1,721,007 research outputs found
DEVELOPMENT OF EPOXIES MATRICES AND STUDY OF PROCESS PARAMETERS ON ELECTRON BEAM CURING OF POLYMERIC COMPOSITES
No full textO objetivo deste trabalho foi desenvolver matrizes epoxídicas e estudar os parâmetros de processo de cura para fabricação de compósitos poliméricos curados por feixe de elétrons. A fabricação de compósitos poliméricos estruturais pelo processo térmico necessita de um ciclo de cura de algumas horas e elevada temperatura, implicando num alto custo de produção. A tecnologia de cura por feixe de elétrons permite curar matrizes epoxídicas em poucos minutos à temperatura ambiente e proporcionar temperaturas de transição vítrea superiores às obtidas pelos sistemas curados pelo processo térmico. Neste trabalho foi utilizada uma resina epoxídica do tipo éter diglicidílico de bisfenol A (DGEBA) e o hexafluoroantimonato de diariliodônio como iniciador catiônico. O tempo do ciclo de cura do compósito polimérico obtido por feixe de elétrons foi de 40 minutos até completar uma dose total aproximada de 200kGy. O grau de cura e a temperatura de transição vítrea foram avaliados por técnicas de análise térmica. A temperatura de transição vítrea determinada por meio da técnica de termodinâmico-mecânica (DMTA) foi de 167ºC (tan ) e o grau cura de 96% obtido pela técnica de calorimetria exploratória diferencial (DSC). A mesma resina epoxídica curada pelo processo térmico por um período de 16 horas, com um endurecedor à base de anidrido, atingiu uma temperatura de transição vítrea de 125ºC (tan ).The aim of this work was to develop epoxies matrices and study of process parameters on electron beam curing of polymeric composites. The production of advanced polymeric composites cured by thermal process needs a cure cycle of quite a few hours as well as high temperatures which result in high production costs. The electron beam curing technology makes possible to cure epoxy matrices in a few minutes at room temperature and it results in glass transition temperatures which are far better than those obtained with systems cured by thermal process. In this work an epoxy resin of the diglycidyl ether of bisphenol A (DGEBA) type was used, as well as, a diaryliodonium hexafluoroantimonate as the cationic initiator. The cure cycle time for the electron beam curing polymeric composite was 40 minutes to complete an approximated total dose of 200kGy. The degree of cure and the glass transition temperature were determined by thermal analysis techniques. The glass transition temperature was determined by a dynamic mechanical analyzer (DMTA) and was 167ºC (tan ) and the degree of cure was 96%, which was determined by differential scanning calorimetry technique (DSC). The same epoxy resin with an anhydride curing agent, took 16 hours when cured by thermal process and its glass transition temperature was 125ºC (tan )
Dynamic translaminar fracture toughness of aeronautical grade titanium-graphite hybrid fiber-metal laminate
No full textDiversos critérios de tenacidade à fratura translaminar dinâmica foram determinados para o laminado híbrido metal-fibra TiGra, empregando-se conceitos e metodologias da Mecânica da Fratura Elástica Linear MFEL (fator-K) e da Mecânica da Fratura Elasto-Plástica MFEP (integral-J). Verificou-se que as tenacidades de iniciação elasto-plástica, Jid, e de carga máxima, Jmd, do TiGra são controladas pelo desenvolvimento, ou supressão de delaminações. Os resultados indicaram que o emprego deste material se justifica mais pela sua resistência à propagação de danos (caracterizada por Jmd) do que à iniciação da fratura dinâmica (por Jid). De modo geral, os requisitos de validade de Jid como verdadeira propriedade do material (JId) foram satisfeitos, embora para Jmd boa parte das restrições quanto ao tamanho mínimo do corpo-de-prova tenha sido violada. Mais freqüentemente, velocidades mais rápidas de impacto beneficiaram as tenacidades-J do TiGra, enquanto que temperaturas mais elevadas afetaram negativamente estas propriedades. Quanto à MFEL, a tenacidade KJd do TiGra foi beneficiada pelo incremento na taxa de carregamento sob temperaturas mais elevadas, enquanto que a tenacidade Kid foi negativamente afetada pela taxa de deformação em todas as temperaturas avaliadas. Temperaturas mais altas também