TOPOLOGY OPTIMIZATION USING A LEVEL SET PENALIZATION WITH CONSTRAINED TOPOLOGY FEATURES

Topology optimization techniques have been applied to structural design problems in order to determine the best material distribution in a given domain. The topology optimization problem is ill-posed because optimal designs tend to have infinite number of holes. In order to regularize this problem, a geometrical constraint, for instance the perimeter of the design (i.e., the measure of the boundary of the solid region, length in 2D problems or the surface area in 3D problems) is usually imposed. In this thesis, a novel methodology to solve the topology optimization problem with a constraint on the number of holes is proposed. Case studies are performed and numerical tests evaluated as a way to establish the efficacy and reliability of the proposed method. In the proposed topology optimization process, the material/void distribution evolves towards the optimum in an iterative process in which discretization is performed by finite elements and the material densities in each element are considered as the design variables. In this process, the material/void distribution is updated by a two-step procedure. In the first step, a temporary density function, ϕ*(x), is updated through the steepest descent direction. In the subsequent step, the temporary density function ϕ*(x) is used to model the next material/void distribution, χ*(x), by means of the level set concept. With this procedure, holes are easily created and quantified, material is conveniently added/removed. If the design space is reduced to the elements in the boundary, the topology optimization process turns into a shape optimization procedure in which the boundaries are allowed to move towards the optimal configuration. Thus, the methodology proposed in this work controls the number of holes in the optimal design by combining both topology and shape optimization. In order to evaluate the effectiveness of the proposed method, 2-D minimum compliance problems with volume constraints are solved and numerical tests performed. In addition, the method is capable of handling very general objective functions, and the sensitivities with respect to the design variables can be conveniently computed

Intrinsic osteoinduction and osteogenesis of biomimetic calcium phosphate scaffolds with different nano-, micro- and macroporosities : ectopic and orthotopic implantation in a canine model

