Estudo e ensaios experimentais destinados à caracterização do comportamento dinâmico de estruturas mecânicas de transformadores de potência quando sujeitos a curto-circuito

Dissertação de mestrado integrado em Engenharia MecânicaDevido ao crescimento da competitividade do mercado global, ao aumento da capacidade da rede de distribuição elétrica e das normas mais restritas ao nível do ruído, os produtores de equipamentos como um transformador de potência, têm de otimizar constantemente os seus produtos. A empresa Efacec Energia S.A. não é exceção. Deste modo, o trabalho apresentado, vem precisamente efetuar uma tentativa de melhoramento estrutural do grande reforço utilizado pela empresa referida nos seus transformadores de alta potência do tipo-Shell. Neste trabalho, apresenta-se a seleção de um novo modelo de grande reforço, cuja geometria e/ou material de fabricação foram alterados para permitir uma redução de massa e um aumento da eficiência estrutural. São também apresentados ensaios mecânicos de materiais, para a validação e determinação dos valores reais das características mecânicas usadas na fabricação destes componentes. Adicionalmente, os dados gerados por estes ensaios, serão usados em simulações numéricas para otimização e determinação do comportamento desta estrutura. Finalmente, neste texto, apresenta-se a caracterização dinâmica do grande reforço, pela apresentação dos resultados decorrentes de análises modais experimentais e numéricas. Estes estabelecem que o componente em questão tem frequências naturais, em corpo livre, próximas das de excitação de um transformador.Due to the increase of competiveness in the global market, the constant increase of the capacity of power lines and the more restrictive noise policies, the manufacturers of equipment such as a Power Transformer, have to constantly update their products. The company Efacec Energia S.A. is no exception. Given this statement, the information presented in the thesis, shows an attempt to optimize the main frame, used the referred company in their Shell-type Power Transformers. In this dissertation, it is shown the selection of a new model of main frame, with a different internal geometry and/or manufacturing material, in order to reduce mass and increase structural efficiency. There are also presented tensile tests to materials used in the manufacturing of the referred structure, in order to determine and validate their real mechanical characteristics. Additionally, the generated data will be used in the optimization and the determination of the dynamic behavior of this structure, recurring it numerical simulations. There are also presents mechanical testing to materials that are being considered to substitute the current ones used in the manufacturing of this structure. Additionally, the generated data will be used to perform numerical simulations, to further optimize and determine the deformation behavior of the main frame. Finally, it will be shown the characterization of the dynamic behavior of the main frame, by the presentation of the information collected by the execution of experimental and numerical modal analysis. The results of these tests determine that, in free body conditions, the main frame has resonance frequencies in the proximity of the excitation frequencies of the power transformer

Vibration damping and acoustic behavior of PU-filled non-stochastic aluminum cellular solids

Aluminum-based cellular solids are promising lightweight structural materials considering their high specific strength and vibration damping, being potential candidates for future railway vehicles with enhanced riding comfort and low fuel consumption. The filling of these lattices with polymer-based (i.e., polyurethane) foams may further improve the overall vibration/noise-damping without significantly increasing their density. This study explores the dynamic (i.e., frequency response) and acoustic properties of unfilled and polyurethane-filled aluminum cellular solids to characterize their behavior and explore their benefits in terms of vibration and noise-damping. It is shown that polyurethane filling can increase the vibration damping and transmission loss, especially if the infiltration process uses flexible foams. Considering sound reflection, however, it is shown that polyurethane filled samples (0.27–0.30 at 300 Hz) tend to display lower values of sound absorption coefficient relatively to unfilled samples (0.75 at 600 Hz), is this attributed to a reduction in overall porosity, tortuosity and flow resistivity. Foam-filled samples (43–44 dB at 700–1200 Hz) were shown to be more suitable to reduce sound transmission rather than reflection than unfilled samples (21 dB at 700 Hz). It was shown that the morphology of these cellular solids might be optimized depending on the desired application: (i) unfilled aluminum cellular solids are appropriate to mitigate internal noises due to their high sound absorption coefficient; and (ii) PU filled cellular solids are appropriate to prevent exterior noises and vibration damping due to their high transmission loss in a wide range of frequencies and vibration damping.This work was supported by Fundação para a Ciência e a Tecnologia FCT under the research Doctoral Grant PD/BD/114096/2015, project UIDP/04077/2020 and UIDB/04436/2020, and Stimulus of Scientific Employment Application CEECIND/03991/2017

