Дисертації з теми "Fatigue aging"
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1
St, Clair Gibson Alan. "Fatigue, aging and the neuromuscular system." Master's thesis, University of Cape Town, 2001. http://hdl.handle.net/11427/3205.
Анотація:
Bibliography: p. 400-445.The aim of this thesis was to investigate the relationship between chronic exercise activity, aging, the neuromuscular system and the symptom of fatigue in a series of studies. The hypothesis of the thesis was that in contrast to the accepted dogma that exercise is beneficial to an individual, increasing longevity and improving quality of life, excessive or chronic exercise activity may accelerate the aging process, lead to neuromuscular damage, and cause the development of pathological symptoms or levels of fatigue.
2
Narynbek, Ulu Kubat. "Fatigue of HNBR - Effects of formulation and thermal aging." Thesis, Ecole centrale de Nantes, 2018. http://www.theses.fr/2018ECDN0006/document.
Анотація:
Cette thèse de doctorat s’intéresse à la fatigue des élastomères, et plus particulièrement des caoutchoucs nitrile hydrogénés (sigle HNBR en anglais). Les HNBR sont des élastomères de haute performance,classiquement utilisés pour des applications industrielles à haute température où une bonne résistance chimique aux hydrocarbures est également requise.La thèse est divisée en trois parties principales.En premier lieu, des améliorations majeures pour les expériences de durée de vie en fatigue des élastomères sont proposées. Une approche expérimentale originale permettant le pilotage des essais en contrainte vraie est détaillée.Grâce à cette méthode, la première « vraie »courbe de Wöhler (contrainte vraie en fonction du nombre de cycles) pour des élastomères est tracée. Ensuite, des outils statistiques, issus de la recherche médicale, permettent une analyse probabiliste des résultats présentant une grande dispersion et des durées de vie censurées.Dans un second temps, ces nouveaux outils sont utilisés pour étudier l'effet de la formulation chimique – les taux d’acrylonitrile et d'hydrogénation notamment - sur la résistance en fatigue du HNBR à 120 oC. Ces résultats sont complétés par des expériences de propagation de fissure de fatigue et par une analyse de l'endommagement. Finalement, l'influence du vieillissement thermique sur la résistance en fatigue du HNBR est étudiée ; deux nouveaux essais sont développés : des essais simultanés de vieillissement et de fatigue, et des expériences de fatigue sur des échantillons préalablement vieillis.De plus, la cristallisation sous tension dans le HNBR est pour la première fois étudiée de manière quantitativeThe present PhD thesis is devoted to investigation of fatigue in elastomers, andmore particularly of carbon black-filled hydrogenated nitrile butadiene rubber (HNBR).HNBR is a high performance elastomer classically used for high temperature industrial applications, where high resistance to industrial solvents is also required.The thesis is divided into three main parts.First, major improvements of testing procedures utilized in fatigue life testing ofelastomers are proposed. An original method to prescribe the true stress throughout fatigue experiments is developed; it permits to plot the first “true” Wöhler curve, i.e. true stress vs.number of cycles, for elastomers. Then,statistical tools, widely applied in medical research, are adopted for a comprehensive probabilistic analysis of fatigue life results that exhibit high scatter and censored lifetimes.Second, these novel tools are used to investigate the effect of chemical formulation -acrylonitrile content, percent hydrogenation,and of a composite blend - on fatigue resistance of HNBR at the service temperature of 120 oC. In addition to these fatigue life tests,fatigue crack propagation experiments and microscopic fatigue damage analysis are carried out. Third, the influence of thermal aging on fatigue performances of HNBR is investigated.To this end, two additional fatigue life tests are carried out: simultaneous aging and fatigue experiments, and fatigue experiments on preaged samples.Additionally, strain-induced crystallization in HNBR is studied; for the first time, quantitative relationships between crystallization, strain,and stress are established for unfilled HNBR blends
3
Che, Yunxiang, and S3145469@student rmit edu au. "Aging structure life prediction and reliability assessment." RMIT University. SAMME, 2008. http://adt.lib.rmit.edu.au/adt/public/adt-VIT20091113.114541.
Анотація:
Confront with the serious aging problem in aircraft structure field, the profession was tasked to unveil the mysterious in the mechanism of aging. In decades, many endeavours were put into different subjects such as, fatigue and crack calculation, corrosion analysis, reliability evaluation, life prediction, structure monitor and protection, structure repair, etc. In an effort of developing a reasonable model for life prediction and reliability evaluation, a wide range of topics in the field of aging structure reliability are reviewed. Many existing methods and tools are carefully studied to distinguish the advantages, disadvantages and the special application. With consideration of corrosion fatigue life, and based on the data obtained through investigating service status of the aging aircraft, a fuzzy reliability approach is proposed and presented. Initially, the thesis presents the literature review in the field, introducing the well-established theories and analysis tools of reliability and points out how such these methods can be used to assess the life and reliability of aging structure. Meanwhile, some characteristic parameters and distributions, as well as some crucial calculation formulations, procedures for aging aircraft reliability/risk analysis are given. Secondly, mathematical models are established to evaluate the initial crack size and to assess both randomness and fuzziness of the variables, which also successfully work out the probability of survival of existing structures over a time period and predict the operation time under specific reliability requirement. As a practical approach to the reliability of aging aircraft structure, example is presented and evaluated. While conduct the calculation, a few programs based on FORTRAN code are developed to solve the none-linear equation, to work out the multi dimension integration and to simulate the survival probability. The crack life prediction software AFGROW is selected for comparison of the calculation results, which also shows the appropriate accuracy of the established model. As conclusion, the effects of some variables including fuzzy factors on reliability and life of aging aircraft structure are finally discussed. It is apparent that the confines of the model are existing as fact because of the huge assumption of the parameters input and model uncertainties. Suggestions on further prospective research are proposed respectively.
4
Varli, Aziz Egemen. "Fatigue Crack Growth Behaviour Of Aa6013 Aluminum Alloy At Different Aging Conditions." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/2/12607440/index.pdf.
Анотація:
The effect of different aging treatments on fatigue crack growth behavior of AA6013 aluminum alloy was investigated. C(T) (Compact Tension) specimens were prepared in L-T and T-L direction for fatigue crack growth tests. Samples were in T651 as received, T42 which is solution heat treated at 538 ºC for 90 minutes, water quenched and aged in room temperature for 96 hours, and one group of samples were overaged at 245 º
C for 12 hours after T42 condition was achieved. Hardness and conductivity measurements were achieved for all conditions after the heat treatments. Fatigue crack growth tests were performed at as received condition T651, T42 and 245 º
C aged samples in laboratory air with sinusoidal loading of stress ratio R=0.1 and at a frequency of 1 Hz. The highest fatigue crack growth resistance is observed for T651 T-L and 245 º
C overaged L-T condition.
5
Avin, Keith Gerard. "Moderators of fatigue: the complexity of interactions." Diss., University of Iowa, 2012. https://ir.uiowa.edu/etd/2815.
Анотація:
Fatigue is a difficult phenomenon to study because the response can vary based upon task-specific (i.e. contraction type, intensity, position– vs. load-matching and muscle group/joint region) and subject-specific (i.e. sex and age) variables. Although numerous investigations have provided insight into muscle fatigue, further efforts were needed to better characterize the influence of age, sex, joint/muscle group, contraction type, and task complexity have upon fatigue. The primary purpose of this series of three studies was to identify and characterize the influences of potential moderating variables (i.e., sex, joint, age, contraction type, and task complexity) upon fatigue resistance during voluntary muscle contraction fatigue tasks through both empirical (systematic review and meta-analysis) and experimental methods. In general, women demonstrated either the same or better fatigue resistance than men (men never better), the sex advantage was joint specific not systematic, old men were more fatigue resistant than young men, task complexity was not an influential factor and fatigue differences were more readily apparent under isometric conditions. The inclusion of empirical and experimental methods helped clarify the driving factors of localized muscle fatigue. This in turn will better direct future study design and power for mechanistic, training and performance response studies.
6
Ali, Mahdi. "Study of the bitumen aging effects on the rheological properties and fatigue behavior." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2018. http://amslaurea.unibo.it/16255/.
Анотація:
A study of bitumen aging effects on the rheological properties and fatigue behavior is carried out on five polymer modified bitumen provided by an oil company. Dynamic Shear Rheometer (DSR), is used to perform advanced experimental investigation. It allows practicing frequency sweep tests, fatigue tests and time sweep tests. Polymer modified bitumen with different percentage of SBS with or without the presence of crumb rubber are tested before and after short and long aging processes.
Master curves were generated based on frequency sweep tests data, they are studied, and a comparison was conducted between each bitumen before and after aging. In this thesis, the DSR is presented, as well as the testing procedure and the tested materials. A fatigue life analysis of the results, based on the use of two different criteria, the first criterion is classically used: it consists in defining the failure as a 50% loss of the initial stiffness. The other criteria based on dissipated energy data.
This thesis provides results which show the effects of several factors on fatigue and healing response such as bitumen type, ageing, the presence of rubber and polymer modification.
7
Yassierli, Yassierli. "Muscle Fatigue during Isometric and Dynamic Efforts in Shoulder Abduction and Torso Extension: Age Effects and Alternative Electromyographic Measures." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/29509.
Анотація:
Aging has been associated with numerous changes in the neuromuscular system. Age effects on muscular performance, however, have been addressed only in limited contexts in earlier research. The present work was conducted primarily to investigate age-related effects on muscle capacity (fatigue and endurance) during isometric and dynamic efforts. This work was also motivated by current theories on muscle fatigue as a potential risk factor for musculoskeletal disorders and recent demographic projections indicating a substantial increase of older adults in the working population. Four main experiments were conducted to investigate development of muscle fatigue during isometric and intermittent efforts in shoulder abduction and torso extension at different contraction levels. Two age groups were involved (n=24 in each), representing the beginning and end of working life. Findings from this study demonstrated that the older group exhibited slower progressions of fatigue, though the age effect was more consistent for the shoulder than the torso muscles. This implied a muscle dependency of the influence of age on fatigue. Several interaction effects of age and effort level were also observed, suggesting that both task and individual factors should be considered simultaneously in job design.
The present investigation also sought to develop alternative electromyography (EMG)-based fatigue parameters for low-level isometric and dynamic contractions, two areas in which improvements are needed in the sensitivity and reliability of existing EMG indices. Several alternative EMG indices were introduced, derived from logarithmic transformation of EMG power spectra, fractal analysis, and parameter estimation based on a Poisson distribution. Potential utility of several of these alternative measures was demonstrated for assessment of muscle fatigue.Ph. D.
8
Cavuoto, Lora Anne. "Evaluating obesity-related differences in upper extremity and trunk muscular capacity." Diss., Virginia Tech, 2012. http://hdl.handle.net/10919/28345.
Анотація:
Work-related musculoskeletal disorders (WMSDs), particularly overexertion injuries, represent a significant economic burden and involve substantial adverse personal outcomes. Two important contemporary changes in workforce demographics may be associated with an increase in the future incidence and cost of WMSDs. First, more than two-thirds of the US adult population is now either overweight or obese, a doubling of the prevalence of obesity over the past 30 years. Second, there has been a shift toward an older worker population, whose injuries often require more time away from work. Obesity and aging can modify job demands and affect worker capacity in terms of muscular and psychomotor function. However, there is a lack of empirical studies quantifying the work-relevant (or ergonomic) impacts related to task demands, capacities, and their potential imbalance. This research assessed obesity- and age-related differences in physical capacity by measuring localized muscle fatigue, endurance, and the effects of fatigue on psychomotor function. Three experiments were completed, progressing from controlled static to more complex intermittent and functional tasks. The work also examined whether obesity and age effects are modified by workplace/workstation configuration, specifically the extent to which body segment masses need to be supported. With obesity, strength was higher, but endurance time was lower, particularly for the more complex tasks. Interaction effects between obesity and age were seen in only a few measures across the studies and did not indicate a consistent effect. Outcomes of this research can facilitate the development of more effective (i.e., inclusive) guidelines to control WMSD risk and contribute to both proactive and reactive interventions to reduce excessive exposures to physical risk factors. Overall, the research goal is to help ensure that ergonomic guidelines and practice are appropriate (or are adapted) to accommodate the diverse and changing workforce.Ph. D.
9
Colas, Damien. "Approche multi-échelle du vieillissement et du comportement cyclique dans le tantale." Phd thesis, Ecole Nationale Supérieure des Mines de Paris, 2013. http://pastel.archives-ouvertes.fr/pastel-00957882.
Анотація:
La conception de structures complexes requiert une connaissance exhaustive des matériaux utilisés à la fois au niveau macroscopique et au niveau microscopique. Dans le cas du tantale, peu d'études ont été menées pour des sollicitations cycliques et pour l'influence du vieillissement (au sens de la diffusion des atomes interstitiels vers les dislocations) sur le comportement macroscopique et sur les champs locaux de déformation. Afin de mieux comprendre les mécanismes microstructuraux régissant la déformation, cette étude mène de front des essais avec un suivi de déformation à l'échelle locale couplés à une modélisation adaptée, autorisant la prise en compte explicite de la microstructure.Dans un premier volet, une étude macroscopique du vieillissement et du comportement cyclique du tantale est présentée. Plusieurs techniques expérimentales ont été utilisées, mettant en évidence la propagation d'une bande de localisation de la déformation lors de l'entrée en plasticité. En parallèle, un modèle phénoménologique EKMC (de Estrin, Kubin et McCormick) rendant compte du comportement macroscopique (notamment du pic de traction dû au vieillissement statique) a été identifié ; permettant ensuite des investigations numériques sur les manifestations de la localisation de la déformation.Ensuite, l'étude a été poursuivie à l'échelle locale à la fois expérimentalement et numériquement. Pour ce faire, des matrices micrométriques de plots en nickel ont été déposées à la surface d'un échantillon. Les images sucessives autorisent le calcul des cartes de déformation expérimentales lors d'un essai de traction interrompu. La modélisation explicite de la microstructure a été rendue possible par la génération d'agrégats polycristallins spécifiques avec des conditions de surface libre. L'étude de l'influence de la prise en compte du vieillissement sur l'hétérogénéité des champs locaux de déformation a permis de confronter ces derniers avec les champs expérimentaux.Enfin, une étude multi-échelles du comportement en fatigue a été effectuée. Un essai de fatigue interrompu avec observations microscopiques et construction de cartes de déformation a été réalisé. L'hétérogénéité de déformation locale a ainsi été identifiée et quantifiée, jusqu'à l'amorçage de fissures. Une simulation d'une sollicitation équivalente a été réalisée sur un agrégat polycristallin, permettant la comparaison directe des champs locaux de surface. Des investigations approfondies ont été menées sur l'agrégat pour mettre en place un critère d'amorçage basé sur des grandeurs physiques en accord avec l'expérience.
10
Bisson, Etienne. "Influence of Neuromuscular Fatigue of the Lower Limb on Postural Control and Associated Central Processes in Young and Older Adults." Thèse, Université d'Ottawa / University of Ottawa, 2012. http://hdl.handle.net/10393/23612.
Анотація:
This thesis investigates the differential effects of muscle fatigue on center of pressure (COP) sway and associated central processes (attentional demands and sensory re-weighting) in older compared with young adults. More specifically, we first sought to determine whether the effect of muscle fatigue on unipedal stance was greater during a dual-task in older versus young adults, and second, to determine whether the effect of muscle fatigue on bipedal stance was greater in a condition with less reliable proprioceptive information in older versus young adults. Our main results show that with different muscle groups fatigued (ankle or hip) and postural tasks with varying difficulty (unipedal stance or bipedal stance on compliant surface), young adults increased their COP sway displacement and velocity with muscle fatigue, but not the associated attentional demands. When the central nervous system needed to increase the weight of the vestibular inputs due to sensory information being less reliable at the ankle joints from standing on a compliant surface (peripheral somatosensory information), COP sway displacement and velocity in young adults were greater with ankle muscle fatigue. We also found that healthy older adults were able to compensate for muscle fatigue just as well as young adults when visual information was available during a unipedal stance or when visual information was not available during a bipedal stance on a firm surface. However, when standing on a compliant surface, older adults showed a greater increase in COP sway displacement compared to young adults and increased attentional demands when visual information was not available during a bipedal stance. Our results suggest that healthy young and older adults are able to compensate for ankle muscle fatigue to limit postural control alterations during quiet standing under different conditions, and that the extent of postural control alterations largely depends on the tasks performed. The compensation strategies may be less efficient for older adults with less reliable proprioceptive information and without vision. Thus, a frailer group of older adults, with already reduced proprioception and/or reduced vision could possibly have more difficulty to efficiently use the same compensation strategies, and may be more at risk of falling when fatigued.