The development of synthetic bone substitutes with enhanced osteogenic properties is urged by the global ageing population. Sintered calcium-phosphate (CaP) ceramics are the most widely used synthetic biomaterials for bone regeneration. However, their clinical performance is inferior to those of autografts, which are still considered the gold standard, despite the serious drawbacks associated with the need of a harvesting surgery. This thesis aims at providing new insights in the development of CaP biomaterials with osteoinductive properties, this is, with the capacity to foster the differentiation of mesenchymal stem cells to bone forming cells, without the need of adding exogenous growth factors. Previous studies pointed to chemical composition, macropore architecture, microstructural topography and specific surface area (SSA) as critical factors in the intrinsic osteoinduction of biomaterials. However, only sintered ceramics with a limited range of porosities and low SSAs had been analyzed so far. In the present thesis, we were able to extend this range to the nanoscale by using biomimetic low-temperature processing routes. Foaming and 3D-printing methods allowed producing biomimetic CaP scaffolds with tailored macropore architectures together with controlled micro and nanoporosity and, hence, high SSAs. In order to evaluate the intrinsic osteoinduction of this new family of biomimetic bone substitutes, nanostructured calcium deficient hydroxyapatite (CDHA) scaffolds with needle-like crystal morphology were implanted intramuscularly in a canine model, and compared with two sintered ceramics, namely biphasic calcium phosphate and beta-tricalcium phosphate (Study I). The results showed that the high reactivity of nanostructured biomimetic CDHA, combined with a spherical concave macroporosity of foamed scaffolds, accelerated and enhanced the osteoinduction potential beyond the limits of conventional, microstructured, sintered ceramics. As a second step, the effect of macropore geometry of nanostructured CDHA on the bone healing capacity was analyzed. The same foamed and 3D-printed CDHA scaffolds were implanted intraosseoulsy in a canine model (Study II). Whereas nanostructured CDHA was shown to be highly osteoconductive irrespective of macropore geometry, a superior osteogenic capacity was observed in the foamed scaffolds, which correlated well with the higher intrinsic osteoinductive potential demonstrated previously. Moreover, foams showed a higher cell-mediated degradation than the 3D-printed constructs, with a simultaneous and progressive replacement of the scaffold by new bone, demonstrating that the control of macropore architecture allows tuning both material degradation and new bone formation. Finally, aiming to further mimic the natural bone apatite, the effect of nanocrystal morphology (plate vs. needle) and carbonate doping on the intrinsic bioactivity of biomimetic CDHA was investigated. To this end, CDHA foams with different nanostructures (Coarse/Fine-CDHA) and carbonated CDHA foams were compared, both in canine ectopic and orthotopic implantation models (Study III). Fine-CDHA foams showed a superior osteoinduction and bone healing potential, as well as a higher degradation than Coarse-CDHA foams, suggesting that there is a threshold value in terms of SSA necessary to activate the cell-mediated resorption and the associated osteoinduction, which determines in turn the osteogenic capacity of the materials in a bony enviroment. Moreover, carbonate dopping of CDHA accelerated both intrinsic osteoinduction and bone healing, simultaneously increasing the cell-mediated resorption. Thus, the increased biomimetism of CDHA allowed the material to enter the natural bone remodelling cycle, this resulting in a tight synchronization between material degradation and bone formation, and ultimately, obtaining bone substitutes with enhanced bone regeneration potential.L’envelliment global de la població exigeix el desenvolupament de nous substituts ossis sintètics amb capacitats osteogèniques optimitzades. Tot i que les ceràmiques de fosfats de calci (CaP) sinteritzades són els biomaterials sintètics més utilitzats en regeneració òssia, la seva eficiència és inferior a la dels empelts d’ós autòleg, els quals continuen sent el tractament de primera elecció malgrat presentar inconvenients importants associats a la necessitat d’una segona cirurgia. Aquesta tesi té com a objectiu optimitzar el desenvolupament de biomaterials de CaP amb propietats osteoinductives, fet que estimula la diferenciació de cèl·lules mare mesenquimals a cèl·lules osteogèniques, sense l’ús de factors de creixement exògens. Estudis recents han identificat diferents factors crítics en l’osteoinducció intrínseca dels biomaterials com ara la composició química, la macroporositat, la microestructura i la superfície específica (SSA). Fins al moment, només s’han analitzat ceràmiques sinteritzades amb un rang limitat de porositats i SSAs. Tanmateix, en la present tesi s’ha aconseguit augmentar aquest rang a la nanoescala per mitjà de rutes de processament biomimètiques a baixes temperatures. L’escumat i la impressió 3D de CaP biomimètics, ha permès l’obtenció d’implants amb arquitectures macroporoses específicament modulades conjuntament amb micro i nanoporositats controlades, i per tant, amb SSAs significativament superiors. Per tal d'avaluar l'osteoinducció intrínseca d'aquests nous materials biomimètics, es van implantar intramuscularment materials nanoestructurats (cristalls tipus agulla) de hidroxiapatita deficient en calci (CDHA) en un model caní, i es van comparar amb dues ceràmiques sinteritzades (Estudi I). Els resultats van mostrar que la gran reactivitat de la CDHA nanoestructurada, combinada amb una macroporositat esfèrica còncava de les escumes, van incrementar el potencial d'osteoinducció més enllà dels límits oferts per les ceràmiques sinteritzades microestructurades. El segon pas va consistir en l’anàlisi de l'efecte de la geometria de la macroporositat dels materials de CDHA sobre la seva capacitat de consolidació òssia, implantant els materials escumats i els impresos en 3D a nivell intraossi en un model caní (Estudi II). Tot i que la CDHA nanoestructurada va demostrar ser altament osteoconductiva independentment de la geometria macroporosa, les escumes van mostrar una capacitat osteogènica superior, correlacionant-se directament amb el major potencial osteoinductiu intrínsec demostrat anteriorment. A més, les escumes van mostrar una reabsorció cel·lular superior als implants obtinguts per impressió 3D, substituint progressivament el material per nou os i, demostrant així que el control de l'arquitectura de la macroporositat permet adequar tant la degradació del material com fomentar la regeneració òssia. Finalment, amb l'objectiu de mimetitzar encara més la fase mineral òssia, es va investigar l’efecte de la morfologia dels nanocristalls (placa vs. agulla) i del dopatge amb ions carbonat sobre la bioactivitat intrínseca de la CDHA biomimètica, implantant escumes de CDHA amb diferents nanoestructures (Coarse/Fine-CDHA) i escumes carbonatades a nivell ectòpic i ortotòpic en gos (Estudi III). Les escumes Fine-CDHA van mostrar un potencial osteoinductiu i osteogènic superiors, i una degradació incrementada respecte a les escumes Coarse-CDHA, suggerint que existeix una SSA mínima per activar la degradació cel·lular dels materials i la conseqüent resposta osteoinductiva, fet que determina la capacitat osteogènica dels materials en un defecte ossi. La carbonatació de la CDHA va accelerar tant el potencial osteoinductiu i osteogènic, com la degradació cel·lular dels materials, suggerint que l’increment del biomimetisme de la CDHA afavoreix la introducció del material dins del cicle de remodelació òssi

