FE - DCA | Dissertações de Mestrado e Teses de Doutoramento
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- Mechanical and Piezoresistive Sensing of GFRP Modified with MWCNTs under Monotonic and Cyclic Loading for Structural Health MonitoringPublication . Limpo, Joana Valente; Pereira, João Pedro Nunes; Silva, Abílio Manuel Pereira da; Parente, João Miguel NunesFiber-reinforced composites, specifically polymer fiber-reinforced composites, have gained increased popularity among scientists and engineers. This results from their optimized properties, including mechanical strength, reduced weight, low density, chemical inertness, freedom in design, and good response to fatigue, among many others. This is reflected in many sectors, including the aeronautical and aerospace fields, where these composites have been replacing the more traditional materials, like metal. However, despite all the advantages of composites, due to their nature, some behavioral aspects, especially damage-wise, are not fully understood, hindering their use for certain functions. One of those functions is the structural health monitoring of components, which goal is to have components and structures be able of in situ monitor of their own condition, without the disadvantages of embedded sensors. With this in mind, this study analyzed the impact of the addition of varying carbon nanotube concentrations, 0.3 wt% and 0.4 wt% MWCNTs, to find an optimized concentration for better piezoresistive properties. Beyond that, the 0.3 wt% and 0.4 wt% MWCNT-reinforced laminates were subjected to multiple flexural tests, from monotonic to cyclic, and with increased deformation, to mimic possible in service conditions and preview their possible behavior under each situation. SEM imaging was done post testing, to link the mechanical and electrical data obtained with the damage suffered by the specimens. Overall, the results pointed to better mechanical properties for the samples with 0.3 wt% nanotubes, which can be a sign of poor interface properties for the higher nanoparticle concentration. On the other hand, the electrical data concluded the samples with 0.4 wt% MWCNT were the most sensitive and presented more stable results. In summary, by adding carbon nanotubes to the glass-fiber-reinforced laminates, it was possible to create a material with self-sensing capabilities, and real-time monitoring of the structure was achieved. The mechanical data reinforced the importance of manufacturing and dispersion processes, especially with increasing nanoparticle concentrations and the electrical resistance results concluded there is a relation between a material’s damage and electrical signal.
- Development of a Long Range Tail-sitter VTOL UAVPublication . Vilaça, António Manuel Babo; Silvestre, Miguel Ângelo RodriguesCurrently, the aerospace industry is undergoing constant evolution, continuously seeking new concepts and technological solutions that enable Unmanned Aerial Vehicle (UAV) operators to carry out their missions more efficiently, with greater versatility, and less constrained by the geographical limitations of their base of operations. In this context, aircraft with Vertical Take-Off and Landing (VTOL) capabilities emerge as one of the most promising alternatives, combining the operational flexibility of helicopters with the cruising efficiency of fixed-wing aircraft. Within the scope of this dissertation, the design of a VTOL unmanned aerial vehicle was developed, aimed at long-range missions with extended endurance. The development process was supported by the construction of two prototypes: one at a 1:2 scale and another at full scale (1:1), allowing for the experimental validation of key design parameters. To support the sizing and system analysis, computational tools were employed, such as Microsoft Excel spreadsheets for preliminary calculations and performance estimates, as well as Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) software, which were essential for the design, fabrication, and assembly of the prototype. This work thus aimed not only to validate the proposed concept but also to explore practical methodologies applicable to the development of UAVs with VTOL capabilities.
- Integrating HAPS: A Strategic Roadmap for Strategic OperationsPublication . Silva, Ana Margarida Camilo Amaro Razões da; Silva, André Resende Rodrigues daHigh-Altitude Platform Stations (HAPS) are aerial systems without pilots that stay in the earth’s atmosphere, around 20 kilometers above the earth’s surface. Such systems, for instance, are used for remote communication, observation, and scientific research. Technology such as solar power, lightweight materials, and autonomous control systems has recently made it possible for them to be flown for several months. As a result, these platforms have greatly extended their operating potential. However, the regulatory structure is quite different from technological progress. Current regulations for civil aviation are suited for manned aircraft below FL600 and, thus, the needs of unmanned, long-endurance stratospheric operations are not catered to. Uncertainties in certification, management of airspace, as well as coordination between civil and military forces are the effects of the absence of specific rules for these activities. This work is an examination of the HAPS technology evolution, a legal and operational environment assessment, and recognition of the technical and regulatory challenges as the main issues. It looks at the European Union Aviation Safety Agency (EASA) and the International Civil Aviation Organization (ICAO) in defining Higher Airspace Operations and at the governance models that would make the global integration feasible. A strategic roadmap is suggested as an instrument to coordinate the move from fragmented, experimental missions to safe, reliable, and commercially viable HAPS operations.