11
Smet, Vanessa. "Aging and failure modes of IGBT power modules undergoing power cycling in high temperature environments." Thesis, Montpellier 2, 2010. http://www.theses.fr/2010MON20075/document.
Анотація:
Cette thèse a pour objet l'étude de la fiabilité de modules de puissance triphasés à IGBTs 200 A - 600 V, destinés à la construction d'onduleurs de traction pour des applications automobiles hybrides ou électriques. Ces travaux visent à évaluer la tenue de ces modules de puissance en régime de cyclage thermique actif à hautes température, en mettant l'accent sur leur résistance à la fatigue thermomécanique. Deux approches complémentaires ont été mises en oeuvre dans ce but: tests de vieillissement accéléré et modélisation numérique. Une compagne d'essais de vieillissement par cyclage actif a été menée avec des profils de température variés, définis par la température ambiante et la variation de température de jonction des IGBTs, utilisés comme facteurs d'accélération des contraintes. Au cours de ces tests, les composants ont électriquement fonctionné dans des conditions semblables à une application réelle (commande MLI). L'objectif était d'identifier les modes de défaillance, d'estimer l'influence des facteurs d'accélération du vieillissement, et d'évaluer la pertinence des indicateurs de défaillance classiques dans ces conditions de stress thermiques sévères. Aussi, afin de mieux comprendre les mécanismes de défaillance responsables de la fatigue de l'assemblage des modules considérés, une modélisation thermomécanique visant à déterminer l'impact des modèles de comportement mécanique sur la durée de vie estimée des brasures, a été développée. La réponse de l'assemblage à des contraintes de cyclage actif similaires à celles appliquées durant les essais a été évaluée par analyse numérique. Les différentes lois de comportement ont été comparées en termes de contraintes, déformations plastiques, et densité d'énergie plastique dans les brasuresThis thesis is dedicated to reliability investigations led on three-phase 200~A~--~600~V IGBT power modules, designed for building drive inverters for hybrid or electric automotive traction applications. The objective was to evaluate the durability of the studied modules when they withstand power cycling in high temperature environments, and especially their resistance to thermo-mechanical fatigue. Two complementary approaches were considered: accelerated aging experiments and numerical modeling.A series of power cycling tests was carried out over a large range of temperature profiles, defined by the ambient temperature and IGBT junction temperature excursion. These quantities are used as thermal stress acceleration factors. Those experiments were led in realistic electrical conditions (PWM control scheme). They aimed at identifying the failure modes of the target devices, assessing the impact of the acceleration factors on their aging process, and evaluating the suitability of standard aging indicators as damage precursors in such harsh loading conditions. Besides, to better understand the failure mechanisms governing the fatigue life of the modules assembly, a thermo-mechanical modeling focusing on solder joints was built. Our simulation efforts concentrated on the appraisal of constitutive modeling effects on solder joints lifetime estimation. Numerical analysis of the assembly response to power cycling in similar operating conditions as practiced in experiments were performed. Behavior laws were then compared on stress, plastic strain, and strain energy density developed within the joints
12
Rahbar-Rastegar, Reyhaneh. "Cracking in Asphalt Pavements| Impact of Component Properties and Aging on Fatigue and Thermal Cracking." Thesis, University of New Hampshire, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10622119.
Анотація:
Cracking in asphalt pavements is one of the most common and critical pavement distresses. Cracks let the water penetrate from the surface to underlying layers resulting in shorter pavement service life and poor riding quality. There are various factors that affect the cracking potential of asphalt mixtures including the properties of asphalt components, mix design factors, loading time and loading mode, temperature, stress state, and aging. While several researchers have conducted studies investigating the cracking of asphalt mixtures, the effective parameters are not all well understood to allow engineers to design and construct more resistant pavements against cracking.
The work presented in this dissertation provides some additional insights into the effects of component properties and aging condition on asphalt cracking. The cracking susceptibility of hot mix asphalt (HMA) is evaluated through the experimental testing and numerical modeling on mixtures produced either in design (laboratory) or production (plant) stage. Various criteria and approaches for the prediction of cracking in asphalt binder and asphalt mixture are assessed and their correlation are discussed. Different levels of aging in laboratory are simulated, and the effects of long term oven aging (LTOA) on linear viscoelastic parameters, fatigue and fracture characteristics of asphalt mixtures are explored. The uniaxial tensile fatigue testing based on simplified viscoelastic continuum damage (SVECD) approach is conducted to characterize fatigue behavior, and semi circular bending (SCB), disc-shaped compact tension (DCT) testing and cohesive zone model are used to evaluate thermal cracking in asphalt mixtures.
This dissertation makes a good contribution in improvement of available approaches for evaluation of cracking potential of asphalt pavements and allows for assessment of different mixtures at early stage of material selection. The results of this study can lead to develop a new parameter to predict fatigue and thermal cracking susceptibility of flexible pavements in performance-based specifications, resulting in a better ride quality and cost saving for contractors and taxpayers.
13
Bryanton, Megan. "The Influence of Muscular Fatigue on Human Multi-Joint Movement: Determinants of Sit-to-Stand Capacity with Aging." Thesis, Université d'Ottawa / University of Ottawa, 2016. http://hdl.handle.net/10393/35369.
Анотація:
The sit-to-stand (STS) is a multi-articular movement of daily living that requires significantly higher knee extensor (KE) efforts compared to ankle and hip musculature, which approach near maximal levels in older adults populations. As well, fatigue may develop more readily with repetitive actions. Consequently, it is understandable how KE strength reserves have been previously correlated with both functional independence and STS strategy characteristics in older persons, and why STS capacity is a significant predictor of disability. However, it is still unclear why compensatory movement strategies manifest when rising from a seated position, and how this may be influenced by aging. The purpose of this thesis compilation was to evaluate alterations in muscular contributions at the ankle, knee, and hip, in relation to STS performance strategies in young and older adults either: a) with repetitive multi-joint STS exercise or b) before and after isolated fatigue of KE musculature. Results showed that aging caused a redistribution of joint torques when ascending from a seated position, and was associated with significantly higher quadriceps muscular efforts in older persons in comparison to their younger counterparts. In contrast, young and older adults exhibited similar compensatory movement and loading strategies during repetitive STS exercise, which appeared to be limited by the ability to sustain KE force output. In turn, lower KE strength reserves of older persons were responsible for their disproportionately higher quadriceps efforts and reduced STS capacities. Young and older persons also appeared to employ motor strategies to compensate for reduced KE force output via increased contribution of the biarticular rectus femoris within the quadriceps KE synergy, as well as through increases in the initially less active ankle plantar flexor and hip extensor musculature. Older adults may benefit from strengthening of thigh extensor musculature to maintain or improve their strength reserves to promote independent living.
14
Robert, Mylène. "Impact of degradation and aging on properties of PFSA membranes for fuel cells." Electronic Thesis or Diss., Université de Lorraine, 2021. http://www.theses.fr/2021LORR0004.
Анотація:
Bien que déjà présentées comme une technologie sûre et propre, les piles à combustible à membrane échangeuse de protons (PEMFC) restent confrontées à de solides verrous en termes de durabilité et de fiabilité, limitant leur commercialisation à large échelle. De nombreuses études ont déjà permis d’affiner la compréhension des phénomènes de vieillissement et ont permis de désigner la dégradation de la membrane comme l’un des principaux facteurs limitant la durée de vie des PEMFC. Cette étude vise à apporter des éléments de compréhension sur les mécanismes de dégradations chimique et mécanique par l’intermédiaire de protocoles de vieillissement ex-situ, ainsi qu’à comprendre l’impact de ces dégradations sur la structure et les propriétés fonctionnelles des membranes. Dans un premier temps, il a été nécessaire de clarifier l’influence de la réaction de Fenton, un protocole de vieillissement ex-situ largement reconnu dans la littérature, sur la dégradation chimique des membranes Nafion™. Les résultats ont confirmé que la concentration en réactifs de Fenton influençait significativement la décomposition chimique du polymère, à la fois d’un point de vue chimique et morphologique. Par la suite, nous avons choisi de suivre l’évolution de la dégradation chimique des membranes Nafion™ en fonction du temps et d’étudier son impact sur la structure de la membrane, ses propriétés de sorption et de diffusion de l’eau ainsi que son fonctionnement en pile. À cet égard, différentes techniques de caractérisation telles que la spectroscopie RMN 19F ou 1H ainsi que la spectroscopie FTIR ont permis de corréler les propriétés physico-chimiques de la membrane à ses caractéristiques structurelles et de mettre ainsi en évidence plusieurs marqueurs de la dégradation chimique. Enfin, un dispositif sur-mesure a été conçu afin d’étudier l’impact des contraintes mécanique et chimique conjointes sur les membranes Nafion™. L’objectif de ce dispositif était de reproduire des conditions de vieillissement proches de celles rencontrées lors du fonctionnement en pileAlthough proton-exchange membrane fuel cells (PEMFC) are nowadays considered as a safe and clean energy technology, they still suffer from durability and reliability issues restricting their widespread commercialization. Innumerable studies have already led to a better understanding of aging phenomena and highlighted membrane degradation as one of the main factors limiting PEMFC lifetime. This study aims at bringing some clarifications on the chemical and mechanical degradation mechanisms of membranes through ex-situ aging protocols as well as understanding the impact of these degradations on the membrane structure and functional properties. First, it was necessary to clarify the influence of Fenton’s reaction, an ex-situ aging protocol widely recognized in the literature, on the chemical degradation of Nafion™ membranes. The results confirmed that Fenton’s reagents concentration significantly influenced polymer chemical decomposition, both from a chemical and morphological point of view. Subsequently, we chose to monitor the evolution of pure chemical degradation of Nafion™ membranes as a function of time and to study its impact on the membrane structure, water sorption and diffusion properties, as well as operability in fuel cells. In that respect, various characterization techniques such as 19F or 1H-NMR as well as FTIR spectroscopies allowed us to correlate physico-chemical properties of the membrane to its structural characteristics and to thus highlight several indicators of chemical degradation. Finally, a custom-made device has been developed to study the impact of conjoint mechanical and chemical stress on Nafion™ membranes. The objective of this device was to replicate aging conditions close to those encountered during fuel cell operation
15
Abdessalem, Abir. "Etude et modélisation de l'endommagement mécanique des matériaux composites sous l'effet de l'humidité : Couplage hydrolyse et endommagement." Thesis, Paris, ENSAM, 2019. http://www.theses.fr/2019ENAM0042.
Анотація:
La présente étude traite la durabilité en milieu humide et l’endommagement par fatigue des matériaux composites à matrice organique (CMO). La diffusion d’eau dans un matériau SMC (matrice polyester renforcée par des fibres de verre et des particules de craies) est tout d’abord analysée expérimentalement. Puis elle est modélisée numériquement sous différentes conditions de vieillissement hygrothermique en immersion. Les mécanismes physiques à l’origine de l’endommagement hygrothermique sont alors identifiés. Des baisses de différentes propriétés mécaniques en fonction du temps et de la teneur en eau absorbée sont mises en évidence avec les phénomènes d’endommagement identifiés. L’effet de l’endommagement hygrothermique sur la durée de vie en fatigue est alors étudié expérimentalement. Un modèle prédictif qui prend en compte les effets d’endommagements d’origine hydrique et mécanique est alors proposé. Un calcul couplé diffusion/mécanique permet de déterminer le comportement global du matériau. On montre qu’il est possible de simuler et de suivre en continu l’évolution de la durée de vie en fatigue du matériau vieilli quel que soit le temps et la températureThe present study deals with the wet durability and fatigue damage of organic matrix composite (OMC) materials. The diffusion of water in a SMC material (polyester matrix reinforced with glass fibers and particles of chalks) is first analyzed experimentally. Then it is modeled numerically under different conditions of hygrothermal aging in immersion. The physical mechanisms causing the hygrothermal damage are then identified. Decreases of different mechanical properties as a function of time and of the absorbed water content are highlighted with the identified damage phenomena. The effect of hygrothermal damage on fatigue life is then studied experimentally. A predictive model that takes into account the effects of waterborne and mechanical damage is then proposed. A coupled diffusion / mechanical calculation makes it possible to determine the overall behavior of the material. It is shown that it is possible to simulate and to follow continuously the evolution of the fatigue life of the aged material whatever the time and temperature
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Walubita, Lubinda F. "Comparison of fatigue analysis approaches for predicting fatigue lives of hot-mix asphalt concrete (HMAC) mixtures." Diss., Texas A&M University, 2003. http://hdl.handle.net/1969.1/3898.
Анотація:
Hot-mix asphalt concrete (HMAC) mixture fatigue characterization constitutes a fundamental component of HMAC pavement structural design and analysis to ensure adequate field fatigue performance. HMAC is a heterogeneous complex composite material of air, binder, and aggregate that behaves in a non-linear elasto-viscoplastic manner, exhibits anisotropic behavior, ages with time, and heals during traffic loading rest periods and changing environmental conditions. Comprehensive HMAC mixture fatigue analysis approaches that take into account this complex nature of HMAC are thus needed to ensure adequate field fatigue performance. In this study, four fatigue analysis approaches; the mechanistic empirical (ME), the calibrated mechanistic with (CMSE) and without (CM) surface energy measurements, and the proposed NCHRP 1-37A 2002 Pavement Design Guide (MEPDG) were comparatively evaluated and utilized to characterize the fatigue resistance of two Texas HMAC mixtures in the laboratory, including investigating the effects of binder oxidative aging. Although the results were comparable, the CMSE/CM approaches exhibited greater flexibility and potential to discretely account for most of the fundamental material properties (including fracture, aging, healing, visco-elasticity, and anisotropy) that affect HMAC pavement fatigue performance. Compared to the other approaches, which are mechanistic-empirically based, the CMSE/CM approaches are based on the fundamental concepts of continuum micromechanics and energy theory.
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Davidson, Bradley. "Experimental and simulation-based assessment of the human postural response to sagittal plane perturbations with localized muscle fatigue and aging." Diss., Virginia Tech, 2007. http://hdl.handle.net/10919/29361.
Анотація:
Falls from heights (FFH) are one of the leading causes of fatalities in skilled labor divisions such as construction, mining, agriculture/forestry, and manufacturing. Previous research has established that localized muscle fatigue (LMF) increases center of mass (COM)- and center of pressure (COP)-based measures of quiet stance. This is important because these increases have been linked to elevated risk of falls, and workers in the construction industry frequently engage in fatiguing activities while working at heights. In addition, the rate of fatality due to an occupational fall increases exponentially with age. Improved methods of fall prevention may be obtained through increased understanding of factors that have a deleterious effect on balance and postural control such as LMF and aging.
An initial study was conducted to investigate the effects of LMF and aging on balance recovery from a postural perturbation without stepping. Sagittal plane postural perturbations were administered to young and older groups of participants before and after exercises to fatigue the lumbar extensors or ankle plantar flexors. Measures of balance recovery were based on the COM and COP trajectories and the maximum perturbation that could be withstood without stepping. Balance recovery measures were consistent with an LMF-induced decrement to recover from perturbations without stepping. Aging was also associated with an impaired ability to recover from the perturbations.
The second study in the series investigated the effects of aging and LMF on the neural control of upright stance during small postural perturbations. Small magnitude postural perturbations were administered to young and older groups before and after fatiguing exercises. A single degree of freedom (DOF) model of the human body was developed that accurately simulated the experimentally collected kinematics during recovery from the perturbations. The model was controlled by invariant feedback gains that operated on the time-delayed kinematics. Feedback gains and time-delay were optimized for each participant, and a novel delay margin analysis was performed to assess system robustness toward instability. Results indicated that older individuals had a longer "effective" time-delay and exhibited greater reliance on afferent velocity information. No changes in feedback controller gains, time-delay, or delay margins were found with LMF in either age group.
The final study investigated the use of a nonlinear controller to simulate responses to large magnitude postural perturbations. A three DOF model of the human body was developed and controlled with the state-dependent Riccati equation (SDRE). Parameters of the SDRE were optimized to fit the experimentally recorded kinematics. Unlike other forms of nonlinear control, the SDRE provides meaningful parameters for interpretation in the system identification. The SDRE approach was successful at stabilizing the dynamical system; however, accurate results were not obtained. Reasons for these errors are discussed, and an alternative formulation to the time-delayed optimal control problem using Roesser state space equations is presented.Ph. D.
18
Abu-Ragheef, Basil. "Polymer Aging Mechanics : An investigation on a Thermoset Polymer used in the Exterior Structure of a Heavy-duty Vehicle." Thesis, Linnéuniversitetet, Institutionen för skog och träteknik (SOT), 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-88887.