Soft Shape-memory Polymers As A Platform For Biomedical Applications

The overall objective of this work was to expand on the previous efforts carried out by other researchers to develop series of smart or stimulus-responsive shape-memory polymers for biomaterials applications. For this purpose, novel shape-memory polymers were fabricated and their macrostructure and microstructure were studied to understand their effects on overall shape-memory characteristics and mechanical properties of these materials. The advent of shape-memory polymers has significantly influenced the development and rapid growth of various functional polymers. Shape-memory polymers are used where the dynamic functions of polymers under an applied stimulus are required, and they find applications as catheters, sutures, drug delivery systems, and scaffolds in tissue regeneration, as well as aerospace applications. Each of these applications demands materials with unique chemical, physical, and mechanical properties to provide efficient functions. Consequently, a wide range of shape-memory polymers have been developed and investigated for these applications, but more research is required to optimize the overall property and function of these polymers. Furthermore, recent advances in the field of polymer science and shape-memory polymers, coupled with the novel characterization methods, necessitate the development of novel functional polymers for specific applications. This dissertation highlights various polymeric materials currently investigated for use in applications requiring shape-memory polymers. Chapter 1 gives an overview of biomaterials along with a background on shape-memory polymers. In the first case described in Chapter 2, epoxy-based triple shape-memory composites (TSMCs) were investigated, and the poly(ε-caprolactone) (PCL) compositional effect on triple shape-memory behavior was explored using heat and water stimuli. The TSMCs were developed using PIPS to achieve particle/matrix morphology. In Chapter 3, two newly TSMCs, one featuring a semicrystalline epoxy and the other featuring an amorphous epoxy, were explored, and the relationships between the morphology of TSMCs and their shape-memory characteristics were studied. Chapter 4 focuses on studying the effect of morphology on shape-memory behavior. This study reveals the effect of particle/matrix and co-continuous fiber/matrix morphology on triple shape-memory behavior of polymers with similar compositions. The knowledge, which was built upon the results of Chapter 2 through 4, would help in optimization of design strategy used for fabrication of triple shape memory polymers with enhanced shape fixing and recovery. In Chapter 5, an innovative smart anisotropic polymeric hydrogel was introduced which can be activated using hydration. The developed anisotropic hydrogel forms helicoids in response to hydration; and the dependence of the radius of curvature and the pitch of the formed helicoids on fiber angle orientation and thickness of hydrogel composites was evident. In Chapters 6 and Chapter 7, another class of shape memory polymers was investigated: liquid crystalline elastomer. Chapter 6 focuses on a new design strategy for fabrication of a hydrogel-forming liquid crystalline elastomer that exhibited soft shape memory properties in response to thermal and water stimuli. This approach involved incorporating liquid crystalline mesogens into the polymer networks to fabricate unique materials. The effect of liquid crystalline mesogens on thermal, mechanical, and shape memory performance of these unique materials was studied. Chapter 7 focuses on epoxy based liquid crystalline elastomers. Similar to Chapter 6, it was confirmed that unique and responsive smart materials can be fabricated using liquid crystalline mesogens. In this study, a better route to synthesize LCEs, based on epoxy chemistry with amenability to open air and catalyst-free synthesis, was introduced. Finally in Chapter 8, another class of responsive smart polymers: near infrared fluorescence shape memory web containing indocyanine green dye, was investigated. This approach presents a new design strategy, for incorporate dyes uniformly without manipulating their properties, to fabricate polymers with unique imaging and shape memory characteristics. For this study, incorporation of ICG dye into PVAc polymer was achieved using the electrospinning technique, yielding near infrared fluorescence polymeric material with high fluorescence intensity and uniform dye incorporation. All the aforementioned polymeric materials have great potentials for different applications and can significantly influence the growth and development of new biomaterials and medical devices. Chapter 9 discusses the conclusions and provides recommendations for future research and development for each chapter of the dissertation

Morphological and Ultrastructural Collagen Defects: Impact and Implications in Dentinogenesis Imperfecta

Abstract of Systematic Review Background: Collagen is the building block for extracellular matrix in bone, teeth and other fibrous tissues. Osteogenesis Imperfecta (OI), or brittle bone disease is a heritable disorder that results from defective collagen type I synthesis or metabolism. The disease manifests as bone fragility that leads to multiple fractures The dental manifestation of OI is Dentinogenesis Imperfecta (DI), a genetic disorder that affects tooth structure and clinical appearance with characteristic greyish-brown discolouration. Management of Dentinogenesis Imperfecta can be difficult. Therefore, understanding the ultrastructural defects in dentinal tissues is of clinical importance. Thus, the aim of the project was to answer the question of what the changes in dentinal collagen macro, micro and ultrastructure in Dentinogenesis Imperfecta. Methods: Three data bases were searched for relevant articles: OVID Embase, OVID Medline, and PubMed Medline. Inclusion criteria were any study, written in English, published after 1990, that examined human dentinal collagen of teeth affected by DI. No exclusions were made by study design or examination protocol. A Cochrane data extraction form was modified to fit project aims and used for data collection. Results: The final dataset of the systematic review included seventeen studies. The most prevalent findings on collagen in DI teeth were increased coarse collagen fibres, and decreased fibres quantity. Other findings included random to parallel fibres orientation and irregular organization. Ultrastructural defects were uncoiled collagen fibres and increased spread of D-banding periodicity. Conclusion: Studies in collagen structure in DI reported changes to the surface topography, quantity, organisation, and orientation of the fibres. Moreover, ultrastructural defects such as the packing/coiling and D-banding of the fibrils, as well as differences in the presence of other collagens are also noted. Taken together, this study provides an understanding of the changes in collagen and its impact on clinical translation, paving the way for innovative treatments in dental management.   Abstract of Scoping Review Background: Dentinogenesis imperfecta is known to affect dental tissues, which is mainly reported to be dentin since it is the most dental tissue abundant. Deformed collagen, abnormal dentinal tubules and reduced mechanical strength are criteria well reported on by studies. In less abundance, enamel has also been described as abnormal, defects as irregularly shaped enamel lamellae and reduced mechanical strength have been described. Cementum, nonetheless, is not a common tissue type to be examined when looking for the effects of this genetic disease. The aim of this scoping review is to identify the effect of dentinogenesis imperfecta on cementum, if present and clarify extent of periodontal involvement in DI patients. Methods: The database of PubMed was searched. Search strategies developed, concepts were: collagen ultrastructure, DI and OI. Inclusion criteria were human or animal studies, of any primary or permanent teeth, affected with dentinogenesis imperfecta, isolated or syndromic DI type, that examined cementum defects. No limitations were applied on study type, date of publication, sample size, age or gender of subjects, nor species in case of animal studies. No restrictions applied on methodology of study as demineralization protocol or examination method. Results: six studies were retrieved, four human and two animal studies. Isolated DI was examined in 3 of which, the observations found were hypomineralized cementum, that is generally hypoplastic but with an increase in cellular cementum in the certain regions. The two papers examining syndromic DI cementum had inconsistent results. Conclusion: cementum defects of isolated DI were reported with consistent results, unlike syndromic DI, this can be because the function and defects of Dentin Sialophosphoprotein (DSPP) are further known than the defects in collagen of DI teeth. The evidence on cementum defects was scarce. The limited number of studies could reflect the limited periodontal involvement in DI patients hence the low clinical importance. From this scoping review we speculate the presence of cementum defects yet on a subclinical level

