Caracterização experimental e modelação numérica de espumas metálicas

As espumas de alumínio são materiais ultraleves, o que as torna atractivas para um largo espectro de aplicações comerciais na área da defesa, na indústria automóvel e aeroespacial, entre outras. Actualmente, há um grande interesse na utilização de espumas de alumínio em componentes estruturais ultraleves, incorporados em sistemas de absorção de energia para protecção contra o impacto. O recurso à simulação numérica para resolver problemas de engenharia em várias áreas é cada vez mais comum. A modelação numérica dos materiais assume vital importância quando o problema envolve a análise de processos tecnológicos como, por exemplo, a conformação plástica de materiais, ou a análise de estruturas. Deste modo, torna-se imprescindível garantir que a modelação dos materiais é de tal forma rigorosa que permite simular o melhor possível o seu comportamento real nas condições concretas da análise a realizar. A forma mais comum de garantir o rigor dos modelos utilizados é a validação dos modelos numéricos tendo por base resultados experimentais. Neste trabalho, fez-se a caracterização do comportamento mecânico das espumas de alumínio com nome comercial ALPORAS!, obtidas pelo processo de fabrico denominado expansão directa do metal fundido por adição de um agente expansor. Esta caracterização consistiu num conjunto de ensaios experimentais quer no regime quasi-estático, quer no regime dinâmico. No regime quasi-estático realizaram-se ensaios de compressão uniaxial e de compressão multiaxial. Para a caracterização no regime dinâmico foram realizados ensaios em barras de Hopkinson de polimetil-metacrilato (PMMA). Com base nos resultados experimentais obtidos determinaram-se os parâmetros dos dois modelos constitutivos para espumas metálicas implementados no programa comercial Abaqus™/Explicit. Estes modelos, e os respectivos parâmetros determinados, foram validados reproduzindo numericamente alguns ensaios experimentais quasi-estáticos e dinâmicos. Assim, verificou-se a adequabilidade dos modelos em diversas condições quer em termos de esforços quer em termos de regime de taxa de deformação Por último, desenvolveu-se uma estrutura inovadora para absorção de energia durante um impacto, constituída por componentes perfilados em liga de alumínio e por componentes em espumas de alumínio. Esta estrutura foi testada exclusivamente com recurso à simulação numérica, utilizando os modelos constitutivos validados anteriormente.