Анотація:
The use of plastic materials in the design of vehicle components is primarily driven by the need for vehicle weight and cost reduction. Additionally, these materials give design engineers freedom in creating appealing exterior designs. However, creating self-carrying exterior structures with polymers must fulfill long-term strength, creep and fatigue life requirements. Thus, the polymer polyDicyclopentadiene (pDCPD) has been chosen for this purpose. Its aging mechanics need to be understood by the design engineers to make the right decisions. This thesis has carried out mechanical tests such as uniaxial tensile testing, fatigue, and creep testing. Digital image correlation (DIC) system has been used to capture strain data from tensile tests. In the final analysis, DIC measurements proved more accurate than extensometer data retrieved from the testing machine. The rise in temperature has been captured using thermal imaging. Several degradation processes have been explored including physical aging, thermo-oxidation, photo-oxidation, chemical- and bio- degradations. Test results showed significant changes in mechanical properties after 17 years of aging. Additionally, severe thermal degradation has been observed in one of the tested panels of pDCPD. Temperature can rise to significant levels during cyclic loading at high stresses, which could have an impact on physical aging effects. Viscoelastic behavior has been explored and changes in dynamic and creep properties have been observed. The investigation also reviled that different defects caused by flawed manufacturing also can affect the material severely as one case has proved in this research.
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Choubey, Anupam. "Microstructural changes under isothermal aging and their influence on thermal fatigue reliability for tin-lead and lead-free solder joints, including microstructural changes under isothermal aging in mixed solder joints." College Park, Md.: University of Maryland, 2007. http://hdl.handle.net/1903/7712.
Анотація:
Thesis (Ph. D.) -- University of Maryland, College Park, 2007.Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Crozatier, Mathilde. "Étude de la durabilité d'un matériau composite bobiné sous chargements sévères." Thesis, Paris, ENSAM, 2017. http://www.theses.fr/2017ENAM0008/document.
Анотація:
Les matériaux composites sont très étudiés depuis des décennies. En effet, l'alliance entre légèreté et résistance de ces structures entraîne le développement de leur utilisation. Leur caractérisation reste un point essentiel dans la plupart des études.Ce travail s’inscrit dans le projet Sollicitern qui vise à concevoir un camion d’hydrocurage en matériaux composites, financé par un fond unique interministériel. L’objectif est d’étudier la durabilité du matériau composite retenu sous chargements sévères. Elle permettra ainsi d’alimenter les modèles numériques des partenaires industriels afin d’ajuster les paramètres pour l’optimisation de la citerne et également d’estimer sa durée de vie face à ces diverses sollicitations.Dans un premier temps, l’attention se porte sur l’intégration de la courbure de la structure et donc du matériau dans l’étude. En effet, ce dernier est fabriqué par enroulement filamentaire, ce qui génère nécessairement une structure tubulaire. Dans un second temps, une caractérisation statique est menée. Cette partie comporte notamment des essais de compression radiale sur l’ensemble de la structure tubulaire où un protocole expérimental est mis en œuvre. En couplant les résultats de ces essais avec ceux d’une simulation numérique reproduisant l’essai, les propriétés élastiques du pli unidirectionnel sont accessibles grâce à une optimisation. Dans un troisième et dernier temps, la durabilité plus spécifique est abordée. Des campagnes d’essais de fatigue, d’impacts, d’usure et de vieillissement sont tout d’abord réalisées indépendamment les unes des autres. L’objectif final est de coupler ces différentes sollicitations et d’appréhender ainsi leur effet combinéComposites materials have been studied for decades. Indeed, the association between lightness and resistance of these structures leads to the development of their use. Their characterization remains an essential point in most researches.This work is part of Sollicitern project, which aims to design a composite water treatment truck, financed by a single interministerial fund. The aim is to study the durability of the composite material retained under severe loads. It thus offers tools to industrialists for adjusting parameters in the tank optimization and the estimation of the lifetime under these various loadings.In the first stage, attention is focused on the integration of the curvature of the structure and therefore of the material in the study. Indeed, it is manufactured by filament winding, which necessarily generates a tubular structure. In a second step, a static characterization is carried out. This part includes, in particular, radial compression tests on the whole tubular structure where an experimental protocol is developed. By coupling the results of these tests with a numerical simulation reproducing the test, the elastic properties of the unidirectional ply are obtained by optimization. In a third and final stage, more specific durability is discussed. Fatigue, impact, wear and aging tests are first carried out independently of one another. The final objective is to associate these different loadings and thus to apprehend their combined effect
21
Mansour, Mustafa. "Effects of Mix, Aging, and Production Types on the I-FIT and IDEAL-CT Cracking Indices." University of Akron / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=akron1597147736637553.
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Emerson, Nadia. "The Effects of 6-weeks of Resistance Training on the Neuromuscular Fatigue Threshold in Older Adults." Master's thesis, University of Central Florida, 2013. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/5631.
Анотація:
Age-related deficits in muscle mass, strength, and function place an increased burden of work on existing skeletal muscle and may lead to early onset of neuromuscular fatigue (NMF) during activities of daily living. Resistance exercise (RE) is the proven method for improving neuromuscular function in healthy older adults. PURPOSE: To investigate the effects of 6 weeks of RE on the NMF threshold as well as strength and functional performance in older adults. METHODS: Twenty-four older adults were randomly assigned to 6 weeks of RE (EXE; n = 12; age 72 ± 6.3 y; BMI 28.4 kg/m2) or control (CONT; n = 12; age 70.3 ± 5.6 y; BMI 27.6 kg/m2). Body fat percent (BF%), lean mass (LM), and fat mass (FM) were measured using DEXA and participants performed a discontinuous cycle ergometer test, physical working capacity at fatigue threshold (PWCFT), to determine the onset of NMF. Functional performance was assessed by time to complete 5 chair rises (CHAIR) and walk an 8-foot course (WALK). Lower body strength was assessed by predicted 1-RM leg extension (1RM). Two-way Analysis of Variance (ANOVA; time [PRE, POST] x group [EXE and CONT]) and magnitude based inferences were used to compare dependent variables. RESULTS: RE significantly increased 1RM (35%; p = 0.001) and CHAIR (20%; p = 0.047). RE had a likely beneficial effect on WALK (15%) and a possibly beneficial effect on PWCFT (14%). There were no significant changes to LM or FM, however, women in EXE significantly decreased BF% (p = 0.020). CONCLUSION: Results suggest that RE improves measures of strength and functional performance and possibly the onset of NMF in older adults.M.S.
Masters
Child, Family, and Community Sciences
Education and Human Performance
Sport and Exercise Science; Applied Exercise Physiology
23
Broudin, Morgane. "Vieillissement thermo-oxydatif d'un élastomère industriel pour applications automobiles antivibratoires : caractérisations, compréhension, outils de dimensionnement." Thesis, Brest, 2017. http://www.theses.fr/2017BRES0079.
Анотація:
Dans les conditions réelles d’utilisation, de multiples facteurs sont à l’origine de l’évolution des propriétés mécaniques des caoutchoucs (température, oxygène, conditions de chargement mécaniques…). Les pièces en élastomère pour l’antivibratoire automobile sont généralement massives (plusieurs millimètres d’épaisseur), le vieillissement conduit alors à des évolutions de propriétés hétérogènes. Ceci peut être induit par plusieurs mécanismes de dégradation liés, par exemple, à la présence ou non d’oxygène. Pour comprendre et identifier les mécanismes de dégradation prépondérants, une campagne de vieillissement accéléré a été menée pour une large gamme de températures représentative des conditions de service rencontrées sur véhicule (de 40°C à 120°C). De nombreux échantillons de géométries différentes (films minces, éprouvettes de caractérisation et éprouvettes de structure) ont été utilisés afin, d’une part, de faciliter l’interprétation, et d’autre part, de viser la transposition des résultats à l’échelle des pièces industrielles. L’étude présentée ici vise à identifier les mécanismes physico-chimiques et/ou évolutions microstructurales à l’origine du processus de dégradation, et de quantifier les conséquences de ceux-ci sur le comportement mécanique et les propriétés en fatigue. Ces résultats permettront de fournir les éléments nécessaires à la modélisation et la prise en compte fine des effets du vieillissement dans le processus de conception de pièces antivibratoires automobilesUnder service conditions, many factors are responsible for the evolution of the mechanical properties of rubber parts (temperature, oxygen, mechanical loadings, etc.). Automotive anti-vibration parts using rubber-like materials are usually massive and ageing can therefore lead to heterogeneous properties. To understand the degradation process and especially the effect of oxygen, aerobic and anaerobic ageing conditions have been studied for a wide range of temperatures (from 40°C to 120°C). Numerous samples have been used with different geometries (from thin films to massive structural samples) to ease the interpretation but also to remain as close as possible from the final applications. The mechanical consequences of the ageing have been investigated for both static and fatigue properties throughout a wide experimental database (about 1000 specimens tested in fatigue, for example). The study aims at identifying the physicochemical mechanisms and/or microstructural evolution that cause the processes of degradation and to quantify the consequences on the mechanical behavior and the fatigue properties. These results will provide the necessary elements needed for the integration of thermo-oxidative effects in the fatigue design loop of automotive anti-vibration parts
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Ng, Andressa Ka Yan. "Evaluation of the fatigue damage behavior of fine aggregate matrices prepared with modified asphalt binders." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/18/18143/tde-03012018-122026/.
Анотація:
The fatigue cracking process occurs by the repeated dynamic loading from the traffic of heavy vehicle. This phenomenon initiates as microcracks and develops under two circumstances: (i) after adhesive failure, when the crack occurs at the interface aggregate-mortar, and/or (ii) after cohesive failure, when the crack develops within the mortar. Based on such interpretation of the cracking phenomenon in asphalt concrete mixtures, researchers have been using the fine aggregate matrices (FAMs) to estimate the fatigue behavior of the asphalt concrete. Good agreement is observed between the properties of the FAM and asphalt concrete properties in studies related to moisture damage, fatigue cracking and permanent deformation. Regarding the fatigue resistance of the flexible pavements, it is important to investigate the effect of the use of modified binders and the binder aging on the fatigue properties of the asphalt concrete, once that the modified binder can enhance the fatigue behavior of the asphalt concrete, and the binder aging hardens the asphalt binder and turns it into a fragile material, with negative effects on the fatigue life of the asphalt concrete. Based on these evidences, this study has the objective of evaluating the effect of modified binders and aging level on the fatigue life of the FAMs, asphalt mastics and asphalt binders. The three scales are comprised of four asphalt binders (neat, AC+PPA, AC+SBS and AC+rubber) aged in short- and long-term. The fatigue properties of the three scales were evaluated by means of the viscoelastic continuum damage (VECD) concepts, once that this theory is able to predict the asphalt concrete behavior independent of loading mode (uniaxial or torsional), control mode (stress-control or strain-control), and amplitude loading applied to induce the damage. The overall results indicate that the addition of modified binder enhances the fatigue behavior and that extended aging is capable of compromise the fatigue performance. At the scales of the binder and the mastic, the AC+rubber presented the best fatigue performance, occupying the first position in the final rank order, and the AC+SBS presented the worst performance, occupying the last position. However, at the FAM scale, the FAMs prepared with the AC+SBS presented the best fatigue performance, occupying the first position in the final rank order, and the FAMs prepared with the neat binder presented the worst behavior, occupying the last position. The best correlation between the three scales regarding the short- and long-term aging was obtained between binder and mastics aged in the PAV with the FAMs aged in long-term for 30 days.O processo de trincamento por fadiga ocorre devido ao carregamento dinâmico repetido do tráfego de veículos pesados. Este fenômeno tem o início por meio de microtrincas e se propaga por meio de duas condições: (i) após a ruptura adesiva, quando a trinca ocorre na interface entre agregado e mástique, e/ou (ii) após a ruptura coesiva, quando o processo de trincamento ocorre no mástique. Com base nesta interpretação para o trincamento por fadiga em mistura asfáltica, pesquisadores vêm usando matrizes de agregado fino (MAFs) para estimar o comportamento da mistura asfáltica completa quanto ao dano por fadiga. Boa correlação é observada entre as propriedades da MAF e da mistura asfáltica completa (MAC) em estudos relacionados ao dano por umidade, fadiga e deformação permanente. Com relação a resistência de pavimentos flexíveis, é importante avaliar o efeito do uso de ligantes asfálticos modificados e do envelhecimento do ligante nas propriedades da mistura asfáltica, uma vez que ligantes modificados podem melhorar o comportamento da mistura asfáltica quanto ao dano por fadiga, e o envelhecimento do ligante asfáltico pode enrijecer o material tornando-o mais frágil, reduzindo a vida de fadiga das misturas asfálticas. Levando em consideração as evidências apresentadas, este estudo tem por objetivo avaliar o efeito de ligantes asfálticos modificados e o nível de envelhecimento na vida de fadiga das MAFs, mástiques e ligantes asfálticos. Estas três escalas da mistura asfáltica completa foram compostas por quatro ligantes asfálticos (CAP 50/70, CAP+PPA, CAP+SBS e CAP+borracha) envelhecidos a curto e a longo prazo. As propriedades das três escalas quanto ao dano por fadiga foram avaliadas por meio dos conceitos da teoria do dano contínuo em meio viscoelástico (VECD), uma vez que esta teoria é capaz de prever o comportamento da mistura asfáltica independentemente do modo de carregamento (uniaxial ou torsional, tensão ou deformação controlada) e da amplitude do carregamento aplicado ao material para induzir o dano. De modo geral, os resultados indicaram que o uso de ligantes asfálticos modificados melhoram o comportamento das MAFs quanto ao dano por fadiga e o envelhecimento é capaz de comprometer o desempenho das MAFs quanto ao trincamento por fadiga. Na escala do ligante e do mástique asfáltico, o CAP+borracha apresentou o melhor desempenho à fadiga, ocupando o primeiro lugar no ordenamento final, e o CAP+SBS o pior desempenho, ocupando a última posição. Entretanto, na escala da MAF, as MAFs preparadas com CAP+SBS apresentaram o melhor desempenho à fadiga, ocupando o primeiro lugar no ordenamento final, e as MAFs preparadas com CAP 50/70 apresentaram o pior desempenho, ocupando o último lugar no ordenamento final. A melhor correlação entre as três escalas com relação ao envelhecimento a curto e a longo prazo, foi obtido entre os ligantes asfálticos e mástiques envelhecidos no PAV com as MAFs envelhecidas a longo prazo por 30 dias.
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Babadopulos, Lucas Feitosa de Albuquerque Lima. "A Contribution to couple aging to hot mix asphalt (HMA) mechanical characterization under load-induced damage." reponame:Repositório Institucional da UFC, 2014. http://www.repositorio.ufc.br/handle/riufc/11164.
