Aplica����o de m��todos num��ricos avan��ados para a previs��o de resist��ncia de liga����es adesivas

Com a necessidade de encontrar uma forma de ligar componentes de forma mais vantajosa, surgiram as liga����es adesivas. Nos ��ltimos anos, a utiliza����o de juntas adesivas em aplica����es industriais tem vindo a aumentar, substituindo alguns m��todos de liga����o tradicionais, por apresentarem vantagens tais como, redu����o de concentra����o de tens��es, reduzido peso e facilidade de processamento/fabrico. O seu estudo permite prever a sua resist��ncia e durabilidade. Este trabalho refere-se ao estudo de juntas de sobreposi����o simples (JSS), nas quais s��o aplicados os adesivos comerciais que variam desde fr��geis e r��gidos, como o caso do Araldite�� AV138, at�� adesivos mais d��cteis, como o Araldite�� 2015 e o Sikaforce�� 7888. Estes s��o aplicados em substratos de alum��nio (AL6082-T651) em juntas com diferentes geometrias e diferentes comprimentos de sobreposi����o (L), sendo sujeitos a esfor��os de trac����o. Foi feita uma an��lise dos valores experimentais fornecidos e uma posterior compara����o destes com diferentes m��todos num��ricos baseados em Elementos Finitos (EF). A compara����o foi feita por uma an��lise de Modelos de Dano Coesivo (MDC) e segundo os crit��rios baseados em tens��es e deforma����es do M��todo de Elementos Finitos Extendido (MEFE). A utiliza����o destes m��todos num��ricos capazes de simular o comportamento das juntas poder�� levar a uma poupan��a de recursos e de tempo. A an��lise por MDC revelou que este m��todo �� bastante preciso, excepto para os adesivos que sejam bastante d��cteis. A aplica����o de uma outra lei coesiva pode solucionar esse problema. Por sua vez a an��lise por MEFE demonstrou que esta t��cnica n��o �� particularmente adequada para o crescimento de dano em modo misto e, comparativamente com o MDC, a sua precis��o �� bastante inferior.

A framework for robustness assessment in the context of corroded RC structures

Structural robustness is an emergent concept related to the structural response to damage. At the present time, robustness is not well defined and much controversy still remains around this subject. Even if robustness has seen growing interest as a consequence of catastrophic consequences due to extreme events, the fact is that the concept can also be very useful when considered on more probable exposure scenarios such as deterioration, among others. This paper intends to be a contribution to the definition of structural robustness, especially in the analysis of reinforced concrete structures subjected to corrosion. To achieve this, first of all, several proposed robustness definitions and indicators and misunderstood concepts will be analyzed and compared. From this point and regarding a concept that could be applied to most type of structures and dam-age scenarios, a robustness definition is proposed. To illustrate the proposed concept, an example of corroded reinforced concrete structures will be analyzed using nonlinear analysis numerical methods based on a contin-uum strong discontinuities approach and isotropic damage models for concrete. Finally the robustness of the presented example will be assessed.

Robustness of corroded reinforced concrete structures

Short Term Scienti c Mission, COST ACTION TU-0601

Efficient Legendre spectral tau algorithm for solving two-sided space-time Caputo fractional advection-dispersion equation

In this paper we present the operational matrices of the left Caputo fractional derivative, right Caputo fractional derivative and Riemann���Liouville fractional integral for shifted Legendre polynomials. We develop an accurate numerical algorithm to solve the two-sided space���time fractional advection���dispersion equation (FADE) based on a spectral shifted Legendre tau (SLT) method in combination with the derived shifted Legendre operational matrices. The fractional derivatives are described in the Caputo sense. We propose a spectral SLT method, both in temporal and spatial discretizations for the two-sided space���time FADE. This technique reduces the two-sided space���time FADE to a system of algebraic equations that simplifies the problem. Numerical results carried out to confirm the spectral accuracy and efficiency of the proposed algorithm. By selecting relatively few Legendre polynomial degrees, we are able to get very accurate approximations, demonstrating the utility of the new approach over other numerical methods.