Significance of cell number on the bulk elastic properties of auxetic reentrant lattices

Auxetics are characterized by a negative Poisson’s ratio, expanding/contracting in tension/compression. Given this behavior, they are expected topossess high shear, fracture and indentation resistance, and superior damping. The lack of natural isotropic auxetics promoted an effort to designstructures that mimic this behavior, e.g. reentrant model. This last is based on honeycombs with inverted protruding ribs. Commonly, this modelis employed in lattices and has been thoroughly studied in terms of mechanical properties and deformation behavior. Given that the amount ofcells has an influence in the overall internal structural behavior, there seems to be an absence of data that determines the minimum number of cellsfor such structure to present internal static bulk properties. Recurring to FEA, this study determines the minimum number of cells to obtain anoverall face constrained auxetic lattice with internal bulk elastic behavior, namely in terms of normalized Young’s modulus and Poisson’s ratio. Itis shown that adding reentrant cells increases the Poisson’s ratio on an exponential rise to maximum function, reducing the normalized Young’smodulus on an exponential decay function. Fundamentally, a minimum number of 13 cells per row to obtain an internal bulk behavior in latticeswith constrained faces.Supported by the project iRAIL Innovation in Railway Systems and Technologies Doctoral Programme funds and by national funds through FCT – Portuguese Foundation for Science and Technology and was developed on the aim of the Doctoral grant PD/BD/114096/2015

The role of acoustic pressure during solidification of AlSi7Mg alloy in sand mold casting

New alloy processes have been developed and casting techniques are continuously evolving. Such constant development implies a consequent development and optimization of melt processing and treatment. The present work proposes a method for studying the influence of acoustic pressure in the overall refinement of sand cast aluminum alloys, using and correlating experimental and numerical approaches. It is shown that the refinement/modification of the α-Al matrix is a consequence of the acoustic activation caused in the liquid metal directly below the face of the acoustic radiator. Near the feeder, there is a clear homogeneity in the morphology of the α-Al with respect to grain size and grain circularity. However, the damping of acoustic pressure as the melt is moved away from the feeder increases and the influence of ultrasound is reduced, even though the higher cooling rate seems to compensate for this effect.This work was supported by FCT funding through the project PTDC/EMEEME/30967/2017 (NORTE-0145-FEDER-030967) and by FEDER funding through COMPETE 2020, NORTE2020, PORTUGAL2020 – Programa Operacional Competitividade e Internacionalização (POCI). Additionally, this work was supported by FCT with the reference project UID/EEA/04436/2019.info:eu-repo/semantics/publishedVersio

Macro-, meso- and microstructural characterization of metallic lattice structures manufactured by additive manufacturing assisted investment casting

Cellular materials are recognized for their high specific mechanical properties, making them desirable in ultra-lightweight applications. Periodic lattices have tunable properties and may be manufactured by metallic additive manufacturing (AM) techniques. However, AM can lead to issues with un-melted powder, macro/micro porosity, dimensional control and heterogeneous microstructures. This study overcomes these problems through a novel technique, combining additive manufacturing and investment casting to produce detailed investment cast lattice structures. Fused filament fabrication is used to fabricate a pattern used as the mold for the investment casting of aluminium A356 alloy into high-conformity thin-ribbed (~ 0.6 mm thickness) scaffolds. X-ray micro-computed tomography (CT) is used to characterize macro- and meso-scale defects. Optical and scanning electron (SEM) microscopies are used to characterize the microstructure of the cast structures. Slight dimensional (macroscale) variations originate from the 3D printing of the pattern. At the mesoscale, the casting process introduces very fine (~ 3 µm) porosity, along with small numbers of (~ 25 µm) gas entrapment defects in the horizontal struts. At a microstructural level, both the (~ 70 μm) globular/dendritic grains and secondary phases show no significant variations across the lattices. This method is a promising alternative means for producing highly detailed non-stochastic metallic cellular lattices and offers scope for further improvement through refinement of filament fabrication.This work was supported by Portuguese FCT, under the reference project UIDB/04436/2020. We are grateful to the funding from the European Research Council through the ERC grant CORREL-CT, number 695638 to enable VHC to visit the Henry Royce Institute to undertake the X-ray CT studies. Tis work was supported by the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P025021/1 and EP/P025498/1 and the Henry Moseley X-ray Imaging Facility funded by EP/T02593X/1