- Oxygen/Hydrogen Combustion under Supercritical Conditions: A Numerical StudyPublication . Martins, Daniel Ribeiro; Silva, André Resende Rodrigues da; Magalhães, Leandro BarbosaThe pursuit of enhanced propulsion system performance has driven the development of combustion technologies operating at increasingly higher pressures. Such conditions tipically exceed the thermodynamic critical thresholds of the propellants, giving rise to complex transcritical and supercritical flow regimes. The oxygen/hydrogen propellant combination has been extensively and successfully employed in liquid rocket propulsion, and, in the context of global decarbonization objectives, the clean, high-energy characteristics of hydrogen make it a promising candidate for future propulsion applications extending beyond rocketry. The modeling of shear coaxial injection of oxygen and hydrogen under supercritical conditions is addressed through a steady-state axisymmetric Favre-averaged Navier-Stokes formulation, with combustion treated using the Eddy-Dissipation-Concept model in conjunction with a detailed kinetic mechanism consisting of eight species and nineteen elementary reactions. The near-field region of the injector constitutes the primary focus of this work, as it is within this zone that the mixing efficiency is established, ultimately governing the overall combustion and engine performance. The simulations are carried out piecewise to ensure consistency thorugh all the simulation process. Firstly, non-reacting simulations are performed, incorporating both ideal- and real-gas models. After that, reacting simulations are performed in order to validate the developed numerical model against experimental data. Two main innovative contributions distinguish the present study from previous works on O2/H2 coaxial injection. The first concerns computational efficiency, achieved through a substantial reduction of the chamber length to the minimum extent necessary to ensure that the outlet boundary conditions do not affect the near-field region. In addition, the omission of the low-velocity recirculation zone located above the injector head was implemented to further reduce computational cost. The second innovation involves the inclusion of heattransfer effects in both the injector and chamber walls. In particular, the influence of the isothermal walls configuration on the injector is analyzed in light of the thermal disintegration mechanism of the inner jet, previously identified in the literature for cold-flow conditions under supercritical regimes. The numerical results demonstrated the capability of the developed framework to accurately model cryogenic O2/H2 injection and combustion under supercritical conditions. The non-reacting simulations established a robust numerical foundation and captured the key flow features, including the dense oxygen core and the surrounding high-velocity hydrogen stream. The incorporation of real-gas effects proved essential, as the ideal-gas assumption led to significant deviations in core length and mixing behavior. In the reacting regime, the real-gas model reproduced the experimental axial temperature profile and flame structure with good accuracy, outperforming the ideal-gas formulation and yielding results comparable to those reported in the literature, thereby confirming its suitability for high-pressure combustion modeling.
- Modelo de avaliação do processo logístico no desmantelamento de aeronaves em entidades certificadas EASA PART-145Publication . Vale, Francisco Gonçalves Candeias do; Saúde, José Manuel Mota Lourenço da; Ros, Sofia ValériaO aumento do número de aeronaves em fim de vida útil exige processos de desmantelamento eficientes e sustentáveis. Esta dissertação centra-se no desenvolvimento de um modelo de avaliação do processo logístico em entidades certificadas EASA PART-145, utilizando a Dassault Aviation Business Services (DABS) como caso de estudo. A metodologia baseou-se numa análise de capacidade por grupos funcionais (Maintenance, Technical Services e Logística) e no desenvolvimento do Índice de Equilíbrio de Fluxo (IEF), uma ferramenta de gestão orientada para o planeamento estratégico e apoio à decisão. A aplicação do modelo revelou um desequilíbrio estrutural entre a capacidade da Produção e da Logística, com o principal estrangulamento na equipa de Maintenance. O IEF demonstrou ser eficaz na análise do estado do sistema e na simulação de cenários futuros. Conclui-se que o modelo proposto foi validado com sucesso, constituindo uma ferramenta útil para a otimização do desmantelamento de aeronaves. O estudo evidencia que a eficiência global depende do equilíbrio e da sincronização do fluxo de trabalho.
