Strength and damage analysis of composite-aluminium adhesively-bonded single-lap joints

With the need to find an alternative way to mechanical and welding joints, and at the same time to overcome some limitations linked to these traditional techniques, adhesive bonds can be used. Adhesive bonding is a permanent joining process that uses an adhesive to bond the components of a structure. Composite materials reinforced with fibres are becoming increasingly popular in many applications as a result of a number of competitive advantages. In the manufacture of composite structures, although the fabrication techniques reduce to the minimum by means of advanced manufacturing techniques, the use of connections is still required due to the typical size limitations and design, technological and logistical aspects. Moreover, it is known that in many high performance structures, unions between composite materials with other light metals such as aluminium are required, for purposes of structural optimization. This work deals with the experimental and numerical study of single lap joints (SLJ), bonded with a brittle (Nagase Chemtex Denatite XNRH6823) and a ductile adhesive (Nagase Chemtex Denatite XNR6852). These are applied to hybrid joints between aluminium (AL6082-T651) and carbon fibre reinforced plastic (CFRP; Texipreg HS 160 RM) adherends in joints with different overlap lengths (LO) under a tensile loading. The Finite Element (FE) Method is used to perform detailed stress and damage analyses allowing to explain the joints’ behaviour and the use of cohesive zone models (CZM) enables predicting the joint strength and creating a simple and rapid design methodology. The use of numerical methods to simulate the behaviour of the joints can lead to savings of time and resources by optimizing the geometry and material parameters of the joints. The joints’ strength and failure modes were highly dependent on the adhesive, and this behaviour was successfully modelled numerically. Using a brittle adhesive resulted in a negligible maximum load (Pm) improvement with LO. The joints bonded with the ductile adhesive showed a nearly linear improvement of Pm with LO.

Comparação de técnicas analíticas e numéricas para previsão de resistência de juntas adesivas de sobreposição simples

As juntas adesivas têm vindo a ser usadas em diversas áreas e contam com inúmeras aplicações práticas. Devido ao fácil e rápido fabrico, as juntas de sobreposição simples (JSS) são um tipo de configuração bastante comum. O aumento da resistência, a redução de peso e a resistência à corrosão são algumas das vantagens que este tipo de junta oferece relativamente aos processos de ligação tradicionais. Contudo, a concentração de tensões nas extremidades do comprimento da ligação é uma das principais desvantagens. Existem poucas técnicas de dimensionamento precisas para a diversidade de ligações que podem ser encontradas em situações reais, o que constitui um obstáculo à utilização de juntas adesivas em aplicações estruturais. O presente trabalho visa comparar diferentes métodos analíticos e numéricos na previsão da resistência de JSS com diferentes comprimentos de sobreposição (LO). O objectivo fundamental é avaliar qual o melhor método para prever a resistência das JSS. Foram produzidas juntas adesivas entre substratos de alumínio utilizando um adesivo époxido frágil (Araldite® AV138), um adesivo epóxido moderadamente dúctil (Araldite® 2015), e um adesivo poliuretano dúctil (SikaForce® 7888). Consideraram-se diferentes métodos analíticos e dois métodos numéricos: os Modelos de Dano Coesivo (MDC) e o Método de Elementos Finitos Extendido (MEFE), permitindo a análise comparativa. O estudo possibilitou uma percepção crítica das capacidades de cada método consoante as características do adesivo utilizado. Os métodos analíticos funcionam apenas relativamente bem em condições muito específicas. A análise por MDC com lei triangular revelou ser um método bastante preciso, com excepção de adesivos que sejam bastante dúcteis. Por outro lado, a análise por MEFE demonstrou ser uma técnica pouco adequada, especialmente para o crescimento de dano em modo misto.

Comparação dos métodos ENF e 4-ENF para determinação da tenacidade ao corte de juntas adesivas

Ao longo destes últimos anos as ligações adesivas têm vindo a verificar um aumento progressivo em aplicações estruturais em detrimento das ligações mecânicas convencionais. Esta alteração de paradigma deve-se às vantagens que as juntas adesivas possuem relativamente aos outros métodos de ligação. A mecânica da fratura e os Modelos de Dano Coesivo (MDC) são critérios comuns para prever a resistência em juntas adesivas e usam como parâmetros fundamentais as taxas de libertação de energia. Pelo facto do ensaio 4-Point End Notched Flexure (4-ENF), aplicado em juntas adesivas, ainda estar pouco estudado é de grande relevância um estudo acerca da sua viabilidade para a determinação da taxa crítica de libertação de energia de deformação ao corte (GIIc). Esta dissertação tem como objetivo principal efetuar uma comparação entre os métodos End- Notched Flexure (ENF) e 4-ENF na determinação de GIIc em juntas adesivas. Para tal foram utilizados 3 adesivos: Araldite® AV138, Araldite® 2015 e SikaForce® 7752. O trabalho experimental passou pela conceção e fabrico de uma ferramenta para realização do ensaio 4-ENF, seguindo-se o fabrico e a preparação dos provetes para os ensaios. Pelo facto do ensaio 4-ENF ainda se encontrar pouco divulgado em juntas adesivas, e não se encontrar normalizado, uma parte importante do trabalho passou pela pesquisa e análise em trabalhos de investigação e artigos científicos. A análise dos resultados foi realizada por comparação direta dos valores de GIIc com os resultados obtidos no ensaio ENF, sendo realizada por série de adesivo, através da comparação das curvas P-δ e curvas-R. Como resultado verificou-se que o ensaio 4-ENF em ligações adesivas não é o mais versátil para a determinação do valor de GIIc, e que apenas um método de obtenção de GIIc é viável. Este método é baseado na medição do comprimento de fenda (a). Ficou evidenciado que o ensaio ENF, devido a ser um ensaio normalizado, por apresentar um setup mais simples e por apresentar uma maior disponibilidade de métodos para a determinação do valor de GIIc, é o mais recomendado. Conclui-se assim que o ensaio 4-ENF, embora sendo uma alternativa ao ensaio ENF, tem aplicação mais limitada.