Nano-engineering of composite material via reactive mechanical alloying/milling (RMA/M)

Attempts to strengthen a chromium-modified titanium trialuminide by a combination of grain size refinement and dispersoid strengthening led to a new means to synthesize such materials. This Reactive Mechanical Alloying/Milling process uses in situ reactions between the metallic powders and elements from a process control agent and/or a gaseous environment to assemble a dispersed small hard particle phase within the matrix by a bottom-up approach. In the current research milled powders of the trialuminide alloy along with titanium carbide were produced. The amount of the carbide can be varied widely with simple processing changes and in this case the milling process created trialuminide grain sizes and carbide particles that are the smallest known from such a process. Characterization of these materials required the development of x-ray diffraction means to determine particle sizes by deconvoluting and synthesizing components of the complex multiphase diffraction patterns and to carry out whole pattern analysis to analyze the diffuse scattering that developed from larger than usual highly defective grain boundary regions. These identified regions provide an important mass transport capability in the processing and not only facilitate the alloy development, but add to the understanding of the mechanical alloying process. Consolidation of the milled powder that consisted of small crystallites of the alloy and dispersed carbide particles two nanometers in size formed a unique, somewhat coarsened, microstructure producing an ultra-high strength solid material composed of the chromium-modified titanium trialuminide alloy matrix with small platelets of the complex carbides Ti2AlC and Ti3AlC2. This synthesis process provides the unique ability to nano-engineer a wide variety of composite materials, or special alloys, and has shown the ability to be extended to a wide variety of metallic materials

Terminological Methods in Lexicography: Conceptualising, Organising, and Encoding Terms in General Language Dictionaries

Os dicionários de língua geral apresentam inconsistências de uniformização e cientificidade no tratamento do conteúdo lexicográfico especializado. Analisando a presença e o tratamento de termos em dicionários de língua geral, propomos um tratamento mais uniforme e cientificamente rigoroso desse conteúdo, considerando também a necessidade de compilar e alinhar futuros recursos lexicais em consonância com padrões interoperáveis. Partimos da premissa de que o tratamento dos itens lexicais, sejam unidades lexicais (palavras em geral) ou unidades terminológicas (termos ou palavras pertencentes a determinados domínios), deve ser diferenciado, e recorremos a métodos terminológicos para tratar os termos dicionarizados. A nossa abordagem assume que a terminologia – na sua dupla dimensão linguística e conceptual – e a lexicografia, como domínios interdisciplinares, podem ser complementares. Assim, apresentamos objetivos teóricos (aperfeiçoamento da metalinguagem e descrição lexicográfica a partir de pressupostos terminológicos) e práticos (representação consistente de dados lexicográficos), que visam facilitar a organização, descrição e modelização consistente de componentes lexicográficos, nomeadamente a hierarquização das etiquetas de domínio, que são marcadores de identificação de léxico especializados. Queremos ainda facilitar a redação de definições, as quais podem ser otimizadas e elaboradas com maior precisão científica ao seguir uma abordagem terminológica no tratamento dos termos. Analisámos os dicionários desenvolvidos por três instituições académicas distintas: a Academia das Ciências de Lisboa, a Real Academia Española e a Académie Française, que representam um valioso legado da tradição lexicográfica académica europeia. A análise inicial inclui um levantamento exaustivo e a comparação das etiquetas de domínio usadas, bem como um debate sobre as opções escolhidas e um estudo comparativo do tratamento dos termos. Elaborámos, depois, uma proposta metodológica para o tratamento de termos em dicionários de língua geral, tomando como exemplo dois domínios, GEOLOGIA e FUTEBOL, extraídos da edição de 2001 do dicionário da Academia das Ciências de Lisboa. Revimos os termos selecionados de acordo com os princípios terminológicos defendidos, dando origem a sentidos especializados revistos/novos para a primeira edição digital deste dicionário. Representamos e anotamos os dados usando as especificações da TEI Lex-0, uma extensão da TEI (Text Encoding Initiative), dedicada à codificação de dados lexicográficos. Destacamos também a importância de ter etiquetas de domínio hierárquicas em vez de uma lista simples de domínios, vantajosas para a organização dos dados, correspondência e possíveis futuros alinhamentos entre diferentes recursos lexicográficos. A investigação revelou que a) os modelos estruturais dos recursos lexicais são complexos e contêm informação de natureza diversa; b) as etiquetas de domínio nos dicionários gerais da língua são planas, desequilibradas, inconsistentes e, muitas vezes, estão desatualizadas, havendo necessidade de as hierarquizar para organizar o conhecimento especializado; c) os critérios adotados para a marcação dos termos e as fórmulas utilizadas na definição são díspares; d) o tratamento dos termos é heterogéneo e formulado de diferentes formas, pelo que o recurso a métodos terminológicos podem ajudar os lexicógrafos a redigir definições; e) a aplicação de métodos terminológicos e lexicográficos interdisciplinares, e também de padrões, é vantajosa porque permite a construção de bases de dados lexicais estruturadas, concetualmente organizadas, apuradas do ponto de vista linguístico e interoperáveis. Em suma, procuramos contribuir para a questão urgente de resolver problemas que afetam a partilha, o alinhamento e vinculação de dados lexicográficos.General language dictionaries show inconsistencies in terms of uniformity and scientificity in the treatment of specialised lexicographic content. By analysing the presence and treatment of terms in general language dictionaries, we propose a more uniform and scientifically rigorous treatment of this content, considering the necessity of compiling and aligning future lexical resources according to interoperable standards. We begin from the premise that the treatment of lexical items, whether lexical units (words in general) or terminological units (terms or words belonging to particular subject fields), must be differentiated, and resort to terminological methods to treat dictionary terms. Our approach assumes that terminology – in its dual dimension, both linguistic and conceptual – and lexicography, as interdisciplinary domains, can be complementary. Thus, we present theoretical (improvement of metalanguage and lexicographic description based on terminological assumptions) and practical (consistent representation of lexicographic data) objectives that aim to facilitate the organisation, description and consistent modelling of lexicographic components, namely the hierarchy of domain labels, as they are specialised lexicon identification markers. We also want to facilitate the drafting of definitions, which can be optimised and elaborated with greater scientific precision by following a terminological approach for the treatment of terms. We analysed the dictionaries developed by three different academic institutions: the Academia das Ciências de Lisboa, the Real Academia Española and the Académie Française, which represent a valuable legacy of the European academic lexicographic tradition. The initial analysis includes an exhaustive survey and comparison of the domain labels used, as well as a debate on the chosen options and a comparative study of the treatment of the terms. We then developed a methodological proposal for the treatment of terms in general language dictionaries, exemplified using terms from two domains, GEOLOGY and FOOTBALL, taken from the 2001 edition of the dictionary of the Academia das Ciências de Lisboa. We revised the selected terms according to the defended terminological principles, giving rise to revised/new specialised meanings for the first digital edition of this dictionary. We represent and annotate the data using the TEI Lex-0 specifications, a TEI (Text Encoding Initiative) subset for encoding lexicographic data. We also highlight the importance of having hierarchical domain labels instead of a simple list of domains, which are beneficial to the data organisation itself, correspondence and possible future alignments between different lexicographic resources. Our investigation revealed the following: a) structural models of lexical resources are complex and contain information of a different nature; b) domain labels in general language dictionaries are flat, unbalanced, inconsistent and often outdated, requiring the need to hierarchise them for organising specialised knowledge; c) the criteria adopted for marking terms and the formulae used in the definition are disparate; d) the treatment of terms is heterogeneous and formulated differently, whereby terminological methods can help lexicographers to draft definitions; e) the application of interdisciplinary terminological and lexicographic methods, and of standards, is advantageous because it allows the construction of structured, conceptually organised, linguistically accurate and interoperable lexical databases. In short, we seek to contribute to the urgent issue of solving problems that affect the sharing, alignment and linking of lexicographic data