Анотація:
BABADOPULOS, L. F. A. L. A Contribution to couple aging to hot mix asphalt (HMA) mechanical characterization under load-induced damage. 2014. 139 f. Dissertação (Mestrado em Engenharia de Transportes) – Centro de Tecnologia, Universidade Federal do Ceará, Fortaleza, 2014.Submitted by Marlene Sousa (mmarlene@ufc.br) on 2015-03-17T12:56:14Z No. of bitstreams: 1 2014_dis_lfalbabadopulos.pdf: 3908887 bytes, checksum: 5aa174295aabf02b82834d29dbed52a8 (MD5)
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Although aging simulation in binder is performed through RTFO and PAV tests, n o considerations of asphalt mixture aging are made in regular laboratory characterization. T he present work is focused in incorporating aging to the modeling of the mechanical behavior of hot mix asphalt (HMA) during load - induced damage. This is accomplished by combining existing models and the adaptation of mixture aging procedures . The aging model used is based on the evolution of an internal state v ariable, associated to oxygen availability, aging temperature a nd four material parameters. The se parameters are related to aging susceptibility , reaction kinetics and dependency on aging history and on aging temperature. The model allows to establish relation ship s between different aging processes. Results at four aging states (using two different temperatures) we re analyzed and the aging model parameters w ere estimated. Capturing aging dependency on temperature constitutes a contribution of the presen t work with respect to previous results reported in the literature . T he aging model is coupled to viscoplasticity and damage, comparing the behavior observed at the different aging states. Concerning the damage models, t his thesis used mechanical models de rived from Schapery's work potential theory to model fatigue behavior . T he Simplified Viscoelastic Continuum Damage (S - VECD) model was selected . U nconfined dynamic creep tests were used to evaluate the effect of aging in the mixture resistance to permanent deformation. In addition to the state - of - the - art modeling of HMA, the characterization methods currently in use in Brazil (tensile strength, resilient modulus and controlled force indirect tensile fatigue tests) were also conducted . T he possibility to sim ulate the material behavior for various loading conditions constitutes a n advantage of the state - of - the - art model over the state - of - the practice method for fatigue characterization, used primarily to rank mixtures. It was concluded that, depending on pavem ent conditions and layer geometry, aging not necessarily affects negatively the fatigue behavior, while certainly improving the permanent deformation characteristics. That happens de s pite the fact that aging produces less damage tolerant materials, i.e., m aterials that fail for less evolved damage states. T he framework ( testing and analysis) for damage characterization of asphalt mixtures was implemented and it is expected to contribute to further developments in aging modeling of asphalt mixtures
Apesar de simulação de envelhecimento ser realizada em ligantes asfálticos através dos ensaios de RTFOT e PAV, nenhuma consideração sobre envelhecimento de misturas é feita na caracterização laboratorial comum. O presente trabalho se concentra na incorporação do envelhecimento na modelagem do comportamento mecânico de concretos asfálticos (CA) para carregamentos que induzem dano. Isto é feito através da combinação de modelos e da adaptação de procedimentos de envelhecimento existentes. O modelo de envelhecimento utilizado se baseia na evolução de uma variável interna de estado e é associado à disponibilidade de oxigênio, à temperatura e a quatro parâmetros materiais. Estes parâmetros são relacionados à susceptibilidade ao envelhecimento, à cinética de reação e à dependência sobre o histórico e sobre a temperatura de envelhecimento. O modelo permite estabelecer relações entre diferentes processos de envelhecimento. Resultados em quatro estados de envelhecimento (em duas temperaturas diferentes) foram analisados, e os parâmetros do modelo estimados. Capturar a dependência do processo quanto à temperatura constitui uma contribuição do trabalho quanto a resultados da literatura. O modelo de envelhecimento é acoplado à resposta viscoplástica e ao dano, comparando-se o comportamento nos diferentes estados. Quanto aos modelos de dano, esta dissertação trata dos derivados da teoria do potencial de trabalho de Schapery para análise da fadiga. O modelo simplificado de dano contínuo em meio viscoelástico (S-VECD) foi selecionado. Ensaios de Creep Dinâmico não confinado foram utilizados para avaliar o efeito do envelhecimento na resistência à deformação permanente. Além da modelagem mecânica do comportamento do CA usando modelos do Estado da Arte, também foram executados métodos de caracterização em uso no Brasil (resistência à tração, módulo de resiliência e ensaios de fadiga por compressão diametral). A possibilidade de se simular a resposta do material em várias condições de carga constitui uma vantagem do método do Estado da Arte sobre o do Estado da Prática, usado principalmente para comparar misturas. Concluiu-se que, dependendo das condições do pavimento e da geometria das camadas, o envelhecimento não necessariamente diminui a resistência à fadiga, embora certamente melhore a resistência à deformação permanente. Isso acontece apesar de o envelhecimento produzir materiais menos tolerantes ao dano, i.e., materiais que rompem para estados de dano menos evoluídos. O procedimento para a caracterização do dano em misturas asfálticas foi implementado e espera-se ter contribuído para um maior desenvolvimento da modelagem de misturas quanto ao envelhecimento.
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Libura, Tomasz. "Étude de la tenue à l’impact après vieillissement en fatigue, des composites stratifiés à base d’acrylique." Electronic Thesis or Diss., Université de Lorraine, 2022. http://www.theses.fr/2022LORR0259.
Анотація:
La résistance à l'impact est l'une des caractéristiques les plus essentielles des structures composites, par conséquent, son examen pour un nouveau matériau a une importance fondamentale. Cette thèse de doctorat est consacrée à la caractérisation de la résine thermoplastique ELIUM Acrylique renforcée par un tissu de fibres de verre tissées, qui appartient à une nouvelle catégorie de matériaux nécessitant des tests poussés avant leur application dans l'industrie. Sa haute résistance, son faible poids ainsi que son faible coût de production offrent d'excellentes opportunités pour de nombreuses applications dans l'industrie automobile en remplacement des stratifiés à base de polymères thermodurcissables. Cette étude présente un travail expérimental en lien avec l'effet de l'endommagement dû au vieillissement en fatigue à faible et haut nombre de cycles, de deux groupes de composites [0°/90°]4 et [45°/ 45°]4, sur leurs résistances à l'impact basse et grande vitesse. Une comparaison entre les données expérimentales et les résultats issues des simulations numériques au moyen d'un modèle macroscopique proposé pour les échantillons GFRE [0°/90°]4 non vieillis de 1.8 mm d'épaisseur est présentée. La réponse des matériaux au chargement en fatigue a été évaluée à travers l'analyse de la variation de rigidité. Ce programme de vieillissement en fatigue a été interrompu pour une valeur donnée de la chute de la raideur, avant la rupture de l'éprouvette. La rigidité initiale a été réduite d'environ 17 % pour le composite GFRE [0°/90°]4, après les tests LCF ou HCF. Cette réduction était beaucoup plus prononcée dans le cas du composite GFRE [45°/45°]4. Elle était estimée à 48 %. La résistance à l'impact basse vitesse a été mesurée en termes de force maximale, d'énergie absorbée et de seuil de pénétration, tandis que la résistance à l'impact grande vitesse a été mesurée en termes de limite balistique et de dissipation de l'énergie totale. Les éprouvettes non vieillies impactées à des énergies de 5 J, 10 J et 30 J n'ont pas été perforées et arborent une grande capacité à faire rebondir l'impacteur. Les composites impactés, ayant été soumis au préalable à la fatigue à faible nombre de cycles (LCF) ont montré la plus forte diminution de la pente initiale sur courbes caractéristiques de la force en fonction du déplacement. Cette réduction significative de la rigidité est causée par la fissuration de la matrice et le décollement de l'interface entre les fibres et la matrice. La réduction de la force maximale pour les essais réalisés à une énergie d'impact de 50 J pour les composites vieillies : GFRE [0°/90°]4 et GFRE [45°/45°]4 était d'environ de 11 % et 19 %, respectivement. L'analyse de l'évolution de l'énergie absorbée lors des tests d'impact a permis de confirmer que les matériaux vieillis GFRE [45°/45°]4 présentaient la plus faible résistance aux chocs. Comme pour les résultats obtenus en impact basse vitesse, l'analyse de la dissipation d'énergie et de l'évolution de la force d'impact lors des essais de perforation a permis de conclure que les plaques composites GFRE [0°/90°]4, ayant au préalable subi un chargement en fatigue, avaient les meilleures performances balistiques et une dissipation d'énergie plus élevée que celle du composite GFRE [45°/45°]4. Il a également été observé que les composites GFRE [45°/45°]4 vieillis en fatigue par chargement cyclique à une contrainte inférieure ou supérieure à la limite élastique avaient des limites balistiques inférieures de 20 m/s à celles de composites vieillies GFRE [0°/90°]4. Les prédictions numériques des limites balistiques, des vitesses résiduelles ainsi que l'analyse de l'absorption d'énergie réalisée à l'aide du code éléments finis ABAQUS/Explicit ont montré un bon accord avec les données expérimentales et ont confirmé la pertinence de l'approche proposéeImpact resistance is one of the most critical features of the composite structures, and therefore, its examination for a new material has a fundamental importance. This thesis is devoted to the characterization of the fully recyclable thermoplastic ELIUM Acrylic resin reinforced by glass fabric woven that belongs to a new category of materials requiring advanced testing before their application in responsible elements of engineering structures. Its high strength, low weight as well as low production cost give an excellent opportunities to its wide application in the automotive industry as a replacement of the thermoset-based laminates. The study presents an experimental work concerning the effect of damage due to low and high cyclic fatigue aging of two groups of specimens with two different woven fabric orientations in relation to the acting load: [0°/90°]4 and [45°/45°]4, on their low and high impact velocity properties. Subsequently, a comparison of the experimental data with the results of numerical simulations predicted by the macroscopic model proposed for the non-aged GFRE [0°/90°]4 specimens of 1.8 mm thickness are shown. A response of the materials on fatigue loading was evaluated in terms of the stiffness variation analysis. The fatigue aging program was interrupted at a given value of the stiffness drop, prior to the specimen failure. The initial stiffness was reduced by about 17 % for the GFRE [0°/90°]4, after either LCF or HCF tests. Such reduction was much more pronounced in the case of GFRE [45°/45°]4. It was equal to 48 %. The low velocity impact resistance was measured in terms of load peak, absorbed energy and penetration threshold, while the high velocity impact resistance using the ballistic limit and total energy dissipation. The non-aged specimens impacted at energy of 5 J, 10 J and 30 J were not perforated and showed a great ability to rebound the impactor. The composites subjected to low cyclic fatigue (LCF) demonstrated the greatest decrease of the initial slope of the force-displacement characteristics. It results from the significant reduction of stiffness caused by the matrix cracking and interface debonding taking place between the fibers and matrix. The maximum force reduction for tests carried out at the impact energy of 50 J for GFRE [0°/90°]4 and GFRE [45°/45°]4 was equal to 11% and 19%, respectively. Analysis of the absorbed energy during impact tests confirmed, that the aged materials containing the woven fabric oriented at the angle of 45° exhibited the weakest impact resistance. Similarly to the tests at low velocity impact, the analysis of the energy dissipation and evolution of the impact force during perforation tests confirmed, that GFRE [0°/90°]4 plates subjected to uniaxial fatigue loading had better ballistic performance and higher energy dissipation than GFRE [45°/45°]4 targets. It was also observed that specimens of GFRE [45°/45°]4 aged by cyclic loading at the stress either below or above the yield point had 20 m/s lower ballistic limits compared to those of GFRE [0°/90°]4. Therefore, one can conclude that the fibers orientation, fatigue aging, and glass fibers concentration significantly affect the elastic properties, and as a consequence, lead to the stiffness reduction and decrease of the energy absorbed and ballistic limit. The results are in agreement with the analyzes of damage phenomena. The numerical predictions of the ballistic limits, residual velocities as well as the analysis of energy absorption performed using the finite element software ABAQUS/Explicit exhibited a good agreement with the experimental data and confirmed the relevance of the approach proposed
27
Kirka, Michael. "Thermomechanical behavior of a directionally solidified nickel-base superalloys in the aged state." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/53458.
Анотація:
Understanding the effects of aged microstructures on the thermomechanical fatigue (TMF) properties of nickel-base (Ni-base) superalloys remains unclear. Few experimental results are currently available in this area, and of the limited results available, some promote aged microstructures as beneficial, while others as detri- mental. The importance of these aged structures arises from the fact that when components used in the hot sections of gas turbine engines remain in service for ex- tended periods of time, the local temperature and stress provides the catalyst for the evolution of the microstructure.
An experimental assessment of a negative misfit directionally solidified (DS) Ni- base superalloy was undertaken to characterize the aging kinetics and understand the influence of the TMF cycle temperature extremum, temperature-load phasing, mean strain, creep-fatigue, orientation effects, and microstructure on TMF fatigue crack initiation. To determine the effects of aging on the TMF response, the as-heat- treated alloy was artificially aged to three unique microstructures identified in the aging kinetics study. The experiments revealed that not all aged microstructures are detrimental to the fatigue life behavior. Specifically, when the γ′ precipitates age in a manner to align themselves parallel to the axis of the applied stress, an increase in the fatigue life over that of the as-heat-treated microstructure is observed for out-of-phase TMF with dwells.
To extend the experimental understanding of the aged microstructures into ser- vice component design and life analysis, a temperature-dependent crystal viscoplas- ticity (CVP) constitutive model is developed to capture the sensitivity of the aged microstructure through embedding additional variables associated with the current state of the γ′ particles. As a result of the adaptations, the CVP model has the ability to describe the long-term aging effects of directional coarsening relevant to the analysis industrial gas turbine hot section components.
28
Rednour, Allison. "A Nutritional Shopping System for Senior Citizens." University of Cincinnati / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1491227694091354.
29
Huang, Zhiyong. "Endommagement des aciers au C-Mn en fatigue oligocyclique et gigacyclique." Thesis, Paris 10, 2010. http://www.theses.fr/2010PA100088.
Анотація:
Dans les générateurs de vapeur des centrales nucléaires à eau pressurisée, les tuyauteries sont soumises à des chargements thermique et mécanique, qui sont variables et divisés en deux régimes différents : la fatigue oligocyclique et la fatigue gigacyclique. Les aciers au carbone – manganèse, type A42, A48 et Tu48 (normes françaises) sont souvent utilisés dans de telles applications. Les propriétés du matériau manifestent certains caractères spéciaux en mécanique et métallurgie comme le vieillissement dynamique conduisant à une augmentation de la valeur de la contrainte maximale et une diminution de la ductilité à la température 200 ℃. Le comportement en fatigue oligocyclique et gigacyclique sont étudiés à température ambiante et 200 ℃. Des essais de fatigue cumulée ont été mis en oeuvre pour étudier l’effet du cumul de dommage combinant des cycles de fatigue oligocyclique suivis de cycles en fatigue gigacyclique. Tous les résultats sont analysés en utilisant la mécanique de l’endommagement des milieux continus et l’analyse microfractographique. Les cycles d’hystérésis en fatigue oligocyclique sont dus à la déformation plastique de durcissement cinématique, ils peuvent être décrits par sous le modèle d’Armstrong – Frederick ; le durcissement isotrope est utile pour prédire l’évolution de l’amplitude de contrainte. Mais avec l’augmentation de la déformation plastique accumulée, le dommage ne peut être négligé. Le modèle de Chaboche d’endommagement par fatigue est utilisé pour décrire l’évolution des dommages oligocyclique et il est étendu au régime gigacyclique. Un modèle de fatigue cumulée des dommages a été développé à partir du modèle de Chaboche et appliqué à l’estimation des dommages de fatigue pour décrire le comportement de l’évolution de la contrainte en fonction du nombre de cycles. En fatigue oligocyclique à la température de 200 ℃, l’acier A48 est sensible au phénomène de vieillissement dynamique et il apparaît un durcissement secondaire, qui peut être prédit par la théorie des dislocations et est simulé dans la thése. L’analyse des surfaces de rupture est effectuée par fractographie au Microscope Electronique à Balayage pour les essais en oligocyclique, gigacyclique et cumul. En fatigue oligocylique, la fissure est initiée en surface. En fatigue gigacyclique, certaines fissures sont initiées sur des inclusions situées à l’intérieur d’éprouvettesIn steam generators of nuclear power plants, typical pipes components are subjected to thermal and mechanical loading which are variable and divided into two different regimes: low cycle fatigue and gigacycle fatigue. Carbon-manganese steels A42, A48 and Tu48 steels (French standards) are often used in such applications. The material properties manifest some special characters in mechanics and metallurgy such as Dynamic Strain Aging, increasing UTS values in 200℃ temperature domain. The LCF and VHCF behaviors are investigated respectively by test method at room temperature and 200℃. The cumulative fatigue tests are implemented through referencing the load as prior LCF following gigacycle fatigue from the steam generator pipes thermal loads in order to obtain the performance of material under accumulated fatigue damage. All the test results are analyzed by using plastic mechanics, continuums damage mechanics and microscopic analysis. Hysteresis loops are due to plastic deformation in LCF which is the effect of kinematic hardening and they can be described by Armstrong – Frederick form models; the isotropy hardening is used to predict the evolution of stress amplitude in LCF. But with rising of accumulated plastic deformation, the damage can not be neglected. The Chaboche fatigue damage model is applied to describe the damage evolution of LCF and extended to VHCF regime. The cumulative fatigue damage model is extended from Chaboche model and applies to the estimation cumulative fatigue damage. The constitutive relationship and isotropy rule are coupled with fatigue damage model that can describe the whole fatigue behavior. In 200℃ for LCF, A48 is sensitive to dynamic strain aging and its secondary hardening behavior is important which can be predicted by dislocation theory and is simulated in the paper. The fractographic analysis is performed by SEM for LCF, VHCF and cumulative fatigue tests. The LCF crack is initiated in surface. Some of cracks of VHCF are given birth from the inclusions located at interior of sample
30
Hoche, François-Xavier. "Vers une prise en compte du vieillissement thermique dans la filière de dimensionnement des structures pour la fatigue thermomécanique." Thesis, Paris Sciences et Lettres (ComUE), 2016. http://www.theses.fr/2016PSLEM051.
