The objective of this study was to investigate the out-of-plane tensile strength of graphite/epoxy laminates as a function of porosity. An experimental test program was designed to apply tension to the faces of circular graphite/epoxy specimens in a direction perpendicular to the laminate mid-plane. The specimens were removed from the webs of angle sections fabricated by Lockheed Georgia Company using (AS4/1806 and AS4/3501-6 graphite/epoxy material systems with a stacking sequence of (±45/90₂/ ±45/0₂)_S or (±45/0₂/ +̅ 45/90₂)_S. The specimen porosities were the result of four distinct processing methods: a baseline hand lay-up, low pressure cure-cycle, a solvent wipe of pre-preg to remove resin, and the addition of water between pre-pregs. The experimental results have shown a significant reduction in the out-of-plane tensile strength as a function of increasing void content. The volume fraction of pores, pore geometry, size, and orientation were determined for a representative number of specimens by metallography and optical analysis methods. This data was combined with the out-of-plane tensile data and used in the theoretical model, prepared by Brown et al, to predict the out-of-plane strength as a function of porosity. The predicted strength values compared very well with the experimental data when the pores were found to be uniformly distributed throughout the laminate. / Master of Science

Submitted in fulfilment of the degree of Doctor of Dental Material Science in the Department of Dental Services, Faculty of Health Sciences, Durban University of Technology, Durban, South Africa, 2008. / Introduction: Incorporating a feldspar chemical bond between alumina filler particles is expected to increase the wear resistant and flexural strength properties, while reducing flexibility of dental composites. Aims and Objectives: An investigation was carried out to evaluate the influence of the feldspar chemical bonding between alumina filler particles on wear, flexural strength and flexibility of experimental alumina/feldspar dental composites. It was hypothesized that wear resistance and flexural strength would be significantly increased with increased feldspar mass, while flexibility was expected to decrease. Methods: Alumina was chemically sintered and bonded with 30%, 40%, 50% and 60% feldspar mass, silanized and infiltrated with UDMA resin to prepare the dental restorative composite material specimens. Results and conclusions: Significantly higher wear resistant characteristics resulted with increased feldspar mass (p<0.5). Improvements in flexural strength characteristics as the feldspar mass was increased was not statistically different (p>0.5). Flexibility characteristics as the feldspar mass was increased was not statistically different (p>0.5). The alumina/feldspar specimens showed lower flexibility (mm displacement) than SR ADORO (p<0.05). Feldspar chemical bonding between the alumina particles may improve on the wear resistance and Flexibility of alumina/feldspar composites when compared to SR ADORO. This study evaluated the influence of a chemical feldspar bond between alumina filler particles.

This thesis mainly treats the fabrication and applications of micro-optical elements in the semiconductor materials gallium arsenide (GaAs) and diamond. The recent trend in high-capacity data transfer using light as the information carrier creates new demands on the optoelectronic systems, such as small size, low cost and the integration of many components. Micro-optical components are key elements for building compact optoelectronic systems and are well suited for integration with other devices. Another area where micro-optical elements can play an important role is the use of lasers in medicine, industrial machining, metrology, etc. In most cases, the laser beam characteristic is not directly suited for the application and external optics is needed to focus, shape or split the laser beam. In the first part of this thesis, the fabrication of continuous-relief diffractive optical elements, such as diffractive lenses and blazed gratings, in GaAs is examined. The manufacturing technology uses electron-beam lithography followed by plasma etching in an inductively coupled plasma etching system. In the next step, these diffractive elements were monolithically integrated with vertical-cavity surface-emitting lasers. In the second part of this thesis a novel topic is examined, diamond micro-optics. Diamond is a unique material in many aspects, it is the hardest material mankind knows, it has an extremely wide optical transmission window, and it possesses the highest thermal conductivity of all solids. Until today, due to difficulties in machining diamond, the realization of diamond optics has been limited. By using the same technology we earlier developed for the fabrication of GaAs optics we demonstrate for the first time continuous-relief structures in diamond of optical quality. Several diamond micro-optical structures are presented; sub-wavelength gratings for reduction of unwanted Fresnel reflections, diffractive fan-out elements used to split a CO2-laser beam and refractive microlens arrays. The accuracy of the fabrication process by plasma etching was evaluated by optical and topographical measurements, in all cases the optical components were of very high quality.