Optimizing high-volume ultrasonic melt degassing using synchronized kinematic translation

Ultrasonic vibration is a physical processing technique that has been gathering support as an environmentally friendly approach to degas light alloy melts. Since metallic sonotrodes promote melt inclusions due to erosion, ceramic sonotrodes have also been shown as a viable solution for ultrasonic melt degassing in industrial scales. This study shows that resonant ceramic sonotrodes are characterized by a complex low-amplitude radial eigen mode, while particle image velocimetry reveals that their efficiency depends on the angular direction. An approach based on synchronized kinematic translation was designed to optimize the degassing efficiency in ultrasonic approaches, assuring its angle with higher cavitation is always facing the center of the crucible. Results show that this approach can reach lower degassing thresholds (Hmin ¼ 0.13 ml/100 g Al) at higher degassing rates, relatively to both Argon inflation (Hmin ¼ 0.22 ml/100 g Al) and static Ultrasound (Hmin ¼ 0.18 ml/100 g Al) methods. An enhanced grain refinement further supports the hypothesis that promoting a synchronized kinematic translation enhances the ultra sonic degassing efficiency. Considering these results, this approach is suggested as a reliable route to implement efficient ultrasonic degassing techniques in industrial light alloy melt treatment.This work was supported by PTDC/EMEEME/30967/2017 and NORTE-0145-FEDER-030967, co-financed by the European Regional Development Fund (ERDF), through the Operational Programme for Competitiveness and Internationalization (COMPETE 2020), under Portugal 2020, and by the Fundação para a Ciencia e a Tecnologia e FCT I.P. national funds. Also, this work was supported by Portuguese FCT, under the reference project UIDB/04436/2020 and Stimulus of Scientific Employment Application CEECIND/03991/2017

Manufacturing methodology on casting-based aluminium matrix composites: systematic review

Ongoing industrial demand for lightweight materials has spiked the research interest in aluminium-based metal matrix composites for its specific properties. The amount of scientific publication available on the matter has led to the vast production of knowledge, which highlights the need for a systematic assessment if further progress is expected. In this paper, a systematic review of the published literature is conducted, according to the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses, on the Scopus and Web of Science databases were used in the literature search, which was completed on the 29 August 2020. The data of the research work is structured in the particle pre-processing stage and the melt processing stage. The present review clarifies the combined pair-wise effect of particles and the melt treatment performed on their wettability or dispersive or de-agglomerative capability, which allows to achieve their final mechanical properties.This work was supported by PTDC/EMEEME/30967/2017 and NORTE-0145-FEDER030967, co-financed by the European Regional Development Fund (ERDF), through the Operational Programme for Competitiveness and Internationalization (COMPETE 2020), under Portugal 2020, and by the Fundação para a Ciência e a Tecnologia—FCT I.P. national funds. Also, this work was supported by Portuguese FCT, under the reference project UIDB/04436/2020, Stimulus of Scientific Employment Application CEECIND/03991/2017, research doctoral Grant 2020.08564.BD

Thin-rib and high aspect ratio non-stochastic scaffolds by vacuum assisted investment casting

Cellular structures are a classic route to obtain high values of specific mechanical properties. This characteristic is advantageous in many fields, from diverse areas such as packaging, transportation industry, and/or medical implants. Recent studies have employed additive manufacturing and casting techniques to obtain non-stochastic cellular materials, thus, generating an in situ control on the overall mechanical properties. Both techniques display issues, such as lack of control at a microstructural level in the additive manufacturing of metallic alloys and the difficulty in casting thin-rib cellular materials (e.g., metallic scaffolds). To mitigate these problems, this study shows a combination of additive manufacturing and investment casting, in which vacuum is used to assist the filling of thin-rib and high aspect-ratio scaffolds. The process uses 3D printing to produce the investment model. Even though, vacuum is fundamental to allow a complete filling of the models, the temperatures of both mold and casting are important to the success of this route. Minimum temperatures of 250 °C for the mold and 700 °C for the casting must be used to guarantee a successful casting. Cast samples shown small deviations relatively to the initial CAD model, mainly small expansions in rib length and contraction in rib thickness may be observed. However, these changes may be advantageous to obtain higher values of aspect ratio in the final samples.This research was supported by the project iRAIL Innovation in Railway Systems and Technologies Doctoral Programme funds and by national funds through FCT—Portuguese Foundation for Science and Technology and was developed on the aim of the Doctoral grant PD/BD/114096/2015.info:eu-repo/semantics/publishedVersio

