University of Macau / Faculty of Science and Technology. / Department of Civil and Environmental Engineering

Thesis (Master)--İzmir Institute of Technology, İzmir, 2004 / Includes bibliographical references (leaves. 67).

The purpose of this study is to investigate the behaviour of functionally graded materials in the coating design through analytical and numerical work. Functionally graded materials are advanced composite materials formed from two or more constituents with a continuously varying composition, which results in a continuous variation of material properties from one surface of the material to the other. The concept of functionally graded material is actively explored in coating design where structural and/or functional failures of the coating can happen due to a mismatch between the material properties of the coating and substrate, particularly at the coating/substrate interface. This work focuses on the performance of coated plates with homogeneous and graded coatings under various types of loading to develop a better understanding of their response. Firstly, the three dimensional elasticity solution for an isotropic coated plate with a stiffness gradient in the coating is extended to cover different types of applied loading and then a three dimensional elasticity solution for transversely isotropic materials with gradients in elastic properties is also developed. Based on the extended/developed solutions, a MATLAB code is created to produce a model that would enable the analysis of coated plates for a range of material, geometric and loading parameters. To test the analytical models, a finite element analysis is performed using the commercial finite element software ABAQUS, in which a user material subroutine is employed to generate a gradient in the material properties within each element and increase the accuracy of the results. All the developed analytical and numerical models are then used to carry out a comparative study of three-dimensional stress and displacement fields in the coated plates with homogeneous and graded coatings and establish the effect of various parameters such as coating thickness, coating position, plate dimensions, stiffness gradient, loading distributions and anisotropy on the coated plate response.

Piezoelectric actuators have been developed in various forms ranging from discrete layered composites to functionally graded composites. These composite actuators are usually made up of differentially poled piezoelectric ceramics. This study presents analyses of thermo-electro-mechanical response of piezoelectric actuators having combinations of metal and ceramic constituents with through thickness gradual variations of the metal and ceramic compositions. This is done in order to achieve better performance. The piezoelectric ceramic constituent allows for electro-mechanical coupling response and higher resistance to elevated temperatures while the metal constituent provides more ductile composites. The gradual variation in the ceramic and metal composition helps to avoid high stress concentrations at the layer interfaces in composites. A functionally graded composite is analyzed with discrete layers of piezoelectric ceramic/metal composite. Each layer in the functionally graded composite has a fixed ceramic/metal composition. The governing equation for such a piezoelectric functionally composite beam is presented based on a multi-layer Euler-Bernoulli beam model and the overall displacement response of the beam under thermal, mechanical and electrical stimuli is predicted. The variation of this response is studied with respect to functional grading parameter, number of layers, thermal and electrical and mechanical stimuli applied. It is found that the displacement due to thermal and mechanical effects can be mitigated to some extent by the application of an electric field. It is also observed that layers of varying thickness may be assumed to model the functional grading more accurately i.e. use thinner layers where the grading changes rapidly and thicker layers where the grading changes gradually. In addition to the above parametric studies, the change in the material properties with temperature is also studied. It is found that the temperature-dependent material parameters are important when the actuators are subjected to elevated temperatures.

Thesis (M.S.) in Mechanical Engineering--University of Maine, 2005. / Includes vita. Includes bibliographical references (leaves 135-142).

This thesis describes the processing and testing of homogeneous nickel and fully yttria stabilized cubic zirconia interpenetrating phase composites. This work was part of a research program investigating step graded Functionally Gradient Materials. This work was focused on understanding the deformation behaviour of the interpenetrating composites near the percolation threshold of the ceramic phase. The composite grades selected for this study included the pure materials, nickel and zirconia, as well as composites with volume fractions of zirconia of 5%, 10%, 15%, 20% and 25%. These compositions were selected to provide data near the zirconia percolation threshold. Processing of the composites involved tape casting, lamination, organic removal, reduction, and hot pressing. All composites except the 5% volume fraction achieved densities greater than 98% of theoretical. Tensile testing was performed on composite grades up to and including the 20% zirconia material, and flexural testing was carried out on the 25% material and pure zirconia. The maximum tensile strength of 530 MPa was obtained in the 10% material resulting from load transfer to the zirconia phase. Ductility decreased as the volume fraction of zirconia increased, with no macroscopic plasticity above 15% volume fraction zirconia. Hardness tests and compression tests were carried out on all composite grades and the yield stress was determined.The compressive yield stress was found to be related to the hardness by the equation: H=6σy This relationship is a result of the constraint imposed on the nickel phase by the zirconia network. Measurements of damage in one pure nickel sample were also performed. The area fraction of voids as a function of local strain was found to follow an exponential relationship. The Young’s modulus of each material was determined ultrasonically and found to be uniform as expected. Modeling of the tensile specimens indicated that materials above the zirconia percolation threshold work harden more rapidly than those below it. This is not accounted for in the model by Ravichandran. / Thesis / Master of Engineering (ME)