degradaram as propriedades de tenacidade-K do TiGra. Em oposição às tenacidades-J, os critérios KJd e Kid não satisfizeram em absoluto os mais exigentes critérios de contenção de plasticidade estabelecidos pela MFEL, se comparados aos propostos pela MFEP. Por fim, o desempenho mecânico do laminado TiGra foi severamente comprometido quando do cômputo da densidade específica para a determinação das tenacidades J e K por unidade de massa, sendo nesta ocasião o laminado híbrido facilmente superado por vários laminados convencionais da classe dos Carbono-Epóxi.Several dynamic translaminar fracture toughness criteria have been determined for TiGr hybrid fiber-metal laminate through Linear Elastic (K-factor) and Elastic-Plastic (J-integral) Fracture Mechanics (LEFM and EPFM, respectively) concepts and methodologies. Instrumented Charpy impact testing was carried out over a wide range of temperatures under two loading rates. It has been discovered that the elastic-plastic initiation toughness, Jid, and the toughness at maximum load, Jmd, of TiGr are controlled by either delamination favoring or suppression. Impact tests revealed that the in-service use of TiGr must rely on its resistance to dynamic fracture propagation (as characterized by Jmd) rather than on fracture initiation (by Jid). In a broad sense, the requirements for Jid data validity as a material property (JId) were fulfilled, whereas many restrictive demands in regard to the minimum testpiece size were violated by the Jmd criterion. Generally, higher impact velocities were beneficial to TiGrs J-toughnesses, inasmuch as higher temperatures impaired these properties. Regarding the LEFM approach, KJd toughness of TiGr laminate was imparted by faster impacts at higher temperatures, whilst the strain rate negatively influenced the Kid toughness over the whole temperature range tested. Higher temperatures also degraded the K-toughness properties of TiGr hybrid laminate. Differently from J-toughnesses values, the KJd e Kid criteria did not satisfy at all the more stringent criteria set forth by the LEFM approach with regard to plastic constraint, as compared to those established by EPFM. Finally, the mechanical performance of TiGr laminate was overwhelmingly compromised as the materials specific gravity was taken in account to obtain K and J toughness values by unit weight, so that TiGr was by far exceeded in this regard by conventional Carbon/Epoxy composite laminates
Laser microesculpture of metallic surfaces to hybrid fiber-metal laminates
No full textEste trabalho objetivou a manufatura de laminados híbridos metal-fibra (LMF) empregando-se chapas com 0,5 mm de espessura de liga-\'TI\'6\'AL\'4\'V\' com superfícies modificadas por laser de fibra de modo a otimizar a sua adesão com polímero termoplástico poli-sulfeto de fenileno (PPS). Observou-se que a microtextura superficial da liga metálica dependeu fortemente da potência do feixe laser, quando potências mais baixas levaram à verdadeira texturização da superfície metálica, enquanto que potências mais elevadas conduziram à ablação da mesma. A texturização superficial metálica sob laser de baixa potência aparentou ser a condição mais apropriada para a adesão metal-polímero por ancoragem mecânica de macromoléculas, o que foi contrabalanceado por elevados níveis de tensão residual das chapas metálicas, gerando grande distorção das mesmas e inviabilizando sua utilização. O emprego de uma potência intermediária (160 W) mostrou-se propício à otimização entre a adesão física entre metal-polímero e o nível de tensões residuais criado nas chapas metálicas. Concluiu-se que os espécimes extraídos do centro dos laminados metal-fibra exibem uma tensão limite média para falha por cisalhamento interlaminar consideravelmente superior à dos espécimes usinados a partir da borda dos LMF. O LMF manufaturado sob maiores pressão e temperatura exibiu uma maior compactação e melhor consolidação, culminando num máximo desempenho médio sob carga de cisalhamento interlaminar. Evidências de uma correlação entre o mecanismo de falha por cisalhamento interlaminar do corpo de prova e o seu nível de resistência a este tipo de carregamento mecânico foram documentadas e discutidas.This work aimed at manufacturing hybrid fiber-metal laminates (FML) by employing 