Carateriza����o das propriedades mec��nicas de um adesivo estrutural de alta ductilidade

A tecnologia de liga����o por adesivos estruturais tem vindo a ser utilizada ao longo de v��rias d��cadas, permitindo solucionar diversos problemas associados a t��cnicas chamadas "tradicionais" de liga����o, como a soldadura, a rebitagem ou a liga����o aparafusada. Esta �� uma alternativa vi��vel para substituir as liga����es mec��nicas, devido a diversos fatores como o menor peso estrutural, menor custo de fabrica����o e capacidade de uni��o de diferentes materiais. O crescente recurso a materiais comp��sitos em diversas ind��strias, nomeadamente a aeron��utica e naval, levaram ao consequente aumento da aplica����o de liga����es adesivas, por serem indicadas como forma de uni��o destes materiais, onde �� de enaltecer a sua elevada resist��ncia �� fadiga. Uma junta adesiva est�� maioritariamente sujeita a esfor��os de corte e arrancamento e portanto o conhecimento dos m��dulos de elasticidade �� tra����o (E) ou corte (G) do adesivo, e ainda as resist��ncias m��ximas �� tra����o e ao corte, n��o �� suficiente quando se pretende prever o comportamento da mesma. Na verdade, torna-se necess��rio abranger na an��lise a plastifica����o progressiva verificada nas juntas adesivas antes da rotura, sendo necess��rio o conhecimento de par��metros tais como a taxa cr��tica de liberta����o de energia de deforma����o �� tra����o (GIc) e corte (GIIc). Este trabalho pretende estudar um adesivo estrutural recentemente lan��ado no mercado, carecendo portanto da sua caracteriza����o, para facilitar a previs��o da resist��ncia de estruturas adesivas ligadas com o mesmo. S��o 4 os ensaios a realizar: ensaios �� tra����o de provetes em bruto, ensaios ao corte com a geometria Thick Adherend Shear Test (TAST), ensaios Double-Cantilever Beam (DCB) e ensaios End-Notched Flexure (ENF). Com a realiza����o dos ensaios referidos, ser��o determinadas as propriedades mec��nicas e de fratura �� tra����o e ao corte, e ser��o fornecidos os par��metros para a previs��o da resist��ncia de juntas adesivas com este adesivo por uma variedade de m��todos, desde m��todos anal��ticos mais expeditos at�� aos m��todos num��ricos mais avan��ados existentes atualmente. Os resultados foram de encontro aos disponibilizados pelo fabricante, sempre que estes se encontravam dispon��veis, obtendo-se discrep��ncias bastante reduzidas nos diversos par��metros determinados.

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.

Compara����o de t��cnicas anal��ticas e num��ricas para previs��o de resist��ncia de juntas adesivas de sobreposi����o simples

As juntas adesivas t��m vindo a ser usadas em diversas ��reas e contam com in��meras aplica����es pr��ticas. Devido ao f��cil e r��pido fabrico, as juntas de sobreposi����o simples (JSS) s��o um tipo de configura����o bastante comum. O aumento da resist��ncia, a redu����o de peso e a resist��ncia �� corros��o s��o algumas das vantagens que este tipo de junta oferece relativamente aos processos de liga����o tradicionais. Contudo, a concentra����o de tens��es nas extremidades do comprimento da liga����o �� uma das principais desvantagens. Existem poucas t��cnicas de dimensionamento precisas para a diversidade de liga����es que podem ser encontradas em situa����es reais, o que constitui um obst��culo �� utiliza����o de juntas adesivas em aplica����es estruturais. O presente trabalho visa comparar diferentes m��todos anal��ticos e num��ricos na previs��o da resist��ncia de JSS com diferentes comprimentos de sobreposi����o (LO). O objectivo fundamental �� avaliar qual o melhor m��todo para prever a resist��ncia das JSS. Foram produzidas juntas adesivas entre substratos de alum��nio utilizando um adesivo ��poxido fr��gil (Araldite�� AV138), um adesivo ep��xido moderadamente d��ctil (Araldite�� 2015), e um adesivo poliuretano d��ctil (SikaForce�� 7888). Consideraram-se diferentes m��todos anal��ticos e dois m��todos num��ricos: os Modelos de Dano Coesivo (MDC) e o M��todo de Elementos Finitos Extendido (MEFE), permitindo a an��lise comparativa. O estudo possibilitou uma percep����o cr��tica das capacidades de cada m��todo consoante as caracter��sticas do adesivo utilizado. Os m��todos anal��ticos funcionam apenas relativamente bem em condi����es muito espec��ficas. A an��lise por MDC com lei triangular revelou ser um m��todo bastante preciso, com excep����o de adesivos que sejam bastante d��cteis. Por outro lado, a an��lise por MEFE demonstrou ser uma t��cnica pouco adequada, especialmente para o crescimento de dano em modo misto.