- Piezoresistive Sensing for Structural Health Monitoring in Liquid Hydrogen Tanks: A Study on Composite Cryotank TechnologiesPublication . Figueiredo, Matilde Rola; Pereira, João Pedro Nunes; Silva, Abílio Manuel Pereira daHydrogen has been emerging as a potential key energy carrier for sustainable aviation, yet its adoption is constrained by distinct engineering challenges, particularly its safe and lightweight storage. The present study investigates the feasibility of using the intrinsic piezoresistive properties of carbon-fibre reinforced prepreg composites, commonly employed in Type V cryogenic hydrogen tanks, for structural health monitoring. Given that matrix microcracking constitutes the predominant damage mechanism in these tanks and typically initiates at strain levels of 0.3%-0.5%, a preventive strain sensing approach was adopted based on these critical deformation thresholds. To achieve a proof of concept, 17-ply multidirectional carbon fibre/epoxy laminates based on Boeing’s cryotank from the Composite Cryotank Technologies and Demonstration program were produced and tested in their as-fabricated state and after controlled cryogenic cycling through five and ten cycles of immersion in liquid nitrogen (-196 ?) followed by reheating (52 ?). Unconditioned specimens showed a clear and repeatable piezoresistive response from surface measurements, with gauge factors ranging from -11.02 to -12.70, confirming their intrinsic self-sensing capability. Cryogenically cycled specimens exhibited moderate mechanical degradation and increased baseline electrical resistance, yet retained self-sensing performance, with gauge factor increases exceeding 85%. While volume (edge-to-edge) measurements were inconclusive, the consistent surface response demonstrates that unmodified carbon fibre/epoxy prepreg composites employed in Type V cryotanks can retain effective strain sensing functionality after cryogenic exposure and cycling, providing insights for the development of integrated structural health monitoring strategies in hydrogen storage applications.
- Efeito da temperatura no comportamento de compósitos de resina epóxi nano reforçados com nano fibras de carbonoPublication . Tavares, Rimaldini da Veiga; Silva, Abílio Manuel Pereira da; Santos, Paulo Sérgio Pina dosA indústria aeronáutica tem se destacado como um fator crucial no desenvolvimento tecnológico, impulsionada pela inovação em compósitos. Os compósitos, especialmente os reforçados com fibras de carbono e nano materiais, têm revolucionado o design e a fabricação de aeronaves, oferecendo melhorias significativas em termos de leveza, resistência e durabilidade. Neste contexto, realizou-se esta dissertação com o prepósito de investigar o efeito da temperatura no comportamento de compósitos de resina epóxi reforçada com nano fibras de carbono (CNF’s), por meio de um estudo numérico e experimental. Visa entender como as variações térmicas afetam as propriedades mecânicas e térmicas desses compósitos avançados, essenciais para aplicações na indústria aeroespacial e aeronáutica. A metodologia utilizada foi a agitação mecânica e a moldação manual de fácil aplicação, custos reduzidos e facilmente escalável para a indústria. Os ensaios de flexão realizados evidenciaram que tanto as matrizes quanto os laminados aditivados com 0,75% em peso de CNF’s apresentaram desempenho superior em relação às respetivas amostras de controlo. A 50 ?, a incorporação de CNF’s na matriz promoveu um aumento de 42,34% na tensão de flexão, um incremento de 44,33% na rigidez e uma redução de 5,77% na deformação, quando comparada à matriz de controlo. Para os laminados submetidos à mesma condição térmica, a adição de 0,75% em peso de CNF’s resultou em um acréscimo de 17,75% na tensão de flexão, um ganho de 5,35% na rigidez e uma diminuição de 7,09% na deformação, em relação ao laminado de controlo. Nos testes de relaxação de tensões, ocorre a redução da tensão ao longo do tempo, e em termos de fluência, o deslocamento aumenta ao longo do tempo e os dados experimentais foram ajustados com sucesso pelo modelo de Kohlrausch-Williams-Watts (KWW).
- Análise de propagação de dano e da resposta piezoresistiva em laminados de CFRPPublication . Pereira, Duarte Amaro; Silva, Abílio Manuel Pereira da; Pereira, João Pedro NunesOs laminados de polímero reforçado com fibra de carbono (CFRP) são amplamente utilizados em estruturas aeronáuticas devido à sua elevada resistência específica, rigidez e durabilidade. Contudo, a sua suscetibilidade a defeitos internos e externos, como a rutura frágil da matriz, delaminação e rutura de fibras, pode comprometer significativamente o desempenho estrutural e a segurança em serviço. Neste trabalho estudou-se experimentalmente a propagação de fissuras em laminados de CFRP através de ensaios de flexão em três pontos e a monitorização piezoresistiva em condições de carga cíclicas. Foram fabricados laminados em fibra de carbono em autoclave com diferentes configurações de empilhamento: unidirecional [0]10, [0]16 e [0]22, cruzada [0/90]5 e simétrica [0/45/90/-45/0]S e realizaram-se ensaios mecânicos de flexão em três pontos e ensaios cíclicos com monitorização em tempo real da resposta elétrica e observação microscópica da evolução dos danos. Os ensaios de flexão em três pontos evidenciaram comportamentos distintos à fratura para cada tipologia de empilhamento. O laminado unidirecional de 10 camadas apresentou a maior resistência máxima à flexão (1469 MPa) e o módulo de elasticidade mais elevado (136 GPa), enquanto os laminados de 16 e 22 camadas exibiram resistências 26 % e 29 % inferiores, respetivamente. O laminado simétrico revelou a maior deformação máxima (1,35 %) e flecha a meio do vão (8,16 mm), evidenciando maior capacidade de absorção de energia, ao passo que o laminado cruzado apresentou resistência de 588 MPa e deformação de 1,27 %, associadas à redistribuição de tensões entre camadas ortogonais. Nos ensaios cíclicos com monitorização piezoresistiva, todos os laminados mostraram correlação direta entre o carregamento e a variação relativa de resistência (?R/R0). O laminado simétrico apresentou a maior sensibilidade eletromecânica, com Gauge Factor (GF) médio de 247,9, aproximadamente 1,56 vezes superior ao do laminado unidirecional de 16 camadas (GF= 158,8) e 2,63 vezes superior ao do laminado unidirecional de 10 camadas (GF= 94,4). O laminado cruzado apresentou o valor mais baixo (GF= 36,4), correspondendo a uma redução de cerca de 85 % face ao laminado simétrico. Estes resultados confirmam que os laminados unidirecionais apresentam o melhor desempenho mecânico, enquanto os laminados simétricos evidenciam maior sensibilidade piezoresistiva, demonstrando o potencial dos CFRP como materiais multifuncionais self-sensing aplicáveis em sistemas de Structural Health Monitoring (SHM) aeronáuticos.