Flexural strengthening by means of a arc overlay in the tension zone

Dissertação para obtenção do Grau de Mestre em Engenharia Civil – Perfil de Estruturas

Impact of health conditions on absenteeism from work

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Economics from the NOVA – School of Business and Economics

What do people do at work when they are not working?

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Management from the NOVA – School of Business and Economics

Ivity internationalization strategy: Integrative work project SME competitiveness

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Management from the NOVA – School of Business and Economics

Tech Frame: Integrative work project in strategy

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Management from the NOVA – School of Business and Economics

What do people do at work when they're not working?

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Management from the NOVA – School of Business and Economics

What do people do at work when they are not working?

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Management from the NOVA – School of Business and Economics

Speed and rhythm in organizations: How do different generations cope with speed and rhythm at work?

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Management from the NOVA – School of Business and Economics

Epidemiologic studies of chagas' disease in the urban zone of Teresina. State of Piauí, northeastern Brazil

The triatomine species Rhodnius nasutus and Triatoma pseudomaculata were captured on palm trees Orbignya martiana "babaçu ", in the urban zone of Teresina. This kind of palm tree is largely distributed in Piauí State. The predominant species was R. nasutus; the young in stars predominated. The infestation index of palm trees and the infection index of triatomines by flagellates were 96.0 ana 29.1%, respectively. Marsupiais, bats and a rodent were captured in palm trees. The flagellates found in both triatomines ana marsupiais were morphologically and biologically indistinguishable from Trypanosoma cruzi. Forty seven percent (481/1,025) of triatomines were found concentrated in six palm trees where marsupiais circulated. Of the total of 1,025 triatomines 230 (22%) were infected by flagellates and 53.0% (123/230) of these infected triatomines were present in the same six palm trees. No evidence of triatomine domiciliation or human transmission was observed in the houses in the vicinity of palm trees. The results suggest that marsupiais play an important role in the life-cycle of T. cruzi in this region. The natural focus of Chagas' disease, demonstrated in the present study could represent a potential epidemiological threat.

The role of management accounting in the process of innovation: A field work perspective

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Finance from the NOVA – School of Business and Economics

Robots and humans as co-workers? The human-centred perspective of work with autonomous systems

The design of work organisation systems with automated equipment is facing new challenges and the emergence of new concepts. The social aspects that are related with new concepts on the complex work environments (CWE) are becoming more relevant for that design. The work with autonomous systems implies options in the design of workplaces. Especially that happens in such complex environments. The concepts of “agents”, “co-working” or “human-centred technical systems” reveal new dimensions related to human-computer interaction (HCI). With an increase in the number and complexity of those human-technology interfaces, the capacities of human intervention can become limited, originating further problems. The case of robotics is used to exemplify the issues related with automation in working environments and the emergence of new HCI approaches that would include social implications. We conclude that studies on technology assessment of industrial robotics and autonomous agents on manufacturing environment should also focus on the human involvement strategies in organisations. A needed participatory strategy implies a new approach to workplaces design. This means that the research focus must be on the relation between technology and social dimensions not as separate entities, but integrated in the design of an interaction system.

Presence of anti-Toxoplasma antibodies in humans and their cats in the urban zone of Guadalajara

Cats are the definitive hosts of Toxoplasma gondii. Infected cats excrete oocysts in their feces, infecting humans and other animals. The objective of the present study was to determine the presence of anti-Toxoplasma antibodies in cat owners and their pets, and determine if there was a relationship between Toxoplasma infection and humans who live with infected cats. IgG anti-Toxoplasma antibodies in sera of 59 cat owners were determined by enzyme-linked immunosorbent assay (ELISA), in 24 sera from their cats, IgG, IgM, and IgA antibodies were found using Burney's ELISA. Thirty-eight (64%) of 59 cat owners were positive to IgG anti-Toxoplasma. Seropositivity for cats was 70.8% IgG, 8.3% IgM, and 62.5% IgA. Cohabitation with cats infected by T. gondii, feeding with leftovers or raw viscera, and lack of control over how their feces were handled are risk factors conducive for humans to become infected by T. gondii.