Laser assisted directional solidification of zirconia-based eutectics

Directionally solidified zirconia-based eutectic (DSE) fibres were obtained using the laser floating zone (LFZ) method. Two systems were investigated: zirconia-barium zirconate and zirconia-mullite. The purpose was to take advantage of zirconia properties, particularly as an ionic conductor and a mechanical rein-forcement phase. The influence of processing conditions in the structural and microstructural characteristics and their consequences on the electrical and mechanical behaviour were the focus of this thesis. The novel zirconia-barium zirconate eutectic materials were developed in order to combine oxygen ionic conduction through zirconia with protonic conduction from barium zirconate, promoting mixed ionic conduction behaviour. The mi-crostructure of the fibres comprises two alternated regions: bands having coarser zirconia-rich microstructure; and inter-band regions changing from a homogeneous coupled eutectic, at the lowest pulling rate, to columnar colony microstructure, for the faster grown fibres. The bands inter-distance increases with the growth rate and, at 300 mm/h, zirconia dendrites develop enclosed in a fine-interpenetrated network of 50 vol.% ZrO2-50 vol.% BaZrO3. Both phases display contiguity without interphase boundaries, according to impedance spec-troscopy data. Yttria-rich compositions were considered in order to promote the yttrium incorporation in both phases, as revealed by Raman spectroscopy and corroborated by the elemental chemical analysis in energy dispersive spectros-copy. This is a mandatory condition to attain simultaneous contribution to the mixed ionic conduction. Such results are supported by impedance spectrosco-py measurements, which clearly disclose an increase of total ionic conduction for lower temperatures in wet/reduction atmospheres (activation energies of 35 kJ/mol in N2+H2 and 48 kJ/mol in air, in the range of 320-500 ºC) compared to the dry/oxidizing conditions (attaining values close to 90 kJ/mol, above 500 ºC). At high temperatures, the proton incorporation into the barium zirconate is un-favourable, so oxygen ion conduction through zirconia prevails, in dry and oxi-dizing environments, reaching a maximum of 1.3x10-2 S/cm in dry air, at ~1000 ºC. The ionic conduction of zirconia was alternatively combined with another high temperature oxygen ion conductor, as mullite, in order to obtain a broad elec-trolytic domain. The growth rate has a huge influence in the amount of phases and microstructure of the directionally solidified zirconia-mullite fibres. Their microstructure changes from planar coupled eutectic to dendritic eutectic mor-phology, when the growth rate rises from 1 to 500 mm/h, along with an incre-ment of tetragonal zirconia content. Furthermore, high growth rates lead to the development of Al-Si-Y glassy phase, and thus less mullite amount, which is found to considerably reduce the total ionic conduction of as-grown fibres. The reduction of the glassy phase content after annealing (10h; 1400 ºC) promotes an increase of the total ionic conduction (≥0.01 S/cm at 1370 °C), raising the mullite and tetragonal zirconia contents and leading to microstructural differ-ences, namely the distribution and size of the zirconia constituent. This has important consequences in conductivity by improving the percolation pathways. A notable increase in hardness is observed from 11.3 GPa for the 10 mm/h pulled fibre to 21.2 GPa for the fibre grown at 500 mm/h. The ultra-fine eutectic morphology of the 500 mm/h fibres results in a maximum value of 534 MPa for room temperature bending strength, which decreases to about one-fourth of this value at high temperature testing (1400 ºC) due to the soft nature of the glassy-matrix.

Seismic vulnerability assessment of slender masonry structures

Slender masonry structures are distributed all over the world and constitute a relevant part of the architectural and cultural heritage of humanity. Their protection against earthquakes is a topic of great concern among the scientific community. This concern mainly arises from the strong damage or complete loss suffered by this group of structures due to catastrophic events and the need and interest to preserve them. Although the great progress in technology, and in the knowledge of seismology and earthquake engineering, the preservation of these brittle and massive structures still represents a major challenge. Based on the research developed in this work it is proposed a methodology for the seismic risk assessment of slender masonry structures. The proposed methodology was applied for the vulnerability assessment of Nepalese Pagoda temples which follow very simple construction procedure and construction detailing in relation to seismic resistance requirements. The work is divided in three main parts. Firstly, particular structural fragilities and building characteristics of the important UNESCO classified Nepalese Pagoda temples which affect their seismic performance and dynamic properties are discussed. In the second part the simplified method proposed for seismic vulnerability assessment of slender masonry structures is presented. Finally, the methodology proposed in this work is applied to study Nepalese Pagoda temples, as well as in the efficiency assessment of seismic performance improvement solution compatible with original cultural and technological value.

