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International audience; Shape Memory Alloys (SMAs) are widely used in endodontics as instruments for root canal preparation namely endodontic rotary files. Despite their high performances, compared to stainless steel instruments, it is still possible to enhance their cutting efficiency by acting on their shape and material properties. As an example, increasing the exhibited reversible martensitic transformation strain makes them more flexible without decreasing their mechanical strength. It becomes possible with single crystal SMAs. Cu-based (Cu-Al-Be, Cu-Zn-Al, Cu-Al-Mn) single crystal SMAs start emerging and can reach about 12% of martensitic transformation strain, in addition to their interesting antimicrobial properties. This study investigates, both numerically and experimentally, the development of endodontic files made of Cu-based single crystal SMAs. The numerical analysis is carried out by the finite element method. The geometry of the instrument is parameterized. Depending on the applied boundary conditions, bending, torsion, or combined bending-torsion loadings are represented. A micromechanical constitutive law is implemented in Abaqus via the UMAT subroutine to describe the thermomechanical behavior of the Cu-based single crystal SMA. Following the thus obtained numerical results, Coltene-Micromega company has manufactured endodontic file prototypes made of Cu-Al-Be single crystal SMA. A specific setup applying the same boundary conditions of torsion-bending loading is used to characterize these SMA file prototypes. The good agreement between experimental and numerical responses, for a combined bending-torsion loading, proves the relevance of the proposed approach and the pertinence of considering Cu-based SMAs for endodontic file applications.

Recent enhancements in canal instrumentation area have significantly improved the endodontic treatment making it faster and more efficient. The improvement of endodontic file quality through the instrument geometry, operating motion, and material properties made the endodontic treatment safer and thereby preserving the tooth and the surrounding periodontal tissues. Nevertheless, few research works investigated the possibility of using an alternative shape memory alloy (SMA) to the Nickel-Titanium (NiTi). This PhD work focuses on investigating the possibility of use in endodontic of Cu-based SMA having promising antimicrobial properties I.e. the Copper-Aluminum-Beryllium (Cu-Al-Be) single crystal SMA. The present study is based on an experimental approach using an original testing device able to apply bending and torsion loading in separate or combined way. Such solicitations are mainly induced by the curvature of the dental canal while the mechanical preparation. A numerical approach completes this experimental part by considering a finite element model representative of the file geometry, the thermomechanical behavior of the Cu-based single crystal or NiTi polycrystal SMAs and the bending- torsion loading and boundary conditions. A developed numerical tool allowed to build a parametrized geometry of an endodontic file and mesh it with tetrahedral finite elements in order to numerically analyze its response under combined bending-torsion loading. Based on these two approaches, an experience plan was carried out in order to analyze the influence of geometrical parameters on the response of endodontic files made of Cu-based single crystal SMA. It could be a precious tool for decision aid in choosing endodontic file geometry and its adapted use for root canal preparation.; Les dernières avancées réalisées dans le domaine de l’instrumentation canalaire ont permis de rendre le traitement endodontique plus rapide et plus efficace. L’amélioration des limes endodontiques au travers des géométries instrumentales, mouvement de travail ou propriétés mécaniques du matériau constitutif de l’instrument ont rendu le traitement plus sûr dans la préservation de l’organe dentaire et des tissus parodontaux environnants. Néanmoins, peu de recherches se sont portées sur l’utilisation d’Alliage à Mémoire de Forme (AMF) différents du Nickel-Titane (Ni-Ti). Ce travail se propose d’étudier une potentielle utilisation en endodontie d’un nouvel AMF doté de propriétés antimicrobiennes très prometteuses : le Cuivre-Aluminium-Béryllium (Cu-Al-Be) monocristallin. Cette étude se fait au travers d’une approche expérimentale, au moyen d’un dispositif d’essais innovant, capable d’appliquer des chargements combinés ou séparés de flexion et de torsion. Ces chargements sont les principales sollicitations appliquées aux limes endodontiques lors du traitement canalaire. Une approche numérique vient compléter les études expérimentales, avec un modèle éléments-finis représentatif des chargements appliqués par le dispositif d’essais. Un programme de génération de modèles géométriques de limes endodontiques est développé et permet de représenter les réponses obtenues expérimentalement à l’aide de deux lois de comportement thermomécaniques adaptées aux AMF polycristallins et monocristallins. Enfin, à l’aide de ces deux outils, un plan d’expérience capable de déterminer l’influence des paramètres géométriques d’une lime endodontique sur sa réponse mécanique est réalisé et apporte une aide précieuse à la décision pour le choix d’une géométrie et d’une utilisation adaptée pour ces limes endodontiques en Cu-Al-Be monocristallin.

Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e Tecnologia O presente trabalho tem como principal objetivo desenvolver um modelo de previsão de vida à fadiga para componentes entalhados produzidos por fusão seletiva por laser submetidos a carregamentos proporcionais combinados de flexão e torção. A geometria da peça em análise é híbrida, sendo constituída por uma secção circular maciça de aço fabricada através de técnicas subtrativas convencionais, no qual está implantada uma secção circular tubular oca, obtida pela técnica aditiva de fusão seletiva por laser, com um furo transversal passante. As relações entre o momento fletor (B) e o momento torsor (T) estudadas foram: B=2T, B=T e B=2T/3.Nesta dissertação desenvolve-se uma metodologia de previsão de vida à fadiga recorrendo ao método dos elementos finitos. O primeiro objetivo compreende a identificação de um modelo constitutivo elastoplástico cíclico, que possibilite uma caracterização precisa do estado de tensão e de deformação na região do entalhe para os diferentes cenários de carregamento considerados. Baseados nestes modelos, pretende-se desenvolver critérios numéricos que permitam prever o local de iniciação da fenda e o ângulo da fenda na fase inicial de propagação. A previsão do local de iniciação é feita com base no nó da parede do furo com maior valor da primeira tensão principal. Relativamente ao ângulo da fenda na fase inicial de propagação, este determina-se com base na primeira direção principal obtida para o nó da parede do furo com maior valor da primeira tensão principal.O segundo objetivo consiste em identificar um critério de falha representativo para os diferentes tipos de carregamentos considerados e prever a vida de iniciação de fenda. Neste trabalho recorre-se ao parâmetro SWT, sendo o carregamento multiaxial reduzido a um carregamento uniaxial equivalente. As previsões de vida à fadiga são obtidas considerando os valores locais efetivos e máximos do parâmetro SWT.As comparações entre os valores previstos e os valores experimentais obtidos para os mesmos cenários de carregamento, sugerem que ambas as abordagens aplicadas permitem obter resultados bastante satisfatórios. Porém, verifica-se que a abordagem baseada no valor máximo do parâmetro SWT conduz a melhores resultados, com 100% dos resultados de previsão localizados dentro das bandas de dispersão com fatores de 2. This work aims to develop a fatigue life prediction model for notched components produced by selective laser melting subjected to proportional bending-torsion loading. The specimen under analysis is a hybrid geometry, consisting of a solid circular cross-section produced by conventional subtractive techniques and a tubular implant obtained by the selective laser melting containing a transversal hole. The bending moment (B) to torsion moment (T) ratios considered in this study are: B=2T, B=T and B=2T/3.This dissertation develops a methodology for fatigue life prediction using the finite element method. The first objective consists of choosing a cyclic elastoplastic constitutive model, which allows an accurate characterization of the stress-strain state at the notch region for the different loading scenarios. Based on this model, it is intended to develop numerical criteria to predict the crack initiation sites and the crack angles at the early stage of growth. The crack initiation site is predicted from the maximum nodal value of the first principal stress at the hole surface and the crack angle at the early stage of growth is determined by computing the first principal direction of the node with the maximum value of the first principal stress along the hole surface. The second objective is the identification of a failure criterium representative of the fatigue damage for the different loading scenarios and predict the fatigue lifetime. In this research, the SWT parameter is selected to correlate the fatigue damage with the fatigue life, by reducing the multiaxial stress-strain state to an equivalent uniaxial stress-strain state. The fatigue life predictions are obtained from both the effective and maximum local values of the SWT parameter. The comparations between the predict values and those obtained experimentally suggest that both approaches can be applied to predict the fatigue life with a quite satisfactory accuracy. However, the approach based on the maximum value of the SWT parameter has better predictive capabilities, with 100% of the results located within scatter bands with factors of 2.

Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e Tecnologia Este trabalho tem como objetivo investigar o comportamento à fadiga da liga de alta resistência AISI 18Ni300 sujeita a carregamentos proporcionais multiaxiais combinados de flexão e torção. O programa de fadiga considerada razões entre tensões normais e tensões de corte iguais a 4/3, 2 e 4 e razão de tensões (R) próxima de zero. A geometria estudada é uma peça bi-material composta por um substrato de secção circular maciça produzido por técnicas subtrativas convencionais e por um implante tubular produzido pela técnica aditiva de Selective Laser Melting (SLM) contendo um furo transversal passante (entalhe). Pretende-se desenvolver critérios que permitam prever o local de iniciação da fenda, as trajetórias da fenda na fase inicial de propagação, e vida de iniciação de fenda para os diferentes cenários de carregamento.No decorrer dos ensaios de fadiga, os provetes foram visualizados periodicamente com recurso a uma câmara digital de alta resolução para relacionar o comprimento de fenda com o número de ciclos aplicados e também para deteção das zonas de iniciação da fenda e respetivos ângulos de iniciação. Após a realização dos ensaios de fadiga, as superfícies de fratura foram examinadas por microscopia eletrónica de varrimento. Numa segunda fase, foi desenvolvido um modelo numérico linear elástico, pelo método dos elementos finitos, que permitiu caracterizar os campos de tensão e deformação na região do furo para as várias condições de carregamento aplicadas nos ensaios experimentais. Os locais de iniciação, os ângulos de iniciação, e as trajetórias de propagação à superfície da peça foram previstos numericamente pelo valor máximo da primeira tensão principal, pelo valor da primeira direção principal no local de iniciação, e pelo campo de tensões principais, respetivamente.O efeito do entalhe no comportamento à fadiga foi estimado com base no critério de Densidade de Energia de Deformação Equivalente e na Teoria das Distâncias Críticas através do Método de Linha (LM) e do Método do Ponto (PM). As previsões de vida à fadiga foram obtidas com a aplicação dos Modelos Smith-Watson-Topper e Coffin-Manson e pelo parâmetro Smith-Watson-Topper. O modelo de CM foi o que permitiu obter melhores resultados, nomeadamente erros menores e desvios-padrão inferiores. This work aims to investigate the fatigue behaviour of the AISI 18Ni300 maraging steel subjected to combined multiaxial proportional loads of bending and torsion. The fatigue behaviour considered in this study has ratios of the normal stresses to the shear stresses equal to 4/3, 2 and 4 and a stress ratio (R) close to zero. The studied geometry is a bi-material specimen composed of a substrate of solid circular cross-section produced by conventional subtractive techniques and by a tubular implant produced by the Selective Laser Melting additive technique containing a through-hole. It is intended to develop criteria to predict the crack initiation sites, as well as the crack initiation trajectories and the crack initiation orientations in the early stage of growth for the different loading scenarios. During the fatigue tests, the specimens were periodically observed using a high resolution digital camera. After the fatigue test campaign, fracture surfaces were examined by Scanning Electron Microscopy to identify the failure micro-mechanisms. In a second phase, a linear elastic numerical model was developed by the Finite Element Method, which allowed the characterization of the stress and strain fields in the hole region for the loading conditions applied in the experimental tests. Crack initiation sites, crack orientation angles and surface crack paths were numerically predicted on the basis of the maximum value of the first principal stress, first principal direction at the initiation site, and from the first principal stress field, respectively.The notch effect on fatigue was accounted for via the Equivalent Strain Energy Density concept, and the Theory of Critical Distances by the Line Method and the Point Method. Fatigue life predictions were obtained using the Smith Watson-Topper and Coffin Manson models as well as the Smith-Watson Topper parameter. The Coffin-Manson models gave the best predictive results, with lower errors and lower standard deviation values.