Анотація:
Les culasses de moteur diesel PSA sont réalisées en alliages d'aluminium de fonderie durcis par précipitation. La culasse atteint une température stable lors de l'utilisation du véhicule mais la succession de démarrages et d'arrêts engendre un phénomène de fatigue thermique au niveau du pontet inter-soupapes. L'exposition à des températures élevées modifie la microstructure de précipitation provoquant une diminution des propriétés mécaniques et donc un vieillissement de l'alliage. Il est donc nécessaire de prendre en compte le vieillissement thermique dans le dimensionnement. Pour y parvenir, nous avons d'abord analysé la microstructure de précipitation (nature des phases précipitées, morphologie et taille) en Microscopie Electronique en Transmission (MET) et le comportement cyclique viscoplastique pour différents vieillissement isothermes. Des essais thermomécaniques anisothermes, simulant le chargement en service, ont été réalisés afin d'analyser les relations entre la déformation plastique et la précipitation. Les paramètres géométriques de la précipitation ont été déterminés en MET pour différents nombres de cales thermomécaniques et ont été comparés à ceux de la précipitation résultant de l'exposition de l'alliage au seul cycle thermique pendant le même nombre de cycles.La microdureté étant un bon indicateur du vieillissement des alliages d'aluminium durcis par précipitation, des modèles d'évolution de microdureté ont été développés afin de simuler sa décroissance dans la structure lors d'un essai représentatif des conditions en service. Ces simulations permettent d'améliorer l'estimation du vieillissement de ces alliages lors de l'utilisation de la culasseCylinder heads of automotive engines are produced by casting of precipitation strengthened Al-Si alloys.The cylinder head quickly reaches a steady state temperature but the succession of starts and stops generates thermomechanical fatigue in the area between the valve seats. At service temperature, the precipitation microstructures evolve from their state after precipitation hardening heat treatment to an aged state, which results in the reduction of their mechanical properties. The increase of thermomechanical stresses in new engines requires taking thermal aging into account for sizing. To that end, the precipitation microstructures (precipitates crystal structures, composition, morphology, and distribution) have been analyzed by Transmission Electron Microscopy (TEM) for various aging conditions and the corresponding cyclic least-viscoplastic behavior has been determined. The effect of plastic deformation on the kinetics of precipitate growth has been studied throughout loadings representative of engine operation. The morphological characteristics of the precipitation microstructure resulting from the thermomechanical aging were determined by TEM for different numbers of cycles and compared with those resulting from the mere thermal aging in order to assess the effect of plastic deformation on the kinetics of precipitate growth. As micro hardness is a good indicator of the aging of precipitation strengthened aluminium alloys, micro hardness evolution models have been developed to calculate its decrease in the cylinder head during a test simulating in service conditions. The simulations give us a better understanding of the in-service aging of these alloys
31
Shabani, Amin. "Thermal and radiochemical of neat and ATH filled EPDM : establishment of structure/properties relationships." Phd thesis, Paris, ENSAM, 2013. http://pastel.archives-ouvertes.fr/pastel-00941289.
Анотація:
EPDM elastomer is widely used as the insulation of low to medium voltage electrical cables used in power plants, for which the life-time prediction has been hampered by the lack of knowledge on structure/mechanical properties, and the nonexistence of pertinent criteria of structural failure. In an attempt to fill this gap, three EPDM matrices filled with 0, 33 and 100 phr of pristine and surface treated ATH were crosslinked by dicumyl peroxide at 170°C and, subsequently, aged thermally at 90,110 and 130°C, and radiochemically under 0.1, 1 and 10 kGy.h-1, in air. A multi-scale approach was employed to analyze the oxidation of EPDM at molecular scale, and to determine its consequences at macromolecular and macroscopic scales by using several complementary characterization techniques: FTIR spectrophotometry, differential calorimetry, rheometry in melt state, swelling test, uniaxial tensile testing, etc. The structure/properties relationships established in this study are capable to explain, in particular, the alteration of elastic and fracture properties of the EPDM matrices due to chain scissions, and the reinforcement of the filler/matrix interphases induced by the specific conditions of oxidation.
32
Pu, Xiaoxue. "Thermomechanical study of the gigacycle fatigue behavior of pure iron and carbon-manganese steels : influence of chemical composition and microstructure on damage and crack initiation mechanism." Thesis, Paris 10, 2019. http://www.theses.fr/2019PA100051.
Анотація:
Ce travail tente de mieux comprendre les dommages par fatigue dans les aciers à ferrite-perlite dans la fatigue à très grand nombre de cycles (VHCF). Les influences de deux paramètres, le pourcentage de phase perlite et le pourcentage d'atomes interstitiels libres dans une solution solide, sont étudiées pour comprendre les mécanismes de dissipation en fatigue à 20 kHz. Une thermographie infrarouge in situ est réalisée pour enregistrer les changements de température, tandis que des observations au microscope sont menées pour étudier le mécanisme de dissipation. Pour les matériaux de BCC, sous fortes amplitudes de contrainte, une augmentation soudaine de la température se produit sans initiation de fissure ni fracture. L’augmentation inévitable de la température jusqu’à des centaines de degrés aux fortes amplitudes de contrainte est principalement due à la mobilité des dislocations vis, qui est l’une des clés permettant d’expliquer le comportement en fatigue observée de la structure du BCC sous un chargement haute fréquence. Par conséquent, les PSB en surface et les micro-vides dans la matrice émergent en masse, accompagnant cette élévation abrupte de la température. Ces phénomènes sont considérés comme une transition du mécanisme de déformation du régime thermique au régime athermique. À faible amplitude, peu de PSB ou de rugosité de surface sont encore observés. Il a été constaté que les PSB sur le fer armco étaient susceptibles d’apparaître avant 1x107 cycles et que le seuil de PSB était inférieur à la limite de fatigue conventionnelle. La présence d'atomes interstitiels libres dans les aciers entraîne l'apparition d'une augmentation secondaire de la température dans la domain de la température stabilisée à 100-200 °C. Ce comportement semble être lié à l'interaction des dislocations coins avec des atomes interstitiels libres. De plus, on pense que le phénomène remarquable de durcissement-adoucissement-durcissement après l'élévation soudaine de la température jusqu'à plus de 300 °C est l'interaction de dislocations à vis multipliées et d'atomes interstitiels libresThis work attempts to a better understanding of the fatigue damage in ferrite-pearlite steels in the Very High Cycle Fatigue (VHCF) domain. The influences of two parameters, pearlite phase percentage and free interstitial atoms percentage in solid solution, are investigated to understand dissipative mechanisms under 20 kHz high frequency fatigue loading. In-situ infrared thermography is carried out to record the temperature changes, while fractography studies and microscope observations are conducted to investigate the dissipative mechanism on the surface of specimens.For body centered cubic (BCC) materials, under high stress amplitudes, a sudden increase of the temperature occurs without a crack initiation and fracture. The inevitable temperature increase up to hundreds of degrees at high stress amplitudes, is caused mainly by the screw dislocations mobility, which is the key to explaining the observed fatigue behavior and thermal response of BCC structure under high frequency loading. Therefore, PSBs on surface and micro-voids in matrix emerge massively, accompanying with this abrupt temperature increase. These phenomena are considered as transition of deformation mechanism from thermal regime to athermal regime. At low amplitudes, few PSBs or surface roughness are still observed on the specimen surface. Through the cycles of PSB appearance on armco-iron, it’s found that PSBs are inclined to appear before 1x10(7)cycles, and PSB threshold lies below the conventional fatigue limit. The increase of pearlite phase content weakens the temperature elevation, and strengthens the fatigue properties. The presence of free interstitial atoms in steels results in appearence of a secondary temperature increase in the stabilized temperature part (100-200 degree). This behavior seems to be related to the interaction of edge dislocations with free interstitial atoms. Moreover, the remarkable hardening-softening-hardening phenomenon after the sudden temperature elevation to above 300 degree is thought as the interaction of multiplicated screw dislocations and free interstitial atoms
33
Tout, Firas. "Étude expérimentale des éléments structuraux multi-matériaux collés sollicités en flexion : effet du fluage, fatigue et durabilité." Phd thesis, Université Claude Bernard - Lyon I, 2014. http://tel.archives-ouvertes.fr/tel-01064338.
Анотація:
Ce travail porte sur la compréhension du comportement en flexion, statique instantané, en fatigue et en fluage des structures multi matériaux du génie civil, constituées d'acier et de béton et assemblées par un adhésif de type époxy. L'analyse et la résolution du problème posé se feront à deux échelles : au niveau de l'interface et à l'échelle d'un élément de structure. De plus, l'attention sera tout particulièrement portée sur le comportement au cours du temps (fluage, relaxation, retrait...), la fatigue et la durabilité sous différents environnements. La première étape consiste à mener une analyse expérimentale sur la caractérisation de la connexion acier-béton, à l'échelle de l'interface, en vue de repérer les principaux facteurs influençant leur comportement mécanique. Pour cela, L'essai push out, analogue à celui utilisé pour caractériser les connecteurs des ponts mixtes, a été plus particulièrement utilisé. Des essais de vieillissement accéléré par immersion dans un bain d'eau chaude ont été réalisés. Ils n'ont pas diminué la contrainte de cisaillement moyenne à rupture de façon significative mais ont en revanche modifié le mode de ruine qui devient mixte (adhésif et cohésif dans la colle). Cette modification est à la fois due au primaire et à l'adhésif lui-même dont les performances intrinsèques diminuent avec le vieillissement. A l'échelle d'un élément de structure, nous avons enrichi les données expérimentales existantes sur le comportement instantané des poutres mixtes acier-béton collées avec la réalisation de 7 essais de flexion avec deux modes de sollicitation (flexion 3 et 4 points). Ces essais nous donnent une base solide pour nos essais en fatigue et sous fluage. On constate que le mode de ruine est mieux prédit en se basant sur une comparaison entre la contrainte de cisaillement dans les poutres avec les contraintes de cisaillement limite déduites des essais push-out. Cette conclusion montre la pertinence des essais push-out dans l'objectif de les prendre comme critère de dimensionnement. Aussi, et contrairement aux travaux antérieurs, nous pouvons conclure que la contrainte moyenne de cisaillement à l'interface entre le béton et l'acier est supérieure à la résistance en traction du béton utilisé. Pour le comportement à long terme, trois poutres ont été testées sous charge cyclique pour plus de 2 million de cycles et à plusieurs niveaux de charge. Une autre poutre a subi une charge constante pendant 7 mois pour étudier l'influence de fluage sur ce type de structure. Après les essais de fatigue et fluage, le comportement des poutres est analogue à celui des poutres de référence. Nous avons toujours une ruine par rotule plastique et non par cisaillement du joint de colle ce qui confirme sa bonne résistance vis-à-vis ces deux phénomènes
34
Deshayes, Maxime. "Influence des stéréotypes sexués et des stéréotypes liés à l’âge sur la performance d’endurance non-technique : un effet positif de l’induction d’un stéréotype négatif ?" Thesis, Université Côte d'Azur (ComUE), 2019. http://www.theses.fr/2019AZUR4108.
Анотація:
La théorie de la menace du stéréotype (Steele, 1997) suggère que lorsqu’un stéréotype négatif à l’encontre d’un groupe social est induit, dans un contexte où ce stéréotype est susceptible de s’appliquer, une baisse de la performance du groupe menacé serait observée. Après avoir été répliqué dans de nombreux domaines comme le domaine cognitif, cet effet négatif sur la performance a été généralisé au domaine physique et sportif. La majorité des études ont investigué cet effet au travers de tâches techniques comme des tâches de dribbles en football par exemple (pour une méta-analyse, voir Gentile, Boca, & Giammusso, 2018). Bien que les capacités techniques représentent des facteurs majeurs de la performance physique, d’autres facteurs régulent également cette dernière, comme les capacités physiques avec en leur centre l’endurance (voir le modèle de Weineck, 1983). L’objectif de ce travail doctoral est d’examiner l’influence de l’induction de stéréotypes sexués et de stéréotypes liés à l’âge sur la performance d’endurance non-technique des femmes et des seniors. Le modèle des processus intégrés (Schmader, Johns, & Forbes, 2008), très largement utilisé dans la littérature, explique les modifications de performance à la suite de l’induction d’un stéréotype négatif lors de tâches techniques. Toutefois, ce modèle n’apporte aucune hypothèse quant à son effet lors de tâches non-techniques. En revanche, le modèle du simple effort (Jamieson & Harkins, 2007), récemment appliqué au domaine physique et sportif, pourrait s’appliquer à ce type de tâche. Il suggère que l’induction d’un stéréotype négatif pourrait entrainer une amélioration de la performance des participant.e.s menacé.e.s lors de tâches d’endurance non-techniques. En accord avec ces prédictions, les études 1, 2, 3 et 4 de cette thèse ont montré une amélioration de la performance d’endurance des femmes, après l’induction d’un stéréotype négatif à leur égard, au travers de tâches à intensité maximale et sous-maximale et sur différents groupes musculaires. Ces études ont notamment observé que les femmes menacées étaient davantage motivées à surpasser les hommes (étude 4) et s’engageaient dans davantage d’effort. Cette amélioration de la performance pourrait également être due à une plus faible fatigue centrale (étude 3) et à une planification plus importante de la commande motrice au niveau du cortex préfrontal (étude 4). Ces quatre études renforcent l’hypothèse selon laquelle les effets des stéréotypes négatifs sexués seraient tâche-dépendants et le modèle du simple effort apparaît alors comme relativement robuste pour expliquer les effets des stéréotypes sexués sur des tâches d’endurance non-techniques. De manière intéressante, ces effets positifs n’ont pas été totalement répliqués chez les seniors. L’induction d’un stéréotype négatif lié à l’âge n’a entrainé aucune modification de la performance maximale des participantes (études 5 et 6) mais une augmentation de la performance lorsque l’intensité de la tâche était réduite (étude 6). Chez les seniors, le modèle du simple effort apparaît comme imparfait et nécessite d’être complété pour expliquer les effets des stéréotypes liés à l’âge dans le domaine physique et sportif. Les résultats de ce travail enrichissent la littérature sur la menace du stéréotype en montrant que l’induction d’un stéréotype peut, contrairement aux prédictions du modèle initial (Steele, 1997), entrainer une amélioration de la performance physique et sportive chez les femmes, et sous certaines conditions chez les seniorsThe stereotype threat theory (Steele, 1997) suggests that when a negative stereotype toward one group is induced, in a context where this stereotype is susceptible to be applied, a decrease in performance would be observed. After being replicated in various domains, this negative effect on performance was generalized to the physical domain. Most studies investigated this effect during technical tasks such as soccer-dribbling tasks (for a meta-analysis, see Gentile, Boca, & Giammusso, 2018). Although technical abilities are major factors in physical performance, other factors also regulate physical performance such as physical abilities with, for example, endurance (Weineck, 1983). The objective of this thesis was to examine the influence of inducing sex stereotypes and aging stereotypes on women’s and older adults’ non-technical endurance performance. The integrated process model (Schmader, Johns, & Forbes, 2008), widely used in the literature, explains performance modifications after a negative stereotype was induced during technical tasks. However, this model makes no assumptions about its effect during non-technical tasks. The mere effort account (Jamieson & Harkins, 2007), recently validated in the physical domain, could be applied during this type of task. This account suggests that inducing a negative stereotype could lead to an increase in threatened participants’ performance during non-technical endurance tasks. In line with these predictions, studies 1, 2, 3 and 4 showed an increase in women’s endurance performance after inducing a negative stereotype toward them during tasks at submaximal and maximal intensities, and on different muscle groups. These studies especially observed that threatened women were more motivated to outperform men (study 4) and they put more effort into the tasks. This performance increase could also be due to a lower central fatigue (study 3) and a greater planning of the central motor command over the prefrontal cortex (study 4). These four studies strengthen the hypothesis that the effects of sex stereotypes would be task-dependent and that the mere effort account appears to be relatively robust to explain the effects of sex stereotypes during non-technical endurance tasks. Interestingly, these positive results were not fully replicated in older adults. Inducing a negative aging stereotype did not influence participants’ maximal performance (studies 5 and 6). An increase in performance was nonetheless observed when the intensity of the task was reduced (study 6). When investigating aging stereotypes, the mere effort account seems to be imperfect and needs to be completed to explain their effects in the physical domain. The results of this thesis enrich the stereotype threat literature by showing that inducing a negative stereotype can, contrary to the predictions of the initial stereotype threat theory (Steele, 1997), lead to an improvement of women’s performances and under certain conditions for older adults
35
Ruffilli, Roberta. "Modes de fatigue des métallisations à base d'aluminium dans les composants MOSFET de puissance." Thesis, Toulouse 3, 2017. http://www.theses.fr/2017TOU30256/document.