A comparative thermoacoustic insulation study of silica aerogels reinforced with reclaimed textile fibres: cotton, polyester and wool

Silica aerogels are highly porous materials with exceptional thermal insulation performance. They become even more attractive if combined thermal and acoustic insulation is achieved. Silica aerogel composites reinforced with fibres are an ingenious way to surpass the fragility stemmed from the aerogel’s intrinsic porosity, and textile fibres are good sound absorption materials. Reclaimed fibres are a relatively low-cost feedstock and were obtained in this work exclusively through mechanical processes from textile wastes, thus promoting the concept of circular economy, namely for cotton, polyester and wool fibres. These reclaimed fibres were used as reinforcement matrices for silica aerogel composites obtained from sol–gel transformation of tetraethyl orthosilicate and isobutyltriethoxysilane/or vinyltrimethoxysilane precursors and dried at ambient pressure after silylation. Silica aerogel composites reinforced with reclaimed cotton fibres had the best sound absorption coefficient (a peak value of 0.89), while the polyester-reinforced composite exhibited the lowest thermal conductivity (k = ~24 mW m−1 K−1, Hot Disk). The better combined results on thermal and acoustic insulation were achieved by the wool-reinforced composites. The thermal conductivity values were less than 27 mW m−1 K−1, and the sound absorption coefficient achieved a peak value of 0.85. Therefore, the aerogel composites developed here can be selected for thermal or/and acoustic barriers by choosing a suitable type of fibre. Their design and preparation protocol followed environmental-friendly and cost-effective approaches.Teresa Linhares acknowledges the PhD grant Ref. SFRH/BD/131819/2017, attributed by Fundação para a Ciência e Tecnologia, I.P. (FCT, Portugal), funded by national funds from MCTES (Ministério da Ciência, Tecnologia e Ensino Superior) and, when appropriate, co-funded by the European Commission through the European Social Fund. Consumables for the syntheses and characterizations performed at CIEPQPF and 2C2T research units were funded by the European Regional Development Fund (ERDF), through COMPETE 2020 Operational Programme for Competitiveness and Internationalization, combined with Portuguese National Funds, through FCT, I.P. under the projects POCI-01-0145-FEDER-006910 and POCI-01-0145-FEDER-007136 (FCT Refs. UIDB/EQU/00102/2020 and UID/CTM/00264/2020, respectively)

Effects of different environmental conditions on the mechanical characteristics of a structural epoxy

With the aim of characterising a commercially available epoxy adhesive used for fibre-reinforced polymers strengthening applications, when submitted to different environmental conditions, mainly thermal (TC), freeze-thaw (FT), and wet-dry (WD) cycles and immersion in pure (PW) and water with chlorides (CW) for periods of exposure that lasted up to 16 months, an experimental program was carried out. Several methodologies were used in its characterization, mainly the scanning electron microscope (SEM), dynamic mechanical analysis (DMA), standard tensile tests (STT) coupled with digital image correlation (DIC). In general the results revealed that the chemical composition was not affected by the environmental conditions. Nevertheless, it was verified through DMA and STT that the modulus of elasticity and tensile strength of the epoxy adhesive increased in the TC, while the specimens submitted to PW and CW faced a high degradation in terms of its mechanical properties. Eventually, the glass transition temperature (Tg) was not affected by the environmental conditions, apart from the specimens subjected to TC and FT, presenting a higher and lower Tg, respectively, when compared with the reference specimens.This work is supported by FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project FPReDur PTDC/ECM-EST/2424/2012. The first and second authors wishes also to acknowledge the grants SFRH/BD/89768/2012 and SFRH/BD/80338/2011, respectively, provided by FCT