Post-fire vegetation regeneration in Portugal : implications for management

This thesis aims at improving the knowledge on the post-fire vegetation regeneration. For that, forests and shrublands were studied, after forest fires and experimental fires. Maritime Pine (Pinus pinaster) recruitment after fire was studied. Fire severity was evidenced as a major effect on this process. High crown fire severity can combust the pines, destroying the seed bank and impeding post fire pine recruitment. However, crown combustion also influences the post-fire conditions on the soil surface, since high crown combustion (HCC) will decrease the postfire needle cast. After low crown combustion (LCC) (scorched rather than torched crowns), a considerable needle cover was observed, along with a higher density of pine seedlings. The overall trends of post-fire recruitment among LCC and HCC areas could be significantly attributed to cover by needles, as well by the estimation of fire severity using the diameters of the burned twigs (TSI). Fire increased the germination from the soil seed bank of a Pinus pinaster forest, and the effects were also related with fire severity. The densities of seedlings of the dominant taxa (genus Erica and Calluna vulgaris) were contrastingly affected in relation to the unburned situation, depending on fire severity, as estimated from the degree of fire-induced crown damage (LCC/HCC), as well as using a severity index based on the diameters of remaining twigs (TSI). Low severity patches had an increase in germination density relatively to the control, while high severity patches suffered a reduction. After an experimental fire in a heathland dominated by Pterospartum tridentatum, Erica australis and E. umbellata, no net differences in seedling emergence were observed, in relation to the pre-fire situation. However, rather than having no effect, the heterogeneity of temperatures caused by fire promoted caused divergent effects over the burned plot in terms of Erica australis germination – a progressive increased was observed in the plots were maximum temperature recorded ranged from 29 to 42.5ºC and decreased in plots with maximum temperature ranging from 51.5 to 74.5ºC. In this heathland, the seed density of two of the main species (E. australis and E. umbellata) was higher under their canopies, but the same was not true for P. tridentatum. The understory regeneration in pine and eucalypt stands, 5 to 6 years post fire, has been strongly associated with post-fire management practices. The effect of forest type was, comparatively, insignificant. Soil tilling, tree harvesting and shrub clearance, were linked to lower soil cover percentages. However, while all these management operations negatively affected the cover of resprouters, seeders were not affected by soil tilling. A strong influence of biogeographic region was identified, suggesting that more vulnerable regions may suffer higher effects of management, even under comparatively lower management pressure than more productive regions. This emphasizes the need to adequate post-fire management techniques to the target regions.

Asymmetric rolling of 5182 aluminium alloy and interstitial free steel sheets

This Ph.D. research focuses on asymmetric rolling (ASR), as an alternative method for improving mechanical responses of aluminium-magnesium alloy and interstitial free (IF) steel regarding industrial requirements. Aluminium alloys are attractive materials in various industries due to their appropriate properties such as low density and corrosion resistance; however, their low formability has limited their applications. As formability of aluminium alloys can be improved through texture development, part of this dissertation is dedicated to producing the desired crystallographic texture with the ASR process. Two types of ASR (i.e. reverse and continuous asymmetric rolling) were investigated. The impact of shear deformation imposed by ASR processes on developing the desirable texture and consequently on mechanical behaviours was observed. The developed shear texture increased the normal and also planar anisotropy. Texture evolution during plastic deformation as well as induced mechanical behaviour were simulated using the “self-consistent” and Taylor models. Interstitial free (IF) steel was the second material selected in this dissertation. Since IF steel is one of the most often used materials in automotive industries it was chosen to investigate the effect of shear deformation through ASR on its properties. Two types of reverse and continuous asymmetric rolling were carried out to deform IF steel sheets. The results of optical microscopy and atomic force microscopy observations showed no significant difference between the grains’ morphology of asymmetric and conventionally rolled samples, whereas the obtained results of transmission electron microscopy indicated that fine and equiaxed dislocation cells were formed through the asymmetric rolling process. This structure is due to imposed shear deformation during the ASR process. Furthermore, the mechanical behaviour of deformed and annealed sheets was evaluated through uniaxial tensile tests. Results showed that at low thickness reductions (18%) the asymmetric rolled sample presented higher stress than that of the conventionally rolled sheet; while for higher thickness reductions (60%) the trend was reversed. The texture analyses indicated that intense rolling texture components which developed through 60% thickness reduction of conventional rolling cause a relatively higher stress; on the contrary the fine structure resulting from ASR appears to be the source of higher stress observed after pre-deformation of 18%.