Анотація:
Cette thèse, effectuée en collaboration entre le CEMES-CNRS, le laboratoire Satie (ENS Cachan) et NXP Semiconductors est motivée par la compréhension des mécanismes de défaillance des dispositifs MOSFET pour les applications dans l'industrie automobile. Un facteur limitant de la fiabilité à long terme des modules de puissance basse tension est le vieillissement électrothermique et/ou thermo-mécanique des parties métalliques de la source: métallisation en aluminium (ou alliage) et fils de connexion. A cause de la différence de coefficient de dilatation thermique entre la métallisation les oxydes et le substrat semi-conducteur, la température atteinte pendant les cycles de fonctionnement (quelques centaines de degrés), induit une déformation plastique inévitable dans le métal, qui est le matériau le plus mou dans l'architecture complexe du MOSFET. Nous avons caractérisé la microstructure métallique avant et après les tests de vieillissement électrothermique accélérés, en utilisant des techniques spécifiques du domaine de la métallurgie physique: microscopie électronique et ionique, cartographie d'orientation de grains et de la composition chimique. Pour la première fois, la métallisation de la source a été caractérisée sous les fils de connexion, qui sont cent fois plus épais que la métallisation. Cet emplacement est critique pour la fiabilité du composant, car le processus de soudure par ultrasons induit une déformation plastique importante qui peut affaiblir la métallisation initiale avant le vieillissement. Ceci est peu étudié dans la littérature en raison de la difficulté à accéder à la métallisation sous les fils sans altérer leur interface, souvent endommagée et fragilisée dans les modules vieillis. Nous avons mis en place des méthodes de préparation d'échantillon, basées sur le polissage ionique, pour étudier cette interface, sans introduire d'artefacts de préparation. Le processus de soudure à froid induit une déformation plastique sévère et non uniforme dans la métallisation sous les fils sans parvenir à recréer un bon contact électrique: de petites cavités et des résidus d'oxyde natif, ont été systématiquement observés à l'interface Al / Al, dans tous les modules analysés, avant et après vieillissement. Le mécanisme principal de défaillance des modules est la génération et la propagation de fissures de fatigue dans l'aluminium, associée à une oxydation locale qui empêche la fermeture de ces fissures. Sous et en dehors des fils de connexion, ces fissures traversent la métallisation perpendiculairement à la surface jusqu'au substrat en silicium en suivant les joints de grains. Cette fissuration est due à la diffusion intergranulaire accélérée des atomes d'aluminium. Dans la zone de soudure, le phénomène de fissuration parallèle à l'interface est favorisé par les imperfections initiales (cavités, oxyde). Les expériences de tomographie ionique ont montré que ces fissures sont confinées à l'interface fil-métal et ne se propagent pas dans le fil malgré sa plus faible résistance mécanique (Al pur, structure à grains plus grands). La propagation de la fissure le long de l'interface Al/Al peut provoquer une diminution du contact entre le fil et la métallisation de la source et éventuellement son décollement. Les fissures dans le métal source peuvent expliquer l'augmentation locale de la résistance et de la température du module qui accélère le processus de vieillissement jusqu'à l'échec. Cette étude a établi de nouvelles techniques dédiées et des méthodes de quantification pour évaluer le vieillissement des parties métalliques des modules MOSFET. La caractérisation complète de l'interface soudée, intrinsèquement défectueuse et la dégradation de la métallisation pendant le vieillissement électrothermique ouvrent la voie à l'amélioration possible les technologies actuelles et au développement potentiel de nouveaux procédésThis thesis, a collaboration between CEMES-CNRS, Satie laboratory (ENS Cachan) and NXP Semiconductors is motivated by the comprehension of the failure mechanisms of low voltage power MOSFET devices produced for ap- plications in the automotive industry. A limiting factor for the long-term reliability of power modules is the electro- thermal and/or thermo-mechanical aging of the metallic parts of the source: Al metallization and bonding wires. At the temperature reached during the on-off operating cycles (few hundred degrees), the difference in the coefficient of thermal expansion between the metallization and the oxide and semicon- ductor parts induces an inevitable plastic deformation in the metal, which is the softest material in the complex MOSFET architecture. We have characterized the metal microstructure before and after accelerated electro-thermal aging tests, by using specific techniques from the field of the physical metallurgy: electron and ion microscopy, grain orientation and chem- ical composition mapping. For the first time the source metallization has been characterized both away and under the bonding connections, which are one hundred times thicker than the metallization layer. The latter is a critical loca- tion for the reliability assessment because the ultrasonic bonding process may weaken the initial metallization microstructure by adding an important plas- tic deformation prior to aging. This is, however, poorly stated in the literature because of the difficulty to access the metallization under the wires without damaging their bonding, which is known to be particularly weak in case of aged modules. In order to investigate the wire-metallization interface, we have set up origi- nal sample preparations, based on ion polishing, that allowed us to disclose the metallization under the bonding wires without introducing preparation artifacts in the microstructure. The bonding process induces a severe and non- uniform plastic deformation in the metallization under the wires without re- creating a good electrical contact: small cavities and native oxide residues, have been systematically observed at the Al/Al interface, in all the analyzed mod- ules, before and after aging. The main mechanism behind the device failure is the generation and propa- gation of fatigue cracks in the aluminum metallization, associated to a local Al oxidation that prevents these crack from closing. Away and under the wire bonds, they run perpendicularly from the surface down to the silicon substrate following the grain boundaries, due to an enhanced intergranular diffusion of aluminum atoms. In the bonding area, the phenomenon of parallel cracking is favored by the initial imperfections in the wire-metallization bonding. Ion to- mography experiments have shown that these cracks are confined to the wire- metal interface and do not propagate in the wire despite its lower strength (pure Al, larger grain structure). Crack propagation along the Al/Al interface can cause a contact reduction between the wire and the source metallization and eventually its failure. Such discontinuities in the metal can explain the lo- cal increase in the device resistance and temperature that accelerates the aging process until failure. This study settled new, dedicated techniques and quantification methods to as- sess the aging of the metal parts of MOSFET devices. The full characterization of the intrinsically defective interface generated by the bonding process and the metallization degradation during electro-thermal aging indicated paths to possible improvements of current technologies and potential developments of new processes
36
Pereira, Andressa Busch Rocha. "Efeito do uso do sistema âncora no controle postural após a fadiga dos músculos flexores plantares em idosos." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/17/17152/tde-29032018-104533/.
Анотація:
O controle postural é perturbado após a fadiga dos músculos flexores plantares. Por outro lado, o uso do sistema âncora reduz a oscilação postural. Entretanto, não se sabe se o uso dessa ferramenta seria capaz de compensar o aumento da oscilação corporal provocada pela fadiga muscular dos músculos flexores plantares. A informação háptica adicional fornecida pelo toque leve e o taping foi capaz de compensar o efeito perturbador da fadiga em adultos jovens. Porém, não se sabe como os idosos respondem a essa perturbação com o uso da informação háptica adicional. O objetivo desse estudo foi investigar se o uso do sistema âncora é capaz de compensar o aumento da oscilação corporal decorrente da fadiga dos músculos flexores plantares em adultos jovens e idosos. Os grupos de participantes foram testados descalços e com os olhos fechados em quatro condições na postura ereta: com e sem o sistema âncora, antes e depois do protocolo de fadiga. O sistema âncora consiste em dois cabos flexíveis com uma massa leve (125 g) fixada em uma das extremidades de cada cabo. Os participantes devem segurar um cabo em cada mão e manter o cabo esticado, sem retirar as cargas do solo. O protocolo de fadiga consistiu em realizar uma única série de flexão plantar bilateral do tornozelo repetidas vezes de forma constante e ininterrupta. Como esperado, o protocolo de fadiga aumentou a oscilação corporal nos dois grupos. Ambos os grupos se beneficiaram do uso das âncoras com a redução da oscilação corporal. Entretanto, esse efeito foi independente da fadiga. Concluímos que o sistema âncora contribuiu com a redução da oscilação corporal dos adultos jovens e idosos. Contudo, o sistema âncora não foi capaz de compensar o efeito perturbador postural em consequência da fadiga dos músculos flexores plantares.The fatigue of the planta flexor muscles disturbs postural control. On the other hand, the use of the anchor system reduces postural sway. However, it is unknown whether the use of this tool would compensate for the increase of the body sway caused by muscle fatigue of the plantar flexor muscles. The additional haptic information provided by light touch and taping was able to compensate for the disturbing effect of fatigue in young adults. However, it is unknown how the old adults would respond to this disturbance with the use of additional haptic information. The purpose of this study was to investigate whether the use of the anchor system is able to compensate for the increase of the body sway due to fatigue of the plantar flexor muscles in young and old adults. The groups of participants were tested barefoot and with eyes closed in four conditions in upright posture: with and without the anchor system, before and after the fatigue protocol. The anchor system consists of two flexible cables with a light load (125 g) attached to one end of each cable. Participants should hold one cable in each hand and the keep the cable taut without removing the loads from the ground. The fatigue protocol consisted of performing a single series of bilateral plantar flexion of the ankles repeatedly and continuously and uninterrupted. As expected, the fatigue protocol increased body sway in both groups. Both groups benefited from the use of anchors with reduction of body sway. However, this effect was independent of fatigue. We conclude that the anchor system contributed to the reduction of body sway in young and old adults. However, the anchor system was not able to compensate for the disturbing effect in postural control created by fatigue of the plantar flexor muscles.
37
Ruffilli, Roberta. "Modes de fatigue des métallisations à base d'aluminium dans les composants MOSFET de puissance." Electronic Thesis or Diss., Toulouse 3, 2017. http://www.theses.fr/2017TOU30256.
Анотація:
Cette thèse, effectuée en collaboration entre le CEMES-CNRS, le laboratoire Satie (ENS Cachan) et NXP Semiconductors est motivée par la compréhension des mécanismes de défaillance des dispositifs MOSFET pour les applications dans l'industrie automobile. Un facteur limitant de la fiabilité à long terme des modules de puissance basse tension est le vieillissement électrothermique et/ou thermo-mécanique des parties métalliques de la source: métallisation en aluminium (ou alliage) et fils de connexion. A cause de la différence de coefficient de dilatation thermique entre la métallisation les oxydes et le substrat semi-conducteur, la température atteinte pendant les cycles de fonctionnement (quelques centaines de degrés), induit une déformation plastique inévitable dans le métal, qui est le matériau le plus mou dans l'architecture complexe du MOSFET. Nous avons caractérisé la microstructure métallique avant et après les tests de vieillissement électrothermique accélérés, en utilisant des techniques spécifiques du domaine de la métallurgie physique: microscopie électronique et ionique, cartographie d'orientation de grains et de la composition chimique. Pour la première fois, la métallisation de la source a été caractérisée sous les fils de connexion, qui sont cent fois plus épais que la métallisation. Cet emplacement est critique pour la fiabilité du composant, car le processus de soudure par ultrasons induit une déformation plastique importante qui peut affaiblir la métallisation initiale avant le vieillissement. Ceci est peu étudié dans la littérature en raison de la difficulté à accéder à la métallisation sous les fils sans altérer leur interface, souvent endommagée et fragilisée dans les modules vieillis. Nous avons mis en place des méthodes de préparation d'échantillon, basées sur le polissage ionique, pour étudier cette interface, sans introduire d'artefacts de préparation. Le processus de soudure à froid induit une déformation plastique sévère et non uniforme dans la métallisation sous les fils sans parvenir à recréer un bon contact électrique: de petites cavités et des résidus d'oxyde natif, ont été systématiquement observés à l'interface Al / Al, dans tous les modules analysés, avant et après vieillissement. Le mécanisme principal de défaillance des modules est la génération et la propagation de fissures de fatigue dans l'aluminium, associée à une oxydation locale qui empêche la fermeture de ces fissures. Sous et en dehors des fils de connexion, ces fissures traversent la métallisation perpendiculairement à la surface jusqu'au substrat en silicium en suivant les joints de grains. Cette fissuration est due à la diffusion intergranulaire accélérée des atomes d'aluminium. Dans la zone de soudure, le phénomène de fissuration parallèle à l'interface est favorisé par les imperfections initiales (cavités, oxyde). Les expériences de tomographie ionique ont montré que ces fissures sont confinées à l'interface fil-métal et ne se propagent pas dans le fil malgré sa plus faible résistance mécanique (Al pur, structure à grains plus grands). La propagation de la fissure le long de l'interface Al/Al peut provoquer une diminution du contact entre le fil et la métallisation de la source et éventuellement son décollement. Les fissures dans le métal source peuvent expliquer l'augmentation locale de la résistance et de la température du module qui accélère le processus de vieillissement jusqu'à l'échec. Cette étude a établi de nouvelles techniques dédiées et des méthodes de quantification pour évaluer le vieillissement des parties métalliques des modules MOSFET. La caractérisation complète de l'interface soudée, intrinsèquement défectueuse et la dégradation de la métallisation pendant le vieillissement électrothermique ouvrent la voie à l'amélioration possible les technologies actuelles et au développement potentiel de nouveaux procédésThis thesis, a collaboration between CEMES-CNRS, Satie laboratory (ENS Cachan) and NXP Semiconductors is motivated by the comprehension of the failure mechanisms of low voltage power MOSFET devices produced for ap- plications in the automotive industry. A limiting factor for the long-term reliability of power modules is the electro- thermal and/or thermo-mechanical aging of the metallic parts of the source: Al metallization and bonding wires. At the temperature reached during the on-off operating cycles (few hundred degrees), the difference in the coefficient of thermal expansion between the metallization and the oxide and semicon- ductor parts induces an inevitable plastic deformation in the metal, which is the softest material in the complex MOSFET architecture. We have characterized the metal microstructure before and after accelerated electro-thermal aging tests, by using specific techniques from the field of the physical metallurgy: electron and ion microscopy, grain orientation and chem- ical composition mapping. For the first time the source metallization has been characterized both away and under the bonding connections, which are one hundred times thicker than the metallization layer. The latter is a critical loca- tion for the reliability assessment because the ultrasonic bonding process may weaken the initial metallization microstructure by adding an important plas- tic deformation prior to aging. This is, however, poorly stated in the literature because of the difficulty to access the metallization under the wires without damaging their bonding, which is known to be particularly weak in case of aged modules. In order to investigate the wire-metallization interface, we have set up origi- nal sample preparations, based on ion polishing, that allowed us to disclose the metallization under the bonding wires without introducing preparation artifacts in the microstructure. The bonding process induces a severe and non- uniform plastic deformation in the metallization under the wires without re- creating a good electrical contact: small cavities and native oxide residues, have been systematically observed at the Al/Al interface, in all the analyzed mod- ules, before and after aging. The main mechanism behind the device failure is the generation and propa- gation of fatigue cracks in the aluminum metallization, associated to a local Al oxidation that prevents these crack from closing. Away and under the wire bonds, they run perpendicularly from the surface down to the silicon substrate following the grain boundaries, due to an enhanced intergranular diffusion of aluminum atoms. In the bonding area, the phenomenon of parallel cracking is favored by the initial imperfections in the wire-metallization bonding. Ion to- mography experiments have shown that these cracks are confined to the wire- metal interface and do not propagate in the wire despite its lower strength (pure Al, larger grain structure). Crack propagation along the Al/Al interface can cause a contact reduction between the wire and the source metallization and eventually its failure. Such discontinuities in the metal can explain the lo- cal increase in the device resistance and temperature that accelerates the aging process until failure. This study settled new, dedicated techniques and quantification methods to as- sess the aging of the metal parts of MOSFET devices. The full characterization of the intrinsically defective interface generated by the bonding process and the metallization degradation during electro-thermal aging indicated paths to possible improvements of current technologies and potential developments of new processes
38
Klass, Malgorzata. "Contribution à l'étude de l'effet du vieillissement sur la fonction neuromusculaire." Doctoral thesis, Universite Libre de Bruxelles, 2007. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/209224.
Анотація:
Résumé en français :L’objectif de nos investigations a été d’étudier, chez le sujet humain, les mécanismes responsables de l'altération des performances motrices au cours du vieillissement. La première étude était destinée d’une part, à comparer la diminution maximale de force observée dans les différents régimes de contraction et, d’autre part, à contribuer à la compréhension des mécanismes à l’origine de la meilleure préservation de force pour le régime excentrique. Etant donné que jusqu’à présent, seuls des facteurs musculaires avaient été proposés afin d’expliquer les différences existant en fonction du régime de contraction, nous nous sommes plus particulièrement intéressés à une éventuelle participation de facteurs nerveux.
Dans la seconde étude, nous avons testé de manière indirecte (via l’enregistrement d’activités réflexes) l’efficacité des afférences périphériques au cours du vieillissement. En effet, sous le contrôle de la commande centrale, celles-ci contribuent également à réguler le niveau d’excitabilité des motoneurones. Une moindre efficacité de celles-ci pourrait donc être un facteur supplémentaire responsable de l’altération de la fonction motrice.
Nous avons ensuite analysé (études III et IV) les propriétés contractiles et le comportement des unités motrices lors de contractions lentes et très rapides (contractions balistiques). Cela nous a permis de vérifier s’il existait chez les seniors une relation entre la diminution de force, de sa vitesse d’installation, et la stratégie d’activation des unités motrices.