0,5 mm-thick \'TI\'6\'AL\'4\'V\'-alloy plaques with fiber laser modified surface in order to optimize metal adhesion with poli-phenylene sulfide (PPS) thermoplastic polymer. The surface microtexture of metallic alloy strongly depended upon the laser power, inasmuch as low-power laser led to true texturization of metal surface, whereas high-power laser light drove to its ablation. Surface metal texturization under low-power laser apparently was the most appropriate condition to metal-polymer adhesion via mechanical entanglement of macromolecules, which was offset by high levels of residual stresses on metallic plaques, bringing them quite warped and useless. The use of an intermediate laser power (160 W) has been shown benign to the optimization between metal-polymer physical adhesion and the residual stress level created in the metal plates. It has been concluded that testpieces machined from the FML central position exhibited average ultimate interlaminar shear strenght considerably higher than those extracted from the FML borders. The FML manufactured under higher pressure and temperature was more compacted and better consolidated, so that it displayed the greatest average performance under interlaminar shear loading. Evidences of a correlation between the failure mechanism by interlaminar shearing of test coupon and its allowance to this type of mechanical loading were documented and discussed
Damage in composite materials: SAFECom (System of Analysis of Fatigue for Composites)
No full textO presente trabalho, inicialmente, apresenta uma revisão a cerca de vários métodos de análise de fadiga e tolerância ao dano em materiais compósitos e, em particular, o conceito de elemento crítico e o método da resistência residual é estudado e aplicado à análise de materiais compósitos poliméricos, reforçados por fibras contínuas, sob cargas cíclicas. Casos de laminados com concentradores de tensão são também abordados, principalmente, devido ao crescente emprego de materiais compósitos. Diante do contexto apresentado, o presente trabalho buscou desenvolver uma ferramenta computacional, que fosse capaz de prever de forma consistente tanto a vida em fadiga de estruturas em material compósito como a resistência residual dessas estruturas sob a ação de carregamentos cíclicos. Dessa forma, um modelo baseado na resistência residual, que envolve integração numérica, é implementado em um programa computacional denominado SAFECom (Sistema de Análise de Fadiga de Estruturas em Compósito), o qual é aplicado a alguns estudos de casos. O SAFECom foi, então, avaliado, comparando resultados obtidos com dados da literatura. Verifica-se uma boa convergência entre os resultados nessa fase de avaliação. Por fim, são listadas possibilidades de aprimoramento do programa, que podem torná-lo aplicável a outros estudos de casos não abordados no presente trabalho, além da avaliação do mesmo através de ensaios experimentais.This work firstly presents an overview about various methods of analysis of fatigue and damage tolerance of composite materials and, in particular, the concept of critical element and the method of residual strength are studied and applied to the analysis of polymer composites reinforced by continuous fibers under cyclic loading. Cases of laminates with stress concentrators are also addressed, mainly due to increasing use of composite materials. In this context, this work aimed to develop a computational tool that was able to consistently predict both the fatigue life of structures in composite material and the residual strength of these structures under the action of cyclic loads. Thus, a model based on residual strength, which involves numerical integration, is implemented in a computer program called SAFECom (System of Analysis of Fatigue for Composites), which is applied to some case studies. The SAFECom is evaluated by comparing results obtained from literature data. There is a good convergence between the results of this assessment. Finally, possibilities for improvement are listed on the program that can make it applicable to other case studies, which were not covered in this study, in addition to its evaluation by experimental tests
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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