- Piezoresistive Sensing for Structural Health Monitoring: A Temperature Dependence CharacterisationPublication . Pinto, Daniel Fernando Moreira; Pereira, João Pedro Nunes; Silva, Abílio Manuel Pereira da; Parente, João Miguel NunesThe aeronautical industry holds safety as its core and its main drive for innovation, with sensors, materials science and self-governance to ensure safe travel through the skies. However, operational costs and maintenance requirements increase proportionally with the increase of number of sensors, which is a growing concern and makes cost-effective solutions desirable. This dissertation aims to advance the knowledge and viability of a promising candidate to resolve this issue, studying a type of what are called self-sensing materials as an alternative to the conventional structural health monitoring (SHM) sensors. This material is a type of piezoresistive nanocomposite (PNC) that, in theory, could replace an airplanes’ complicated arrays of sensors while providing even better performance. However, PNCs have a glaring fault that is hindering their commercial availability, namely their perceptual conditioning of operational temperature. Current state of the art has mostly neglected this correlation, especially in the SHM sector, an oversight that this dissertation aims to rectify. For this reason, electromechanical properties of a PNC specimen made of glass-fibre epoxy resin with 0.5%𝑤𝑡 multiwalled carbon nanotubes (MWCNT) as filler were thoroughly tested at three different test temperatures: 30°𝐶, 50°𝐶 and 70°𝐶. The experimental analysis, static and cyclical tests, revealed that temperature has a profound effect on sensing performance, outlining key factors of response change with temperature variation. Three gauge factors (GF) were calculated, one for each temperature, with values ranging from a maximum of 2.42 to a minimum of 0.83. For a broader scope on this effect, multiple variations of the basic three-point bending (3PB) tests were performed to determine which factors were application-specific and which were constant.
- Effects of Wing-Fuselage Interface Position on Lift and DragPublication . Bombas, Rodrigo Manuel Cação do Carmo; Gamboa, Pedro VieiraThe interference effect between two aircraft components has become increasingly relevant in the design process, in parallel with the advances in aerodynamic efficiency observed over the last decades. In order to understand how this phenomenon manifests itself in small aircraft operating at low Reynolds numbers, a study was carried out on the influence of wing position relative to the fuselage, as well as the effect of fairings at the interface between the two components and the fuselage cross-section on interference drag. To determine the most favourable wing–fuselage interface positions in terms of drag and lift—considering also two types of fairings and fuselage cross-sections—the aircraft provided by the UBIAT team was modelled in CATIAV5. A simplified model was developed that nonetheless preserved the essential features of the real aircraft. The geometry was then prepared in Ansys Discovery to define the body of influence, the simulation volume, and the boundary conditions. This setup was subsequently imported into Ansys Fluent, where the computational mesh was generated according to a previously conducted mesh-independence study. The CFD simulations were performed using the k − ω SST turbulence model with standard parameters, including corrections for transitional flow and low Reynolds number regimes. Four alternative wing–fuselage interface configurations, in addition to the baseline case, were analysed: two vertical and two horizontal. The vertical configurations were further assessed with fairings and with a circular fuselage cross-section. The results regarding wing–fuselage positioning for both fuselage geometries were consistent with expectations and supported by literature. As for the introduction of fairings, both were found to improve aerodynamic efficiency, although the results did not fully align with published references. The main conclusions drawn from this work are the advantage of a high-wing configuration when interfaced with a square fuselage cross-section, and of a mid-wing configuration when interfaced with a circular fuselage cross-section. The addition of fairings yielded improvements of up to 4.7% in aerodynamic efficiency for the high-wing configuration with a square-section fuselage, while their effect was negligible for the other vertical positions.