Enfin, dans l’étude V, nous avons analysé la fatigabilité neuromusculaire chez les seniors, comparativement aux sujets jeunes. Plus spécifiquement, nous avons étudié les contractions concentriques et excentriques maximales fatigantes. La fatigue n’a en effet pas encore été étudiée lors de ces deux types de contraction chez les adultes jeunes et âgés. L’objectif de cette dernière étude était surtout d’analyser la part relative des mécanismes nerveux et musculaires à l’origine de la fatigue ainsi que leur décours dans le temps en fonction de l’âge des sujets.
L’approche méthodologique utilisée nous a permis d’analyser les modifications fonctionnelles du muscle et de ses unités motrices avec l’âge. Un intérêt tout particulier a été porté aux ajustements de la commande nerveuse et à la compréhension des mécanismes sous-jacents. L’ensemble de nos résultats expérimentaux documentent de manière originale l’interaction étroite entre les adaptations nerveuses et musculaires au cours du vieillissement. Plus largement, notre travail doctoral contribue à une meilleure connaissance de la remarquable plasticité du système neuromusculaire chez l’homme.
Résumé en anglais :\
Doctorat en Sciences de la motricité
info:eu-repo/semantics/nonPublished
39
Solignac, Marie-Paul. "Etude du comportement a la fissuration en fluage et en fatigue d'aciers inoxydables austenitiques z2 cnd 17-12 (316 l) a 550 c." Paris 6, 1986. http://www.theses.fr/1986PA066540.
Анотація:
Description du comportement du materiau en fluage par une loi puissance. Etude des relations entre la vitesse de propagation et les parametres mecaniques. Prevision de la duree de vie. En fatigue, etude de l'influence des parametres experimentaux et de certains traitements (vieillissement, ecrouissage) sur la fissuration. Etude par microscopie electronique des modes de rupture, ainsi que de la microstructure necessaire a la comprehension du comportement de l'acier
40
Khoury, Georges. "Investigation de l'évolution de la microstructure et de la plasticité des joints de brasure SAC sous effets du vieillissement thermomécanique." Thesis, Bordeaux, 2020. http://www.theses.fr/2020BORD0124.
Анотація:
Actuellement, les alliages SAC sont largement utilisés en tant que matériaux de brasure. Les joints de brasure SAC présentent une microstructure dynamique et qui évoluent excessivement sous différentes conditions de vieillissement. La caractéristique de visco-plasticité du SAC étant extrêmement sensible de sa microstructure complique l’évaluation et l’estimation de l’espérance de vie des joints.Ce travail s’adresse à l’amélioration des prédictions de la fiabilité des joints de brasure SAC en contribuant à la compréhension de l’évolution de la microstructure et de la caractéristique mécanique de visco-plasticité du SAC. Ce travail consiste à étudier séparément l’évolution de la microstructure et de la plasticité sous effet du vieillissement.L’étude de la microstructure a permis d’acquérir de nouvelles connaissances sur le coarsening sous différentes conditions de vieillissement. Cette étude a aussi permis d’évaluer l’hypothèse de l’accumulation linéaire du dommage sur la base d’une analyse microstructurale et sur la base d’un cycle thermique avec les différentes conditions qu’il présente.L’étude de la plasticité a permis d’évaluer l’effet du vieillissement par cyclage thermique sur la tenue mécanique du SAC. Cette étude a aussi permis d’évaluer l’effet de la non prise en compte de l’évolution de la caractéristique mécanique de plasticité sur l’estimation de l’espérance de vie des joints de brasure en utilisant les simulations par EFNowadays, SAC alloys are widely used as soldering materials. The SAC solder joints have a dynamic microstructure which evolve excessively under different aging conditions. The characteristic of visco-plasticity of the SAC being extremely sensitive to its microstructure complicates the evaluation and the estimation of the life expectancy of the joints.This work is aimed at improving the predictions of SAC solder joints reliability by contributing to expand knowledge on the evolution of the microstructure and the mechanical characteristic of visco-plasticity of SAC. This work consists in studying separately the evolution of microstructure and plasticity under the effect of aging.The study of the microstructure allowed the gain of new knowledge on the coarsening under different aging conditions. This study also allowed the evaluation of the hypothesis of linear damage accumulation on the microstructural analysis basis and on the basis of a thermal cycle with all the different conditions that it presents.The study of plasticity allowed the assessment of the effect of aging by thermal cycling on the mechanical strength of the SAC. This study also allowed the evaluation of the effect of not taking into account the evolution of the mechanical characteristic of plasticity on the estimation of the life expectancy of the solder joints using finite element simulations
41
Naanani, Soumaya. "Etude du comportement à hautes températures d'un alliage TiAl performant densifié par frittage flash." Thesis, Toulouse 3, 2018. http://www.theses.fr/2018TOU30359.
Анотація:
L'un des enjeux principal de l'industrie aéronautique est de développer des systèmes plus performants et plus légers. Cette volonté s'inscrit, entre autres, dans une démarche de réduction du carburant consommé par les avions. Les alliages à base d'aluminures de titane (TiAl) présentent une résistance au fluage et à l'oxydation remarquables, ainsi qu'une masse volumique plus faible que les matériaux utilisés usuellement. Ces propriétés sont propices à la fabrication de pièces industrielles telles que des aubes de turbine à basses pression de moteurs d'avion et d'aubes de turbocompresseurs de moteurs automobiles. L'alliage IRIS (Ti49.92-Al48-W2-B0.08) a été développé par le CEMES en 2013 afin de répondre à ce type d'application industrielle. Le procédé d'élaboration (Spark Plasma Sintering), basé sur la métallurgie des poudres permet la fabrication rapide de pièces proches des cotes finales, et offre la possibilité de contrôler avec précision la microstructure. Compte tenu des propriétés intéressantes en traction et en fluage, nous avons décidé d'investiguer dans cette thèse les propriétés mécaniques de cet alliage à une température de service plus importante, soit 800°C, qui est considéré actuellement comme la température ultime à laquelle les alliages TiAl pourraient être utilisés industriellement. Pour cela nous avons abordé un nouveau volet dans l'histoire d'IRIS ; le comportement en fatigue oligocyclique. Ces essais ont été complétés par des investigations en fluage et en traction aux mêmes températures. Un autre volet de ce travail a consisté à identifier les mécanismes microscopiques de plasticité pour ces différentes sollicitations, afin d'interpréter les propriétés macroscopiques observées. Enfin, en raison du domaine de température particulièrement élevé, une caractérisation de la stabilité des microstructures lors de traitements de vieillissement a été entrepriseOne of the major challenges of the aircraft industries is to develop more effective and lighter systems. This will fits, among others, in a process of reducing fuel consumption by airplanes. Titane aluminum based alloys present a remarkable creep resistance and a high level of resistance to oxidation and low density. These properties are conductive to industrial manufacturing parts, as low pressure turbine for aircraft engines and turbocharger wheels for automotive engines. IRIS alloy (Ti49.92-Al48-W2-B0.08) has been developed by a CEMES team in 2013 in order to meet these industrial expectations. It is processed by Spark Plasma Sintering, a powder metallurgy technique which allows a fast processing with parts near to the final dimensions and a precise control of the microstructure. First tensile and creep tests on IRIS show interesting results which lead us in this work to explore more mechanical properties at a considerable service temperature of 800°C, which is considered as the ultimate operating temperature of TiAl alloys. For this purpose, we explored a new chapter in IRIS' life: the low cycle fatigue behavior. These studies have been supplemented by tensile and creep tests at the same level of temperature. In order to understand and explain these mechanical behaviors, plasticity studies have been performed on each type of solicitation. Finally, we achieve a final study on aged microstructures, to complete our understanding of the impact of the temperature on the alloy
42
Nuñez, Javier Yesid Mahecha. "Caracterização à fadiga de ligantes asfálticos modificados envelhecidos a curto e longo prazo." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/18/18143/tde-19112013-171029/.
Анотація:
O trincamento por fadiga é um dos principais mecanismos de deterioração dos pavimentos flexíveis e, mesmo assim, é, ainda hoje, um dos defeitos mais difíceis de serem controlados. Uma das alternativas para a melhoria das características de fadiga das misturas asfálticas é o emprego de ligantes asfálticos modificados, porém, há dúvida se o enrijecimento provocado pela adição dos modificadores pode influenciar a resistência à fadiga da mistura asfáltica. Com o intuito de mensurar características do ligante asfáltico que tenham relação com o trincamento por fadiga da mistura asfáltica, alguns ensaios de laboratório foram desenvolvidos recentemente, destacando-se dois: o ensaio de varredura de tempo (time sweep) e o ensaio de varredura de amplitude linear (LAS). O objetivo deste trabalho é caracterizar o comportamento à fadiga de 12 ligantes asfálticos modificados de mesmo grau de desempenho (PG 76-xx). As formulações CAP+PPA, CAP+SBS, CAP+SBS+PPA, CAP+Borracha, CAP+Borracha+PPA, CAP+PE, CAP+PE+PPA, CAP+EVA, CAP+EVA+PPA, CAP+SBR, CAP+SBR+PPA e CAP+Elvaloy+PPA foram preparadas tendo como base um ligante asfáltico 50/70 de classificação PG 64-xx. Com base na caracterização realizada neste estudo, as principais conclusões são: (a) o material que apresentou melhor vida de fadiga foi o CAP+EVA; (b) o material menos sensível à deformação foi o CAP puro; (c) o menos sensível à temperatura foi o CAP+PPA; (d) o efeito do PPA nos ligantes asfálticos com outros aditivos provocou diminuição da vida de fadiga para a maioria das formulações, aumento da sensibilidade aos níveis de deformação para todas as formulações, diminuição da sensibilidade à temperatura e aumento da sensibilidade ao envelhecimento para a maioria das formulações; (e) tomando por base o ordenamento final das melhores características à fadiga dos ligantes asfálticos segundo os parâmetros analisados, o CAP+Borracha apresenta o melhor desempenho globalFatigue cracking is a major mechanism of deterioration of flexible pavements, and even then, it is still one of the defects that are more difficult to control. One alternative to improve fatigue characteristics of asphalt mixtures is the use of modified asphalt binders; however, there is no doubt that the stiffening caused by the addition of modifiers can influence the fatigue resistance of asphalt mixture. In order to measure characteristics of asphalt binders that are related to the fatigue cracking of asphalt mixture, some laboratory tests have been recently developed such as the time sweep and the linear amplitude sweep (LAS) test. The objective of this study is to characterize the fatigue behavior of 12 modified asphalt binders with the same high-temperature performance grade (PG 76-xx). The formulations AC+PPA, AC+SBS, AC+SBS+PPA, AC+Rubber, AC+Rubber+PPA, AC+PE, AC+PE+PPA, AC+EVA, AC+EVA+PPA, AC+SBR, AC+SBR+PPA and AC+Elvaloy+PPA were prepared with a 50/70 base asphalt binder graded as PG 64-xx in the Superpave specification. Based on the characterization made in this study, the main conclusions are the following: (a) the material that showed better fatigue life was the AC+EVA; (b) the material that is less sensitive to deformation was the neat AC; (c) the less sensitive one to temperature was the AC+PPA; (d) the effect of PPA in asphalt binders prepared with other additives caused a reduction in the fatigue life for many formulations, an increase in the sensitivity levels of deformation for all formulations, a reduction in the sensitivity to temperature and an increase in the sensitivity to aging for several formulations; (e) on the final ranking of the characteristics of asphalt binders according to the parameters analyzed in this research study, the AC+Rubber showed the best overall performance
43
Castro, Danilo Borges Villarino de. "Estudo do comportamento em fadiga do aço ASTM A297 Gr HP modificado com nióbio em altas temperaturas." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/88/88131/tde-20122012-092456/.
Анотація:
Este trabalho teve por objetivo estudar o comportamento mecânico em elevadas temperaturas do aço ASTM A-297 Gr HP modificado ao nióbio. Este tipo de aço é utilizado em suportes de tubos que permanecem longos períodos de tempo expostos em temperaturas de operação próximas a 900ºC. Nesta pesquisa, foram estudadas a microestrutura, a resistência mecânica, a tenacidade ao impacto e a resistência à fadiga, considerando o efeito da temperatura de ensaio. Para a avaliação do efeito da temperatura e tempo de permanência na microestrutura e, consequentemente, nas propriedades mecânicas, foram realizados ensaios de dureza, tração e impacto para cada uma das condições apresentadas. Todas as condições do material envelhecido apresentaram a presença de precipitados nos contornos interdendríticos e precipitados secundários no seio da matriz austenítica, fazendo com que o material elevasse seus parâmetros de resistência mecânica, quando comparado com a condição de como-recebido (CR). Após esta etapa, foi estudado o comportamento mecânico em temperaturas elevadas, apenas para as condições CR e T1000 (envelhecido a 927°C durante 1000 horas). Ensaios mecânicos de tração, impacto e fadiga (E-N e da/dN-\'delta\'K) foram realizados nas seguintes temperaturas: 25ºC, 871ºC, 899ºC, 927ºC, 954ºC e 982ºC. Para a avaliação dos ensaios de impacto, utilizaram-se corpos de prova Charpy do tipo convencional (presença do entalhe) e pré-trincados (entalhe mais pré-trinca). Em todas as temperaturas de ensaio de tração, observou-se que a condição T1000 resultou em aumento do valor dos parâmetros de resistência mecânica e diminuição da sua ductilidade, em relação à condição CR. Em ensaios nas temperaturas elevadas, observou-se que a precipitação de carbetos secundários na matriz austenítica da condição T1000 é capaz de evitar uma queda acentuada dos parâmetros de resistência mecânica, quando comparada com a condição CR. As duas condições (CR e T1000) mostraram-se sensíveis à presença de um concentrador de tensão (entalhe mais pré-trinca), quando submetidas a ensaios de impacto. Os resultados de tração e impacto corroboram as evidências fractográficas encontradas para cada condição, indicando que há uma maior parcela de fratura dúctil nas amostras ensaidas em temperaturas elevadas do que na temperatura ambiente. Em relação ao comportamento em fadiga do material, observou-se que a vida em fadiga do material ensaiado em 25ºC foi muito maior do que a 927ºC, principalmente na região de fadiga de alto ciclo. Este fato pode ser atribuído a dois fatores dependentes do tempo: nucleação prematura decorrente de uma significativa redução do limite de escoamento e devido ao efeito da oxidação superficial das bandas de deslizamento permanentes. Os ensaios de propagação de trinca por fadiga mostraram que a taxa da/dN foi maior na temperatura de 927ºC do que em 25ºC, devido ao fato da maior ductilidade que o material apresenta em elevadas temperaturas, associado aos efeitos de oxidação e de fluência. Alguns resultados de fadiga de baixo ciclo foram confrontados com modelos teóricos de plasticidade. As curvas teóricas foram capazes de simular relativamente bem os resultados experimentais.This work aims to study the mechanical behavior at high temperatures of ASTM A-297 Gr HP steel changed with niobium. This type of steel is used for pipe supports that remain exposed long periods of time at operating temperatures near 900°C. In this study, was studied the microstructural composition, the mechanical strength, impact toughness and fatigue resistance, considering the effect of temperature test. To evaluate the effect of temperature and residence time in the microstructure and hence the mechanical properties were performed hardness, tensile and impact test for each of the conditions presented. All conditions of the aged material showed the presence of precipitates in the interdendritic boundaries and secondary precipitates within the austenitic matrix, causing the material to elevate its mechanical strength, compared to CR condition. After this step was studied the mechanical properties at elevated temperatures only for the conditions CR and T1000 (aged at 927°C for 1000 hours). Tensile, impact and fatigue tests (E-N and da/dN-\'delta\'K), were performed at the following temperatures: 25°C, 871ºC, 899ºC, 927ºC, 954ºC and 982°C. To evaluate the impact tests, were used Charpy specimens of the conventional type (presence of the notch) and pre-cracked (pre crack more notch). In all test temperatures of traction, it was observed that the T1000 condition resulted in an increase of the value of mechanical strength and reduction of its ductility, compared to CR conditions. In tests at elevated temperatures, it was observed that the precipitation of carbides in the austenitic matrix of the secondary condition T1000 is able to avoid a sharp decline in mechanical strength compared with the proviso CR. The two conditions (CR and T1000) were sensitive to the presence of a stress raiser (notch further pre-crack) when subjected to impact tests. The results of tensile and impact fractographies corroborate the evidence found for each condition, indicating a greater share of ductile fracture in testing samples at elevated temperatures than at room temperature. Regarding the fatigue behavior of the material, it was found that the fatigue life of the material tested at 25°C was much greater than 927°C, especially in the region of high cycle fatigue. This fact can be attributed to two factors dependent on the time: premature nucleation due to a significant reduction in yield strength and due to the effect of surface oxidation of permanent slip bands. The tests of crack propagation by fatigue showed that the rate da/dN was greater at a temperature of 927°C than at 25°C, because of higher ductility that the material exhibits at high temperatures associated with the effects of oxidation and creep.