Avaliação da vulnerabilidade sísmica de edifícios escolares

A grande motivação para este trabalho resultou na avaliação da vulnerabilidade sísmica das escolas básicas e secundárias que fazem parte integrante do parque escolar de Portugal Continental. Apesar de até ao momento apenas se terem estudado a vulnerabilidade de algumas escolas em algumas zonas particulares do nosso país, para este trabalho de investigação tivemos uma ambição muito maior e obviamente fizemos um esforço muito maior. Estabelecemos assim como meta, a avaliação de todo o parque escolar. Embora todo o parque escolar possua na ordem das três centenas de escolas em todo o território nacional e sendo este projeto de reabilitação, um projeto com a duração de 2007 a 2015. Em 2011, por motivos da crise económica, todo o projeto congelou sendo reabilitadas até à data cerca de apenas um terço das escolas. Esta quantidade foi o número de escolas que avaliamos em todo o país. As escolas, sendo edifícios públicos com uma importância fundamental, tanto pela elevada concentração de jovens, como pela função essencial como centros de aprendizagem para as gerações vindouras, como também pela ameaça que representam na eventualidade dum cenário sísmico pela enorme densidade de utilizadores, e pela vantagem de nesse cenário de catástrofe a importância estrutural ser superior em relação à maior parte dos edifícios correntes, devidamente demonstrado pelos argumentos enumerados, consequentemente as escolas podem servir como instalações de proteção civil perante uma catástrofe sísmica para apoio das populações circundantes afetadas. Portanto para cada uma das escolas cordialmente fornecidas pelo Parque Escolar, E.P.E., foi feito um estudo exaustivo e muito individual de cada uma das escolas, onde cada análise foi desenvolvida por uma metodologia simplificada, sendo cada análise sempre individual, e nunca tendo sida aplicada em série, este fator melhora substancialmente a eficácia da avaliação para a quantificação das vulnerabilidades e da determinação do grau de dano e das frações de perda para os requisitos fundamentais de limitação de danos, de não colapso, e de colapso iminente que correspondem a ações com períodos de retorno de 95, 475 e 975 anos. Este trabalho é fundamental para as entidades competentes terem a consciência da vulnerabilidade das escolas secundárias, para puderem atuar a nível estrutural e diminuir assim a vulnerabilidade sísmica, e mesmo que por impossibilidade económica o poder governamental não intervenha, então pode e principalmente deve, elaborar planos de emergência tanto com engenheiros civis qualificados como com a total colaboração das corporações de bombeiros que fazem parte das forças de operação e socorro da Autoridade Nacional de Proteção Civil (ANPC).

Barrier properties of poly(lactic acid) based films

In recent years, the search for a environmentally friendly products has increased. One of the major challenges has been the demand for biodegradable materials that can replace plastic. If a few decades ago, plastic replaced, for example, the ivory in billiard balls, and in other products, saving the lives of thousands elephants, nowadays a replacement for that plastic is being searched, to prevent the change of the environmental conditions, essential to life in harmonly with the fauna and flora that the human specie has, in recent years, destroyed. Plastic is a petroleum derivate, whose price has been growing exponentially, mainly due to the fact of beind a cheap material and also to enable the production of products that are essential to modern life. Therefore, the petrochemical era is going to come to an end and a new environmentally sustainable era, based on biodegradable materials from renewable sources, will follow. The change to green routes only will be possible with the support of the major companies, and the implementation of drastic governmental law. Poly(lactic acid), PLA, is produced from the lactose present in the corn or sugarcane and has been intensively studied in recent years because if some limitants properties required its extrusion are overcome, it has the potential to replace the traditional polymers. PLA have high brittleness, low toughness and low tensile elongation. In this work, natural antioxidant (alpha-tocopherol) and synthetics antioxidants (BHT ant TBHQ) were added to the PLA with the aim not only to improve their flexibility, but also to create an active packaging to extend the shelf life of the foods and improve the organoleptic properties by preventing food losses. The impact of the addition of antioxidants into the PLA films, in its mechanical, thermal and barrier properties were studied by FTIR, DSC, SEM, AFM, DMA, TGA, QCM and time-lag techniques.