Effects of multi-pass friction stir processing on aluminium

Abstract: Friction stir processing (FSP) is a new solid-state processing technique for microstructural modification based on friction stir welding (FSW) developed by The Welding Institute (TWI) in 1991. Since its invention, the process has continually been improved, and its scope of application expanded leading to industrial applications and commercialisation in the microstructural modifications of materials to achieve desired mechanical properties. The literature review investigates studies already conducted in the field of FSP on different aluminium materials. However, no published work on the correlation of microstructural homogeneity in multi-friction processed zone with mechanical properties seem to exist despite the fact that inhomogeneity in the friction processed zone might be a reason for the degradation in mechanical properties of single-pass FSP processed materials. This dissertation focuses on the effect of multi-pass FSP on aluminium. FSP was conducted using constant rotational and transverse speeds of 1600 rpm and 40 mm/min respectively. A tool plunge depth of 5.3 mm and a 3° tilt angle was used. AA6061-T6 was the selected matrix alloy, and its microstructural homogeneity in correlation with the evolving mechanical properties after each successive FSP pass was studied in detail. Macrostructural and microstructural characterisation was carried out with an optical microscope (OM) and a scanning electron microscope (SEM), while tensile testing, and microhardness profiling was carried out to evaluate the mechanical properties of the processed materials using a tensile testing machine and a Vickers microhardness tester respectively. A study of the underlying thermodynamics occurring during the FSP process was also conducted using molecular dynamics (MD) simulation. The macrostructural and microstructural evaluations of the processed samples revealed an increase in microstructural homogeneity as the number of FSP passes increases. The correlation of this homogeneity with the resulting mechanical properties indicates that a nearly 100% homogenous friction processed zone improved the mechanical properties in the processed aluminium materials. However, the BM was found to have better mechanical v properties with an ultimate tensile strength (UTS) of 338 MPa, a yield strength of 311 MPa, and an average Vickers microhardness of 99 HV, compared to the fully homogenous processed zone with a UTS of 177 MPa, yield strength of 172 MPa, and average Vickers microhardness of 67 HV. The resulting microstructural evolution and grain sizes after each FSP pass have also been observed to be strongly dependent on the processing parameters, thermal cycle, and presence of second-phase precipitates, rather than only on microstructural homogeneity. The results obtained from the MD simulation prove that it is possible to adequately represent MD simulations of FSP on aluminium alloys. The underlying thermodynamics was explained, and consistency between experimental FSP and the simulation process was achieved.M.Eng. (Mechanical Engineering