Tesi per compendi de publicacions. La consulta íntegra de la tesi, inclosos els articles no comunicats públicament per drets d'autor, es pot realitzar prèvia petició a l'Arxiu UPC / Tetragonal polycrystalline zirconia stabilized with 3 mol% of yttria (3Y-TZP) is a biocompatible ceramic showing superior mechanical properties, which are partly the consequence of phase transformation: the tetragonal metastable phase can transform, with a net volume increase, to the stable monoclinic phase by a martensitic transformation. The transformation can be activated either mechanically by the application of high stresses, or chemically by the diffusion of water species, when the material is exposed to humid environment and moderate temperatures. In the first case, the local transformation at the crack tip under tension hinders their propagation, producing a phenomenon known as transformation toughening, which allows the design of damage-tolerant ceramics. On the other hand, the spontaneous and progressive surface transformation in presence of humidity represents an aging phenomenon known as low-temperature degradation (LTD), hydrothermal degradation or aging. Affecting a surface layer of only few micrometers, this phenomenon is accompanied by a substantial impairment of the surface integrity, representing a serious issue for the use of 3Y-TZP in load-bearing biomedical applications. It was shown in orthopedics that zirconia femoral heads used in arthroplasty may be vulnerable to LTD, leading in some cases to premature failures. Likewise, some recent publications devoted to dental zirconia have highlighted that it is susceptible to LTD if exact processing conditions are not followed. The first subject of the thesis is the study of reliable solutions to avoid hydrothermal degradation of 3Y-TZP. Two novel methods are proposed, which allow a strong enhancement of the aging resistance through limited changes in the processing. These methods are based on co-doping the material from the surface with CeO2 by two different approaches. One includes infiltration of CeO2 precursors in the pre-sintered porous material. During thermal decomposition and sintering, CeO2 is trapped into the pores and diffused into 3Y-TZP, obtaining a functionally-graded material with decreasing CeO2 content from the surface to the bulk. The co-doping profile has been optimized by studying the pre-sintering, infiltration and sintering parameters in order to avoid a drop in the mechanical properties generally found in CeO2-doped zirconia. The second approach is applied to the dense material, where surface roughness has been created for adhesion or osseointegration purposes. After pressure infiltration of CeO2 precursors into the surface defects and pores created during the roughening process, a few-micrometers thick co-doped layer is obtained with a diffusion treatment. This helps sealing the surface defects and avoids hydrothermal degradation, without affecting the color or the mechanical properties, being so directly applicable in the manufacturing of dental crowns, abutments and dentures. The second subject focuses on small-scale testing of zirconia near-surface regions. The existence of extrinsic size effects on the mechanical properties has been investigated, and the mechanical response of the degraded and non-degraded state has been compared in bending and compression. By testing micropillars and micro-cantilevers milled with focused ion beam (FIB), higher strength and strain at failure have been recorded with respect to the bulk state, as a result of transformation-induced plasticity and the absence of processing defects. No size effect has been found in terms of strength among small-scale samples, whereas the "yield" stress for phase transformation is lower for smaller samples. Hydrothermal degradation produces a microcrack network which controls the behavior and strongly impairs the mechanical properties of small-scale samples milled inside the degraded layer. In bending, a different response in terms of strength and stiffness has been measured depending on sample orientation, proving that that the induced damage is anisotropic. / El óxido de zirconio tetragonal estabilizado con 3 mol% de itria (3Y-TZP, o simplemente circona) es un material cerámico biocompatible de altas prestaciones mecánicas. Esto se debe, en parte, a la transformación de la fase tetragonal metastable en la fase monoclínica, acompañada por un incremento de volumen. Esta transformación martensítica se puede activar mecánicamente mediante esfuerzos elevados, o bien químicamente por la difusión de especies acuosas, cuando el material está expuesto a ambiente húmedo y temperaturas moderadas. En el primer caso, la transformación local que ocurre en la punta de las grietas bajo tensión permite el diseño de cerámicos tolerantes al daño (aumento de tenacidad por transformación). Por otra parte, la transformación espontánea y progresiva de la superficie en presencia de humedad es un fenómeno conocido como degradación