Oxidation and glycoxidation of phosphatidylserine and their effect in immune response : a lipidomic approach

Phosphatidylserine (PS) is a member of the class of phospholipids, and is distributed among all cells of mammalians, playing important roles in diverse biological processes, including blood clotting and apoptosis. When externalized, PS is a ligand that is recognized on apoptotic cells. It has been considered that before externalization PS is oxidized and oxPS enhance the recognition by macrophages receptors, however the knowledge about oxidation of PS is still limited. PS, like others phospholipids, has two fatty acyl chains and one polar head group, in this case is the amino acid serine. The modifications in PS structure can occur by oxidation of the unsaturated fatty acyl chains and by glycation of the polar head group, due to free amine group, thus increasing the susceptibility to oxidative events. The main goal of this work was to characterize and identify oxidized and glycoxidized PS, contributing to the knowledge of the biological role of oxidation products of PS, as well as of glycated PS, in immune and inflammatory processes. To achieve this goal, PS standards (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho- L-serine (POPS), 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS), 1- palmitoyl-2-linoleoyl-sn-glycero-3-phospho-L-serine (PLPS) and 1-palmitoyl-2- arachidonoyl-sn-glycero-3-phospho-L-serine (PAPS)) and glycated PS (PAPS and POPS) were induced to oxidize in model systems, using different oxidant reagents: HO• and 2,2'-azobis-2-methyl-propanimidamide dihydrochloride (AAPH) . The detailed structural characterization of the oxidative products was performed by ESI-MS and MS/MS coupled to separation techniques such as off line TLC-MS and on line LC-MS, in order to obtained better characterization of the larger number of PS and glycated PS oxidation products. The results obtained in this work allowed to identify several oxidation products of PS and glycated PS with modifications in unsaturated fatty acyl chain. Also, oxidation products formed due to structural changes in the serine polar head with formation of terminal acetamide, terminal hydroperoxyacetaldehyde.and terminal acetic acid (glycerophosphacetic acid, GPAA) were identified. The mass spectrometric specific fragmentation pathway of each type of oxidation product was determined using different mass spectrometry approaches. Based on the identified fragmentation pathways, targeted lipidomic analysis was performed to detect oxidation products modified in serine polar head in HaCaT cell line treated with AAPH. The GPAA was detected in HaCaT cells treated with AAPH to induce oxidative stress, thus confirming that modifications in PS polar head is possible to occur in biological systems. Furthermore, it was found that glycated PS species are more prone to oxidative modifications when compared with non glycated PS. During oxidation of glycated PS, besides the oxidation in acyl chains, new oxidation products due to oxidation of the glucose moiety were identified, including PS advanced glycation end products (PSAGES). To investigate if UVA oxidative stress exerted changes in the lipidome of melanoma cell lines, particularly in PS profile, a lipidomic analysis was performed. The lipid profile was obtained using HILIC-LC-MS and GC-MS analysis of the total lipid extracts obtained from human melanoma cell line (SKMEL- 28) after UVA irradiation at 0, 2 and 24 hours. The results did not showed significant differences in PS content. At molecular level, only PS (18:0:18:1) decreased at the moment of irradiation. The most significant changes in phospholipids content occurred in phosphatidylcholines (PC) and phosphatidylinositol (PI) classes, with an increase of mono-unsaturated fatty acid (MUFA), similarly as observed for the fatty acid analysis. Overall, these data indicate that the observed membrane lipid changes associated with lipogenesis after UVA exposure may be correlated with malignant transformations associated with cancer development and progression. Despite of UVA radiation is associated with oxidative damage, in this work was not possible observe oxidation phospholipids. The anti/pro-inflammatory properties of the oxidized PLPS (oxPLPS) versus non-oxidized PLPS were tested on LPS stimulated RAW 264.7 macrophages. The modulation of intracellular signaling pathways such as NF-kB and MAPK cascades by oxPLPS and PS was also examined in this study. The results obtained from evaluation of anti/pro-inflammatory properties showed that neither PLPS or oxPLPS species activated the macrophages. Moreover only oxidized PLS were found to significantly inhibit NO production and iNOS and il1β gene transcription induced by LPS. The analysis at molecular level showed that this was the result of the attenuation of LPS-induced c-Jun-N-terminal kinase (JNK) and p65 NF-kB nuclear translocation. Overall these data suggest that oxPLPS, but not native PLPS, mitigates pro-inflammatory signaling in macrophages, contributing to containment of inflammation during apoptotic cell engulfment. The results obtained in this work provides new information on the modifications of PS, facilitating the identification of oxidized species in complex samples, namely under physiopathologic conditions and also contributes to a better understanding of the role of oxPS and PS in the inflammatory response, in the apoptotic process and other biological functions.