Synthesis, CFD simulation and characterization of direct laser metal deposition (LMD) of Al-Cu-Ti coatings on Ti-6Al-4V alloy for aerospace application

Abstract: This research study was aimed at researching the synthesis, simulation and characterization of Direct Laser Metal Deposition (DLMD) of Al-Cu-Ti Coatings on Ti-6Al- 4V Alloy for aerospace application. This investigation was accomplished by numerical analysis and experimental techniques. In the category of manufacturing techniques (Additive Manufacturing), the Direct Laser Metal Deposition (DLMD) technique is quite a new field in the industry. This manufacturing technique is flexible, in the sense that it can be used to manufacture working parts or components for bigger assemblies, freeform shapes according to the designer specifications, advanced coatings on the surfaces of working components, and fully solid structures. This research also enquires into the enhanced properties (mechanical, hardness and corrosion) of the modified coating powders (Al-Cu-Ti) on the Titanium alloy (Ti-6Al-4V) and its applications for components and parts enhancements in aerospace industries...M.Ing. (Mechanical Engineering Science

The Effect of Ultrasonic Vibration on the Solidification of Light Alloys

This exposition presents the novel thermodynamical and microstructural modification to light alloys, such as aluminum alloys and magnesium alloys, by ultrasonic vibrations during their solidification processes. Ultrasonic vibration has proven to be effective in controlling columnar dendritic structure, reducing the size of equiaxed grains, and under some conditions, producing globular non-dendritic grains. Despite this, the solidification process under the effect of ultrasonic vibration was not clear. Not only was there no such research on how ultrasonic vibration affected its solidification thermodynamically, but also its effects on the as-cast microstructure, including the primary fcc phase, the eutectics, and the secondary phases, were not systematically studied. In addition, most studies had been empirical and phenomenological rather than quantitative. Prior to the experiments, thermodynamic simulations were carried out using the Scheil model to determine the temperature versus solid fraction curve of the alloys. The starting temperature for ultrasonic processing and the casting temperature were predetermined according to the simulation result. An experimental apparatus which supplied a powerful 1500 Watts at 20 KHz of ultrasonic power was designed and built. Thermal analysis experiments were performed. The result shows that, with ultrasonic vibration, the steady growth temperature and the minimum supercooling temperature have been elevated; while the recalescence time decreased, which indicates a much slower growth rate of primary fcc aluminum grains. The difference between dendrites nucleation/growth and thickening is not significant in the casting with ultrasonic vibration, which might suggest dendrites formation might not present in this solidification process. The mechanisms for ultrasonic influence on solidification have been discussed. Two types of ultrasonic processing techniques were developed and attempted. The first one related to introducing the vibration into the solidifying specimen through the liquid, while the second through the formally solidified part. For the first ultrasonic processing technique, the treatment was employed isothermally, intermittently, and continuously. In contrast to the fully developed dendrites up to several millimeters in length in untreated A356 alloy, fine globular primary fcc Al grains sized less than 200 mm were obtained in the specimen treated with 5 second intermittent ultrasonic vibrations. However, dendrites were not completely broken down into fine grains in the isothermally or continuously processed specimens. It may imply that there is limited effect of dendrite fragmentation on the formation of globular/non-dendrite microstructure in the acoustically processed melt, and acoustically induced heterogeneous nucleation seems to be the dominant mechanism for the formation of a globular microstructure. For the second approach, ultrasonic treatment was performed continuously. During the treatment, grain refinement reached an unprecedented level. The average grains were globular with size ranges from 20 to 40 mm. Superfine globular grains of size less than 20 mm were obtained in the area near the ultrasonic radiator. Similar grain refinement could only be reached by using a quenching method with a much faster cooling rate. The main parameters of ultrasonic processing, such as casting temperature, ultrasonic intensity, and the distance from the radiator, have been investigated. It is concluded that high acoustic amplitude/intensity favors the formation of small, spherical primary aluminum grains. The casting temperature of 630°C brings about best grain refinement result. The primary aluminum grain size in a casting increases with the increasing distance from the acoustic radiator. In order to examine the feasibility of ultrasonic vibration for SSM processing, high intensity ultrasonic vibration has been applied during the casting of A356 alloy at high volume. Non-dendritic/globular grains have been obtained. Grain refiner can further refine A356 alloy structure, with the combination of ultrasonic vibration. Experiments on the grain refinement of other aluminum alloys have been carried out. Fine globular grains were obtained in various aluminum alloys, including A354, 319, 6063, 6061, 2618 alloys. It was found that 670 °C is the optimum casting temperature for grain refinement of 2618 with the aid of ultrasonic vibration. The effect of ultrasonic vibration on the modification of eutectic silicon in aluminum-silicon alloys has been studied. The introduction of ultrasonic vibration into A356 alloy modified the morphology of eutectic silicon from a coarse acicular plate-like form to a finely dispersed rosette-like form. The length, width, and aspect ratio of eutectic silicon all reduced significantly. This modification is beneficial to the mechanical properties. Ultrasonic grain refinement and secondary phases modification to magnesium AM60B alloy have been examined. With ultrasonic vibration, alloy experienced a reduction in size of primary α-Mg grains from 760 µm to about 25~48 µm in diameter, which is much better than other traditional grain refinement methods. The morphology of eutectic phases was modified from a mainly fully divorced blocky morphology dispersed among dendrite arms, to a mainly lamellar/script morphology across the grain boundaries. Furthermore, the volume fraction of the eutectic morphology is less. Ultrasonic processing of solidifying metals can have a number of applications. Incorporating ultrasonic vibration into a die casting machine would dramatically increase the integrity and properties of die castings. Ultrasonic vibration may be used for producing semisolid feedstock directly from molten metal. Ultrasonic techniques can also find applications in forging industries for processing alloys that are difficult to cast. Ultrasonic treatment has the advantages of being environmentally favorable, cost effective, and ready to be combined with other known physical processing technologies for liquid and solidifying metal. It is expected that the results of this study will impact a wide range of alloy processing including DC casting, continuous casting, vacuum arc remelting, and foundry processing in the areas of grain refinement, semi-solid metalcasting (SSM), and the production of new and novel microstructures. It is highly recommended to continue both the research reported in this study and the application and commercialization of this technology