Target-specific multiphysics modeling for thermal medicine applications

Dissertation to obtain the degree of Doctor of Philosophy in Biomedical Engineering

Study of AC losses in medium-sized high temperature superconducting coils

The study of AC losses in superconducting pancake coils is of utmost importance for the development of superconducting devices. Due to different technical difficulties this study is usually performed considering one of two approaches: considering superconducting coils of few turns and studying AC losses in a large frequency range vs. superconducting coils with a large number of turns but measuring AC losses only in low frequencies. In this work, a study of AC losses in 128 turn superconducting coils is performed, considering frequencies ranging from 50 Hz till 1152 Hz and currents ranging from zero till the critical current of the coils. Moreover, the study of AC losses considering two different simultaneous harmonic components is also performed and results are compared to the behaviour presented by the coils when operating in a single frequency regime. Different electrical methods are used to verify the total amount of AC losses in the coil and a simple calorimetric method is presented, in order to measure AC losses in a multi-harmonic context. Different analytical and numerical methods are implemented and/or used, to design the superconducting coils and to compute the total amount of AC losses in the superconducting system and a comparison is performed to verify the advantages and drawbacks of each method.

Monte Carlo valuation of worst-of auto-callable equity swaps

This thesis proposes a Monte Carlo valuation method for Worst-of Auto-callable equity swaps. The valuation of this type of swap usually requires complex numerical methods which are implemented in ���black-box��� valuation systems. The method proposed is an alternative benchmark tool that is relatively simple to implement and customize. The performance of the method was evaluated according to the variance and bias of the output and to the accuracy when compared to a leading valuation system in the market.

Designing next generation car washing machines with rotary brushes

Tese de Doutoramento - Leaders for Technical Industries (LTI) - MIT Portugal

Contact force models for multibody dynamics

"Series: Solid mechanics and its applications, vol. 226"

In Search of a theory of debt management

A growing literature integrates theories of debt management into models of optimal fiscal policy. One promising theory argues that the composition of government debt should be chosen so that fluctuations in the market value of debt offset changes in expected future deficits. This complete market approach to debt management is valid even when the government only issues non-contingent bonds. A number of authors conclude from this approach that governments should issue long term debt and invest in short term assets. We argue that the conclusions of this approach are too fragile to serve as a basis for policy recommendations. This is because bonds at different maturities have highly correlated returns, causing the determination of the optimal portfolio to be ill-conditioned. To make this point concrete we examine the implications of this approach to debt management in various models, both analytically and using numerical methods calibrated to the US economy. We find the complete market approach recommends asset positions which are huge multiples of GDP. Introducing persistent shocks or capital accumulation only worsens this problem. Increasing the volatility of interest rates through habits partly reduces the size of these simulations we find no presumption that governments should issue long term debt ? policy recommendations can be easily reversed through small perturbations in the specification of shocks or small variations in the maturity of bonds issued. We further extend the literature by removing the assumption that governments every period costlessly repurchase all outstanding debt. This exacerbates the size of the required positions, worsens their volatility and in some cases produces instability in debt holdings. We conclude that it is very difficult to insulate fiscal policy from shocks by using the complete markets approach to debt management. Given the limited variability of the yield curve using maturities is a poor way to substitute for state contingent debt. The result is the positions recommended by this approach conflict with a number of features that we believe are important in making bond markets incomplete e.g allowing for transaction costs, liquidity effects, etc.. Until these features are all fully incorporated we remain in search of a theory of debt management capable of providing robust policy insights.

Optimal Martingales and American Option Pricing

Pricing American options is an interesting research topic since there is no analytical solution to value these derivatives. Different numerical methods have been proposed in the literature with some, if not all, either limited to a specific payoff or not applicable to multidimensional cases. Applications of Monte Carlo methods to price American options is a relatively new area that started with Longstaff and Schwartz (2001). Since then, few variations of that methodology have been proposed. The general conclusion is that Monte Carlo estimators tend to underestimate the true option price. The present paper follows Glasserman and Yu (2004b) and proposes a novel Monte Carlo approach, based on designing "optimal martingales" to determine stopping times. We show that our martingale approach can also be used to compute the dual as described in Rogers (2002).

Adaptive Continuous time Markov Chain Approximation Model to General Jump-Diusions

We propose a non-equidistant Q rate matrix formula and an adaptive numerical algorithm for a continuous time Markov chain to approximate jump-diffusions with affine or non-affine functional specifications. Our approach also accommodates state-dependent jump intensity and jump distribution, a flexibility that is very hard to achieve with other numerical methods. The Kolmogorov-Smirnov test shows that the proposed Markov chain transition density converges to the one given by the likelihood expansion formula as in Ait-Sahalia (2008). We provide numerical examples for European stock option pricing in Black and Scholes (1973), Merton (1976) and Kou (2002).