Propriedades intrínsecas e de processamento de termoplásticos elastoméricos incorporando borracha de pneu reciclada

O trabalho apresentado tem por objetivo contribuir para a valorização da borracha proveniente de pneus em fim de vida, assente em princípios de sustentabilidade ambiental. A abordagem adotada para a concretização deste objetivo consiste na incorporação de borracha de pneus em formulações de base termoplástica e elastomérica (TPE), adequadas ao processo de moldação por injeção. São desenvolvidos estudos sobre a morfologia, propriedades mecânicas, térmicas e reológicas das ligas poliméricas à base de granulado de borracha de pneu (GTR). A falta de adesão entre o GTR e a matriz polimérica leva à degradação das propriedades mecânicas dos materiais obtidos. A estratégia explorada passa pela utilização de um elastómero para promover o encapsulamento do GTR e, desta forma, procurar obter ligas com propriedades mecânicas características de um TPE. São analisadas ligas ternárias (TPEGTR) compostas por polipropileno (PP) de elevada fluidez, GTR e elastómero virgem. O efeito da presença de diferentes elastómeros nas ligas é analisado neste trabalho: um elastómero de etilenopropileno- dieno (EPDM), e um novo elastómero de etileno-propileno (EPR) obtido por catálise metalocénica. O estudo da morfologia das ligas obtidas mostra haver interação entre os materiais, sendo possível inferir a viabilidade da estratégia adotada para promover a adesão do GTR. A incorporação de elastómero promove o aumento da resistência ao impacto e da extensão na rotura nas ligas, o que é atribuído, fundamentalmente, ao encapsulamento do GTR e ao aumento da tenacidade da matriz termoplástica. Com o objetivo de avaliar a influência da estrutura cristalina das ligas TPEGTR no seu comportamento mecânico, procede-se à análise do processo de cristalização sob condições isotérmicas e não isotérmicas. Neste estudo, é avaliado o efeito da presença dos materiais que constituem a fase elastomérica na cinética de cristalização. Para cada uma das ligas desenvolvidas, recorre-se ao modelo de Avrami para avaliar o efeito da temperatura no mecanismo de nucleação, na morfologia das estruturas cristalinas e na taxa de cristalização. Recorre-se à reometria capilar para estudar, sob condições estacionárias, o comportamento reológico das ligas TPEGTR. O modelo de Cross-WLF é utilizado para avaliar o comportamento reológico de todos os materiais, obtendo-se resultados similares àqueles obtidos experimentalmente. O comportamento reológico dos polímeros PP, EPR e EPDM é do tipo reofluidificante, tendo o EPR um comportamento reológico similar ao do PP e o EPDM um comportamento reo-fluidificante mais pronunciado. Em todas as ligas analisadas o comportamento reológico revela-se do tipo reo-fluidificante, sendo que a presença de GTR promove o aumento da viscosidade. Os parâmetros obtidos do modelo de Cross-WLF são utilizados para realizar a simulação da etapa de injeção recorrendo a um software comercial. Os resultados obtidos são validados experimentalmente pelo processo de moldação por injeção, evidenciando uma boa adequabilidade da aplicação deste modelo a estas ligas. O trabalho desenvolvido sobre ligas TPEGTR, constitui um contributo para a valorização da borracha proveniente de pneus em fim de vida, assente em princípios de sustentabilidade ambiental.