Constitutive model for fibre-reinforced composite materials exposed to high temperature

A la pàgina XIX del Sumari manquen: la pàgina 333: "Original publications" i les pàgines 335 a 361 "Bibliography"The high strength-weight ratio of composite materials have made them one of the best materials for the design of light-weight structures. However, its special complexity has made them not suitable for the design of structures with a relative complexity or with numerous structural component and pieces. Hence, the importance in the development of adequate constitutive models which allow simulating the micro-macro scale interaction of composites, and to address the intrinsic and natural flexibility of composites that is not as relevant in traditional materials. Meanwhile, the mechanical development of these materials is a mature research branch with more than four groundbreaking decades of life, this is not certainly met at the thermo-mechanical level which is still in an early stage and, consequently, limiting the extensive use of composites in real world and complex structures, particularly structures in which a strong and detailed fulfilment of fire criteria is necessary. E.g., this is the very situation in the large-length ship design sector, where the share in the market for ships built using composite material, tends to be very reduced and closely accompanied by tools which serve to perform structural health monitoring, in order to palliate, the amount of high uncertainty of the present thermo-mechanical response, found in the design of these structures. The present thesis focuses on the development, formulation-wise and computational implementation, of a numerical model in order to predict the non-linear constitutive behaviour of fibre-reinforced plastic (FRP) composites exposed to thermal degradation due to high temperatures. This very model is cemented in the groundbreaking development of constitutive mechanical formulations specially tailored for composites also known as rule of mixtures -- in this present context, the formulation is the so-called serial-parallel rule of mixtures -- which establish a set of closure equations to obtain the suitable micro-macro scale interaction of the composite structure and, at the same time, to take into account the characterisation of the internal and state variables of the constituent phases. Apart, the ultimate objective of this thesis, in this special context -- where a structure is under thermal loads or, what is the same, exposed to fire -- it is mandatory to develop a consistent formulation and tool to perform what is referred to as a fire collapse assessment analysis. The utilisation of a more sophisticated thermal degradation or pyrolysis formulation, based on the present existing formulations, will be employed in order to obtain the internal and state variables of the thermal degradation process. Thus, the outcome of this analysis will serve as means to obtain the unknown thermal state of the structure and complete the thermo-mechanical analysis. The formulation of the thermo-mechanical problem is adapted to be used in laminated non-linear constitutive shells. The use of shells is a necessity for the right optimisation of the computational cost of analysing structures with a high number of structural reinforcements or divisions, such as the ones that appear regularly during the ship design process of large ship structures.Per als materials compostos, la seva relació esforç-pes elevada ha fet d'ells un dels millors materials per al disseny d'estructures lleugeres. No obstant això, la seva especial complexitat, fa d'ells un difícil treball quan es tracta del disseny d'estructures amb una certa complexitat, o, en l'existència de nombroses divisions estructurals i peces. En conseqüència, el desenvolupament de models constitutius adients és de vital importància, en especial aquells que permeten la simulació de la interacció per la micro-macro escala dels compostos, i que resolguin la flexibilitat natural i intrínseca d'aquests materials avançats, qüestió que no és tan rellevant per al disseny de materials tradicionals. Mentrestant, el desenvolupament de teories mecàniques per aquests materials es troba ja a la seva maduració, amb més de quatre dècades de descobriments en aquesta branca. D'altra banda, en qüestions que involucren l'anàlisi termo-mecànica, el paradigma es considera relativament verd, el qual limita l'aplicació extensiva dels compostos en aplicacions pràctiques i d'estructures complexes, de fet, és particularment limitant en el disseny d'estructures que requereixen del compliment d'exigents i detallats criteris relatius al foc. E.g., això mateix succeeix en el disseny d'embarcacions de grans eslores, on la quota de mercat dels vaixells construïts mitjançant materials compostos sol ser reduïda, i estretament acompanyada per eines de monitoratge de la integritat estructural, per així poder pal·liar la gran incertesa vinculada a la resposta termo-mecànica, fruit de les capacitats del disseny comercial actual. L'actual tesi se centra en el desenvolupament, de manera teòrica, i amb corresponent implementació computacional, d'un model numèric capaç de predir el comportament no-linear constitutiu de compostos plàstics amb fibra embedida (FRP) quan aquests són exposats a altes temperatures i en conseqüència a la degradació tèrmica. Aquest mateix model està inspirat en els desenvolupaments, pioners i excepcionals, de models constitutius mecànics, els quals estan pensats per a compostos. Aquestes