Published Article / This paper looks at the development of composite materials in layered manufacturing. It is known that Rapid Prototyping (RP) using a single material compares poorly with other conventional manufacturing processes when making parts from similar materials. For example, injection moulded parts are over 30% stronger than RP fabricated parts of the same material. The incorporation of secondary materials can result in a composite that can improve this situation. This paper will discuss different composites that are commercially available as well as some into which research is being conducted. An advantage of RP is that composites do not have to be manufactured in a homogeneous manner. Functionally graded parts may be fabricated where reinforcing material can be added in appropriate locations and in required orientations.

No description available.

A functionally-graded Al(subscript)2TiO(subscript)5/ZrO(subscript)2-Al(subscript)2O(subscript)3 (AT/zirconia-alumina) composite has been successfully synthesized by an infiltration process involving an alpha-Al(subscript)2O(subscript)3-ZrO(subscript)2 (90:10 by weight) green body and a solution containing titanium chloride. The mass gain after infiltration has been used to estimate the amount of new phase introduced into the system. The phase composition character of the functionally-graded material (FGM) has been determined by x-ray diffraction. The Rietveld "whole pattern" refinement method was applied to diffraction patterns of the sample which were collected from the surface and at several depths which were made by polishing away the material. Absolute weight fraction determination using the Rietveld external standard method showed that the concentration of AT reduces linearly from the surface to the core. In contrast, the alpha-alumina content increases with depth in a complementary manner. Low level amorphous phase was also observed. Other functionally-graded microstructural profiles examined were x-ray characteristic line intensity of Ti, Ti dot-mapping, and alpha-alumina grain size. The FGM also exhibits graded character in both thermal and mechanical properties, i.e. thermal expansion, microhardness, and Young's modulus. The thermal expansion coefficient (TEC) of the FGM increased with polishing-depth and approached that of the zirconia-alumina reference sample at a depth of 0.5 mm. / Relatively lower thermal expansion and softer surface layer in comparison to those of the core (TEC value of 5.9 x 10(subscript)-6 degrees celsius(subscript)-1 and microhardness of 6 GPa compared to 7.4 x 10(subscript)-6 degrees celsius(subscript)-1 and 12 GPa, respectively) render possibilities to implement the material to which thermal shock resistance surface but hard core, such as a metal melting crucible, are required. Load-dependent microhardness was obviously observed on the surface of the material but only slight dependence was observed in the core. This observation indicated that the material exhibit "quasi-ductile" surface but brittle core. In comparison to the reference specimen, the FGM displayed damage-tolerance and remarkable machinability.

The use of metallic foams as a filler in thin-walled structures can enhance their crashworthiness characteristics. It is believed that, tailoring the properties of the foam filler would enhance the effectiveness of these characteristics. This view is also supported by recent works in the literature. It is the objective of this study to examine the crush behaviour of functionally graded foam-filled tubes and evaluate the effect of discretely graded density upon the specific energy absorbed. Nonlinear parametric finite element simulations of the foam-filled tube were developed to estimate the most favourable foam density gradient in the lateral and axial directions. The effect of various design parameters such as density grading, number of grading layers, and thickness of the interactive layer upon the resulting specific energy absorption was investigated. The results show that the specific energy absorption of a tube filled with functionally graded foam is better than uniform density foam.