged microplastic exposure C. helgolandicus produced smaller eggs with reduced hatching success, and had reduced survival owing to declining energetic reserves. Microplastics egested by copepods significantly altered the properties and sinking rates of faecal pellets, with potential repercussions for marine nutrient flux. This investigative work highlights that microplastics pose a significant risk to the health of animals at the base of the marine food web.

Call number: LD2668 .T4 1979 T62 / Master of Science