hidrotérmica o envejecimiento. Aunque afecta una capa superficial de unas pocas micras, el envejecimiento reduce de manera substancial la integridad superficial, representando un problema para aplicaciones biomédicas estructurales. Se ha demostrado en ortopedia que las cabezas femorales de 3Y-TZP pueden ser vulnerables a la degradación hidrotérmica, llegando en algunos casos al fallo de la prótesis. En años recientes, la circona está siendo empleada en el campo dental, donde se ha visto que puede sufrir envejecimiento. La primera parte de esta tesis trata de desarrollar soluciones viables para evitar el envejecimiento. Se han propuesto dos nuevos métodos, que con pequeños cambios en el procesado permiten una fuerte mejora en la resistencia a la degradación. Estos están basados en el co-dopaje del material desde la superficie con CeO2 con dos estrategias diferentes. Una incluye la infiltración de precursores de CeO2 en el material presinterizado, que es poroso. Durante la descomposición térmica y el sinterizado, parte del CeO2 se queda atrapado en los poros y difunde en el retículo de la TZP, obteniendo un gradiente de composición con contenidos decrecientes de CeO2 desde la superficie hacia el interior. Se ha optimizado el gradiente estudiando los parámetros de pre-sinterizado, infiltración y sinterizado para evitar una posible caída en las propiedades mecánicas que se encuentra generalmente en la circona estabilizada con CeO2. La segunda estrategia se aplica en materiales densos donde se haya inducido rugosidad superficial para favorecer la adhesión o la osteointegración. Después de infiltrar a presión los precursores de CeO2 en los defectos y poros presentes en la superficie, se ha creado una capa co-dopada de pocas micras de espesor con un tratamiento de difusión. Esto contribuye en eliminar los defectos superficiales y evitar el envejecimiento sin modificar el color o las propiedades mecánicas, siendo directamente aplicable al procesado de coronas dentales, pilares y dentaduras postizas. La segunda parte de la tesis está enfocada en estudiar las las propiedades mecánicas de regiones superficiales de 3YTZP mediante ensayos de muestras microscópicas, para evaluar la presencia de efectos de escala y comparar la respuesta del material antes y después del envejecimiento. En micropilares y microvigas mecanizados con haz de iones focalizados se han medido resistencias y deformaciones de rotura mucho más altas que en probetas macroscópicas, gracias a la plasticidad inducida por deformación y a la ausencia de defectos de procesado. No se han encontrado efectos de escala entre las muestras microscópicas en cuanto a la resistencia, mientras que el límite de elasticidad donde aparece la transformación de fase es menor para muestras más pequeñas. El envejecimiento produce una red de microgrietas que controla el comportamiento y afecta las propiedades mecánicas de muestras microscópicas mecanizadas dentro la capa degradada. En flexión se ha medido una respuesta diferente en términos de resistencia y rigidez según la orientación, demostrando que el daño producido es anisotrópico

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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The dynamic response of subsurface cracks in fiber reinforced composites is analytically studied. The response of layered half-space and three-layered plate with two interface cracks excited by a plane SH-wave and line load respectively are studied by formulating the problem as integral equations in the frequency domain. The governing equations along with boundary, regularity and continuity conditions across the interface are reduced to a coupled set of singular integral equations by using Betti's reciprocal theorem along with the Green's functions. In addition, the transient response of an orthotropic half-space with a subsurface crack subjected to inplane line load at an arbitrary angle is analyzed. Two new Green's functions for the uncracked medium are developed and used along with the representation theorem to derive the scattered field. Satisfaction of the traction free condition at the crack surfaces gives rise to a system of singular integral equations. Singular integrals involved in the analysis are computed numerically by removing the poles. Part of the integrals containing the poles are then obtained analytically by using residue theorem. The solution of singular integral equations are obtained by expanding the unknown crack opening displacements (COD) in terms of a complete set of Chebychev polynomials. The problem is first solved in the frequency domain, the time histories are then obtained numerically by inverting the spectra via Fast Fourier Transform (FFT) routine. Numerical results are presented for isotropic and anisotropic materials for several different crack geometries. The results show significant influence of crack geometries and material properties on the COD and surface response of composites.

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