44
Stitt, Alice C. "A physics-based maintenance cost methodology for commercial aircraft engines." Thesis, Cranfield University, 2014. http://dspace.lib.cranfield.ac.uk/handle/1826/13134.
Анотація:
A need has been established in industry and academic publications to link an engine's maintenance costs throughout its operational life to its design as well as its operations and operating conditions. The established correlations between engine operation, design and maintenance costs highlight the value of establishing a satisfactory measure of the relative damage due to different operating conditions (operational severity). The methodology developed in this research enables the exploration of the causal, physics-based relationships underlying the statistical correlations in the public domain and identifies areas for further investigation. This thesis describes a physics-based approach to exploring the interactions, for commercial aircraft, of engine design, operation and through life maintenance costs. Applying the "virtual-workshop" workscoping concept to model engine maintenance throughout the operating life captures the maintenance requirements at each shop visit and the impact of a given shop visit on the timing and requirements for subsequent visits. Comparisons can thus be made between the cost implications of alternative operating regimes, flight profiles and maintenance strategies, taking into account engine design, age, operation and severity. The workscoping model developed operates within a physics-based methodology developed collaboratively within the research group which encompasses engine performance, lifing and operational severity modelling. The tool-set of coupled models used in this research additionally includes the workscoping maintenance cost model developed and implements a simplified 3D turbine blade geometry, new lifing models and an additional lifing mechanism (Thermo-mechanical fatigue (TMF)). Case studies presented model the effects of different outside air temperatures, reduced thrust operations (derate), flight durations and maintenance decisions. The use of operational severity and exhaust gas temperature margin deterioration as physics based cost drivers, while commonly accepted, limit the comparability of the results to other engine-aircraft pairs as the definition of operational severity, its derivation and application vary widely. The use of a single operation severity per mission based on high pressure turbine blade life does not permit the maintenance to vary with the prevalent lifing mechanism type (cyclic/steady state).
45
Olivier, Nelson Cárdenas. "Análise de falha da camada polimérica externa de cabos umbilicais." Universidade de São Paulo, 2007. http://www.teses.usp.br/teses/disponiveis/3/3132/tde-14012008-111221/.
Анотація:
No presente trabalho é proposto um método para estabelecer um critério de falha da camada polimérica externa dos cabos umbilicais usados em operações offshore. O método proposta baseia-se na determinação do incremento da quantidade de água absorvida pela camada polimérica externa do cabo devido à deterioração provocada pelos carregamentos cíclicos e o envelhecimento originados pelo meio marinho. Este método propõe a realização de um ensaio de fadiga à tração uniaxial no polímero que compõe a camada externa do cabo. O teste é feito simulando as solicitações a que é submetida a camada externa do umbilical durante os ensaios de fadiga feitos, segundo a norma API 17E, durante o processo de homologação do cabo. Além disso, se propõe fazer um ensaio de envelhecimento artificial no polímero da camada externa simulando as condições de serviço do cabo no meio marinho. Com o fim de avaliar a influência dos carregamentos cíclicos e o os agentes do meio marinho nas propriedades do polímero, são feitos ensaios de absorção de água segundo a norma ASTM D 570, ensaios de tração segundo a norma ASTM D 638, ensaios de dureza segundo a norma ASTM D 2240 e observações num microscópio eletrônico de varredura. Finalmente a partir do incremento de massa sofrido pelo polímero no ensaio de absorção de água foi estimado o dano no material da camada externa do cabo. Os resultados obtidos no trabalho mostraram que os carregamentos cíclicos e o meio marinho aceleraram o processo de degradação da camada polimérica externa do umbilical demonstrando assim a eficiência do método proposto.In the present work is proposed a method to establish a failure criterion of the polymeric external layer of umbilical cables used in offshore operations. The proposed method is based on the determination of the increment of the quantity of absorbed water by the polymeric external layer of the cable due to deterioration caused by cyclic loads and the aging process originated by the marine environment. The method proposes to carry out a uniaxial fatigue test on the polymer that composes the external layer of the cable. This test is made simulating the cyclical deformation that the external layer is submitted in the umbilical during the fatigue test done, according to the API 17E standard, during the process of cable homologation. It is also intended to make an artificial aging test in the polymeric external layer simulating the service conditions of the cable in the marine environment. In order to evaluate of the cyclical loads and the sea set aging influence on the properties of the polymer, carry out water absorption test according to the ASTM D 570 standard, traction test according to the ASTM D 638 standard, hardness test according to the ASTM D 2240 standard and observations in an scanning electron microscopy. Finally starting from the mass increment suffered by the polymer in the water absorption test it was estimated the damage in the external layer material of the cable. The results obtained in this work showed that the cyclical loads and marine environment accelerated the degradation process of polymeric external layer of umbilical cable demonstrating the efficiency of the proposed method.
46
Pocheron, Mickaël. "Life-time prediction of solder joints used in surface mount assemblies during thermo-mechanical and isothermal aging." Thesis, Bordeaux, 2015. http://www.theses.fr/2015BORD0245.
Анотація:
Les directives ROHS et WEEE banniront, dans les années qui viennent, le plomb de l’industrie électronique. Seulement, les assemblages électroniques de Schlumberger destinés à des applications hautes températures, tels que les ceux faisant intervenir des composants montés en surface, font intervenir des brasures à forte teneur en plomb. C’est pourquoi, Schlumberger investit énormément afin de trouver de nouvelles brasures sans plomb pour les remplacer. Ce projet, qui s’inscrit dans ce cadre, a pour objectif de prédire la durée de vie d’assemblages utilisant ces nouvelles brasures avec un substrat et des composants en céramique. Cette prédiction se fait en deux étapes. La première est expérimentale. Les assemblages sont soumis à des vieillissements accélérés thermomécaniques et isothermes. En plus de la durée de vie, ces tests apportent des connaissances sur les effets du vieillissement, sur les mécanismes et les zones de défaillances, sur l’interaction de ces brasures avec les finitions du substrat et des composants et enfin sur l’évolution de la microstructure et des phases d’intermétalliques créées lors du vieillissement.La seconde étape est la modélisation de ces assemblages afin de comprendre leur comportement sous sollicitations thermomécaniques. Les simulations aident à comprendre les phénomènes locaux qui apparaissent dans l’assemblage et à extraire des paramètres de fatigue pour diverses conditions thermomécaniques. Enfin, une corrélation entre les résultats de défaillance expérimentaux et la fatigue calculée grâce à la simulation va permettre d’estimer la durée de vie des assemblages pour différentes sollicitations thermomécaniques. Les simulations permettent donc de diminuer le nombre d’essais expérimentaux souvent chers et longs. Seulement, pour faire des simulations fiables, il est nécessaire de connaitre les paramètres mécaniques de tous les matériaux. Pour la brasure, cela veut dire le comportement élastique, plastique et en fluage. Donc, un bénéfice supplémentaire pour Schlumberger est la détermination de ses paramètres pour les nouvelles brasuresBecause of ROHS or WEEE directives, in a close future, lead materials will be banned from electronicindustry. Unfortunately, Schlumberger is using high-lead content solders for surface mount devices forhigh temperature applications. Considering this issue, Schlumberger puts in place high amount of investments to replace these solders by lead-free solders. The topic of the work is to study lead free candidates destined to support Schlumberger high temperature mission profiles. The device under test chosen for this project is a surface mount device composed of a passive component connected to a ceramic substrate by solder joints. The predictive study of reliability of these new assemblies for high temperature applications needs two complementary analyses. The first study is to characterize, experimentally, the life time of surface mount assemblies using these new lead free solders submitted to accelerated thermomechanical and isothermal aging tests. Hence, the first benefits for Schlumberger are knowledge on thecompatibility of these new alloys with their current finishes with the microstructure and intermetallic compounds evolution. More over, the main effects due to aging are investigated like failure sites and mechanisms. The second goal of the project is to perform thermo-mechanical simulations of surface mount assembly under thermal cycling. Simulations help to understand local phenomena and estimatefatigue parameters under other thermal conditions. Then, a correlation between experimental results about failure and calculated fatigue leads to an estimation of the life time of the assemblies. Thus, simulations have the advantage to reduce the number of time-consuming and expensive thermo-mechanical agingtests. To perform a simulation, the physical parameters of each solder material are needed like elastic,plastic and creep data. Additional benefits for Schlumberger involve mechanical properties which are, at the moment, unknown for these new high temperature lead free materials
47
Mrozowski, Nicolas. "Mécanismes de propagation de fissure dans un superalliage base nickel polycristallin en régime d’interaction fatigue-fluage-oxydation-vieillissement métallurgique Aging of γ′ precipitates at 750°C in the nickel-based superalloy AD730TM: a thermally or thermo-mechanically controlled process?" Thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2020. http://www.theses.fr/2020ESMA0010.
Анотація:
Dans un objectif d’optimisation des relations « microstructure-propriétés » des superalliages base nickel γ/γ' utilisés notamment pour la fabrication de disques de turbine, le comportement en propagation de fissure à 750°C a été étudié dans la version polycristalline de l’AD730TM. En particulier, le phénomène de ralentissement de la propagation avec l’augmentation du facteur d’intensité des contraintes ΔK est au coeur de la problématique abordée. Par le biais d’une campagne d’essais de fissuration à 750°C, on met en évidence que ce phénomène apparaît dans des conditions de chargement de fatigue-fluage. L’influence de l’histoire de chargement thermomécanique sur les courbes de vitesse de propagation est analysée en modifiant différents paramètres expérimentaux tels que la microstructure initiale (précipitation γ'), la valeur initiale de ΔK, l’environnement ou les temps de maintien. L’origine du phénomène de ralentissement est alors attribuée aux effets conjoints et « protecteurs » en pointe de fissure de la relaxation des contraintes, de la croissance d’oxydes et du vieillissement métallurgique. Afin d’enrichir les résultats de fissuration, des analyses complémentaires du vieillissement de la précipitation γ' ont été conduites à 750°C. Il en résulte notamment que les cinétiques de vieillissement (grossissement des précipités γ', précipitation de carbures intergranulaires) sont accélérées dans des conditions de chargement cyclique. En outre, le taux de relaxation des contraintes est d’autant plus élevé que la microstructure a vieilliThe 750°C crack propagation behavior in the polycristalline nickel-base superalloy AD730TM is studied. This study falls within the overall objectives of optimizing the “microstructure – mechanical properties” relations in nickel-base superalloys used for example in turbine disks. The deceleration of the crack propagation with the increase in ΔK value is specifically at the core of the PhD thesis work. Via a crack propagation test campaign, this phenomenon is systematically identified when dwell-fatigue loading are performed. The influence of the thermomechanical history of the material in investigated by varying several experimental parameters such as the initial microstructural state (γ’ precipitates), the initial value of ΔK, the environment or the dwell durations. The origin of the deceleration phenomenon is therefore shown to be linked with combined and protective effects of stress relaxation, oxide growth and metallurgical aging at the crack tip. To improve the understanding of the crack propagation results, complementary analysis of the 750°C γ’ precipitates aging is performed. One of the main outcomes is that aging kinetics (γ’ growth, carbide precipitation) are enhanced thanks to a cyclic loading. Besides, the more the microstructure is aged, the higher the stress relaxation rate at 750°C
48
Burch, Judith Gulliver. "Dementia garden design: a framework to facilitate Kaplans’ attention restoration theory (A.R.T.) in environments of care." Thesis, Kansas State University, 2012. http://hdl.handle.net/2097/13665.
Анотація:
Master of Landscape ArchitectureDepartment of Landscape Architecture/Regional and Community Planning
Timothy D. Keane
This thesis documents an exploratory design process that examines the efficacy of a framework for designing dementia gardens based on: theory, Stephen and Rachel Kaplan’s Attention Restoration Theory (A.R.T.), (Kaplan and Kaplan, 1989) and Roger Ulrich’s Theory of Supportive Gardens (Ulrich, 1999); John Zeisel’s (2007) process for designing dementia gardens; and design details, Claire Cooper Marcus’ Garden Audit Tool (2007) and Moore’s analysis of exemplary dementia gardens (2007). It documents the integration of theory that is not specific to dementia gardens (Kaplans’ A.R.T. and Ulrich’s Theory of Supportive Gardens) with process (Zeisel) and programming elements that are specific to dementia gardens (Cooper Marcus’ Garden Audit Tool Kit and Moore’s exemplary dementia gardens). The framework was developed during an illustrative courtyard design project for a retirement center whose clientele included patients with varying need levels. Throughout the illustrative design project, knowledge of the four A.R.T. characteristics (Being Away, Fascination; Compatibility and Extent) guided design decision-making in an effort to create an engaging environment, where improved health outcomes and restorative person-environment interactions could occur.
49
Douzi, Chawki. "Effet du vieillissement par fatigue électrothermique sur la compatibilité électromagnétique des composants de puissance à base de SiC." Thesis, Normandie, 2019. http://www.theses.fr/2019NORMR002/document.
Анотація:
Ce travail de recherche porte sur l’étude de l’effet du vieillissement par fatigue électrothermique sur la compatibilité électromagnétique des composants de puissance à base de carbure de silicium. Il est axé sur deux grandes parties ; une partie expérimentale et une autre plus orientée modélisation. Sur le plan expérimental, cette thèse étudie l’effet du vieillissement des transistors à base de carbure de silicium utilisés dans les convertisseurs statiques sur les perturbations électromagnétiques générées par ces convertisseurs. La deuxième partie porte sur la modélisation de ces transistors afin d’émuler l’effet de leur vieillissement sur les perturbations électromagnétiques des modules qu’ils composent. Cette dernière étape repose sur une étude de l’évolution des caractéristiques électriques statiques et dynamiques effectuées sur le composant sous test pour extraire les principaux paramètres intrinsèques du transistor de puissance dégradé après les séries de stress appliquées. En effet, ces paramètres intrinsèques dégradés émulent l’effet du vieillissement et sont représentatifs des principaux phénomènes pouvant influencer les convertisseurs de puissance étudiés. De ce fait, le changement de leurs valeurs dans le modèle du dit composant, décrit en VHDL-AMS et implémenté sur le simulateur de type circuit ANSYS SIMLORER, nous permet d’obtenir un modèle d’un tel composant après vieillissement. Cette étape a permis de valider la méthodologie développée pour la simulation des perturbations électromagnétiques conduites d’un composant sain dans un premier temps et d’un composant vieilli dans un second. Globalement, cette approche de modélisation innovante développée dans ce travail permet d’aider les concepteurs des convertisseurs statiques à prédire les perturbations électromagnétiques conduites avant et après vieillissement sans passer par la mesure et ses points faibles. Ceci apporte des informations complémentaires sur l’évolution des signatures CEM de tels modules durant sa durée de vie et d’estimer donc le risque lié au vieillissement des composantsThis research work focuses on the electrothermal aging effect on the electromagnetic compatibility of power components based on silicon carbide SiC. It focuses on two major parts ; an experimental part and another more oriented modelization. Experimentally, this thesis studies the aging effect of SiC transistors used in static converters on the electromagnetic interferences EMI generated by these converters. The second part deals with the modeling of these transistors in order to emulate the effect of their aging on the EMI of the modules they compose. This step made it possible to validate the methodology developed for the simulation of the conducted EMI of a healthy SiC MOFSET at first and of an aged SiC MOSFET in a second time. Overall, this innovative modeling approach developed in this work helps the designers of static converters to predict the conducted EMI before and after aging without going through the measurement. This provides additional information on the evolution of the EMC signatures of such modules during its lifetime and thus to estimate the risk associated with the aging of the components
50
Gualandi, Mattia. "Fatigue response of CFRP subjected to hygroscopic ageing." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017.
Анотація:
The purpose of this publication is to analyse the hygroscopic ageing effects of a CFRP material subjected to fatigue. The tested material is provided by the company RiBa Composites. Because of the lack of data about fatigue in composites, there is not a specific test approved by any regulation for this analysis. Therefore, the tests are inspired by the ASTM D2344/D2344M, which is the test characterizing the short-beam strength and the moisture absorption of CFRP aged in deionized water. The difference, other than the type of applied load (the original test is performed by using quasi-static loading condition, meanwhile the fatigue test is performed by using CA loading), is in the number of specimens used. In fact, because of the long time spent by specimens to fail under low amplitude loadings, the use of five specimens per loading level for each ageing period is restricted to three specimens. Moreover, because of the definition of fatigue, for some ageing case more specimens are used and, sometimes, even different loading levels are considered.
The final outcome is the analysis of the fatigue life. During the data analysis has been observed that even for CFRP materials can be made some fatigue life prediction by the use of the well know Wohloer diagram. Moreover, coefficients for the Basquin’s law have been obtained for all the ageing periods.