teories formen part de la família de les regles de barreges, en particular, la formulació escollida és la famosa regla de barreges sèrie-paral·lel, la qual estableix un conjunt d'equacions de tancament per així obtenir l'adequada interacció del material compost a la micro-macro escala. Aquesta formulació, a la mateixa vegada, té en compte la caracterització i evolució de tant variables internes com d'estat, per a les constitutives, en aquest context es tractaria de la fibra i la matriu. Per una altra banda, l'objectiu últim d'aquesta tesi, dins d'aquest context particular, on una estructura és sotmesa a càrregues tèrmiques, o en altres paraules, s'exposa al foc, és de forçosa necessitat el desenvolupament d'una formulació consistent i una eina capaç de verificar el que es podria batejar com una anàlisi de col·lapse al foc. L'ús d'una formulació més sofisticada per la degradació tèrmica o piròlisi, basada en formulació existent, serà empleat per així aconseguir les variables internes i d'estat dels processos de degradació tèrmica. En conseqüència, els resultats d'aquesta anàlisi tèrmica serveixen per a obtenir el desconegut estat tèrmic de l'estructura, la distribució de temperatura a través de l'espessor del laminat, i complementar l'anàlisi del model termo-mecànic. La formulació del problema termo-mecànic és adaptada per ser usada en làmines no lineals de materials compostos. Fer servir làmines és una necessitat per a la correcta optimització del cost computacional derivat de l'anàlisi d'estructures amb un alt nombre de reforços o divisions, anàlisis que són freqüentment trobats dins del procés del disseny d'embarcacions de grans eslores.Para los materiales compuestos, su relación esfuerzo-peso elevada ha hecho de ellos uno de los mejores materiales para el diseño de estructuras ligeras. No obstante, su especial complejidad, hace de ellos un arduo trabajo cuando se trata del diseño de estructuras con una cierta complejidad, o, en la existencia de numerosas divisiones estructurales o piezas. Consecuentemente, el desarrollo de modelos constitutivos adecuados es de importancia, en especial aquellos que permiten la simulación de la interacción para la micro-macro escala de los compuestos, y que resuelven la flexibilidad natural e intrínseca de estos materiales avanzados, cuestión que no es tan relevante para el diseño de materiales tradicionales. Mientras tanto, el desarrollo de teorías mecánicas para estos materiales se encuentra en su madurez, con más de cuatro décadas de hallazgos en esta rama. En contraposición, en cuestiones que atañen el análisis termo-mecánico, el paradigma se encuentra relativamente verde, lo cual limita la aplicación extensiva de los compuestos en aplicaciones prácticas y estructuras complejas, de hecho, es particularmente limitante en el diseño de estructuras que requieren del cumplimiento de exigentes y detallados criterios relativos al fuego. E.g., esto mismo sucede en el diseño de embarcaciones de grandes esloras, donde la cuota de mercado de los buques construidos mediante materiales compuestos suele ser reducida, y estrechamente acompañada por herramientas de monitorización de la integridad estructural, para así poder paliar la gran incertidumbre vinculada a la respuesta termo-mecánica, fruto de las capacidades del diseño comercial actual. La actual tesis se centra en el desarrollo, de manera teórica, y con su correspondiente implementación computacional, de un modelo numérico capaz de predecir el comportamiento no-lineal constitutivo de compuestos plásticos con fibra embebida (FRP) cuando estos son expuestos a altas temperaturas y en consecuencia a la degradación térmica. Este mismo modelo está inspirado en los desarrollos, pioneros y excepcionales, de modelos constitutivos mecánicos, las cuales están pensadas para compuestos. Estas teorías forman parte de la familia de las reglas de mezclas, en particular, la formulación escogida es la renombrada regla de mezclas serie-paralelo, la cual establece un conjunto de ecuaciones de cierre para así obtener la adecuada interacción del material compuesto en la micro-macro escala. Esta formulación, a su misma vez, tiene en cuenta la caracterización y evolución de tanto variables internas como de estado, para las fases constituyentes, en este contexto se trataría de la fibra y la matriz. Por otra banda, el objetivo último de esta tesis, dentro de este contexto particular, donde una estructura se somete a cargas térmicas, o, en otras palabras, se expone al fuego, es de forzosa necesidad el desarrollo de una formulación consistente y una herramienta capaz de verificar lo que se puede acuñar como un análisis de colapso al fuego. El uso de una formulación más sofisticada para la degradación térmica o pirolisis, basada en formulación existente, será empleado para así obtener las variables internas y de estado de los procesos de degradación térmica. En consecuencia, los resultados de este análisis térmico sirven para obtener el desconocido estado térmico de la estructura, la distribución de temperatura a través del espesor del laminado, y complementar el análisis termo-mecánico. La formulación del problema termo mecánico es adaptada para ser usada en láminas no lineales de materiales compuestos. Usar láminas es una necesidad para la correcta optimización del coste computacional derivado del análisis de estructuras con un alto número de refuerzos o divisiones, análisis que son frecuentemente encontrados en el proceso de diseño de embarcaciones de grandes esloras.Postprint (published version