Pour améliorer la caractérisation et le dimensionnement des disques de turbines pour les moteurs d’avion, le motoriste Safran Aircraft Engines (SAE) développe des modèles de comportement, des lois d’endommagement et des critères de fatigue plus adaptés aux chargements réels. Pour aider à cette démarche, le but de cette étude est de développer un modèle de plasticité adapté à l'Inco718DA (un alliage à base nickel utilisé dans la fabrication des turbines haute pression), capable de représenter différents chargements (monotone, cyclique symétrique et non-symétrique). La proposition puis l'identification du modèle a été possible grâce à une campagne expérimentale favorisant des tests complexes et innovants aux essais de fatigue nombreux et coûteux. Les essais faits incluent un essai monotone avec décharges élastiques, un essai multi-niveau à Rε=-1 et deux essais multi-niveau à Rε=0 pour mieux caractériser la relaxation de la contrainte moyenne et un essai pour quantifer le rochet. Le comportement cyclique du matériau a été identifié en utilisant un écrouissage cinématique non saturant avec des éléments s'inspirant de la surface mémoire de Chaboche. Un des défis a été d'obtenir des boucles stabilisées "pointues" dans un régime de plasticité cyclique saturante, en utilisant une évolution du paramètre Γ en fonction de la déformation plastique équivalente maximale (prefacteur du terme de rappel de la loi d'écrouissage cinématique).Une deuxième difficulté apparaît dans la description de la relaxation de la contrainte moyenne, phénomène complexe avec un impact considérable sur la durée de vie en fatigue. Dans le chapitre 3, un modèle est proposé pour la caractérisation de la relaxation partielle de la contrainte moyenne. Une originalité du modèle est l'idée que la relaxation incomplète est une conséquence directe de la différence entre la charge et la décharge de la boucle de hystérésis. Le paramètre choisi pour décrire cette différence a été le préfacteur du terme de rappel Γ, pour lequel la thermodynamique donne de la liberté. Par rapport à d'autres lois d'écrouissage confirmées, notre modèle présente l'avantage d'utiliser un seul terme de rappel, mais avec une formulation plus complexe. En plus, le modèle est incrémental (écrit en taux/en vitesse), il peut donc prendre en compte des chargements complexes tels que aléatoires ou plus simplement tels que dans les essais multi-niveaux pilotés en déformation. Dans le dernier chapitre de la thèse, une campagne biaxiale vaste est présentée, avec les développements pour réaliser des essais biaxiaux pilotés en déformation. La campagne biaxiale a été réalisée sur des éprouvettes cruciformes en utilisant des capteurs LASER et des mesures de champs mono et stéréo analysées en utilisant la Corrélation d'Images Numeriques (CIN). Pour analyser la relaxation de la contrainte moyenne en biaxial un moyen de mesure et de contrôle fiable a dû être développé, adapté aux déformations plastiques élevées qui apparaissent dans la région d'intérêt de l'éprouvette. En utilisant la corrélation d'images intégrée (I-CIN) avec des fonctions de forme adaptées sur un seul élément et des calculs sur GPU, on a obtenu des fréquences de mesure de 100~Hz. En plus, avec sa précision et vitesse, I-CIN a été une technique adaptée pour contrôler une machine d'essais multiaxiale hydraulique. Un résultat important obtenu quand on a réalisé des essais equi-biaxiaux pilotés en déformation a été l'observation d'une relaxation de la contrainte moyenne très faible par rapport au cas uniaxial. Ce résultat doit être pris en compte dans les études futures avec des calculs éléments finis sur l'éprouvette complète. / To improve the characterization and design of aircraft engine turbine disks, the propulsion systems manufacturer Safran Aircraft Engines (SAE) develops constitutive equations, damage laws and fatigue criteria that are more adapted to real loadings. As part of this effort, the purpose of the current study is to develop a plasticity model for Inco718DA (a nickel-based alloy used in the manufacturing of high-pressure turbine disks), capable of representing several loading conditions (monotonic, symmetrical and non-symmetrical cyclic loading). The identification of the model was possible thanks to a uniaxial campaign, favoring a few but complex, innovative, tests to numerous costly fatigue tests. The tests we performed include a monotonic test with elastic discharges, a multi-level Rε = -1 test and two multi-level Rε = 0 tests that better quantify the mean stress relaxation and a test to identify ratcheting. The cyclic behavior was identified using a non-saturating kinematic hardening law with elements of Chaboche's memory surface. One of the challenges was to obtain sharp stabilized loops in a saturated cyclic plasticity regime, which was possible using parameter Γ evolving with respect to the maximum equivalent plastic strain, in the back-stress of kinematic hardening rule.A second difficulty appears in the description of mean stress relaxation, which has a considerable impact on fatigue lifetime. In chapter 3, a model is proposed for the description of the incomplete mean stress relaxation. One of the originalities is the idea that incomplete mean stress relaxation is a direct consequence of the difference between the loading and the unloading part of the hysteresis loop. The parameter we used to describe this difference, was the prefactor of the back-stress term Γ, for which the thermodynamics allows liberty. When compared to confirmed kinematic hardening laws that model non-zero mean stress relaxation, our model presents the advantage of using only one backstress, even if its description is more complex. Moreover, the model is incremental (written in a rate form in chapter 3 section 5) so it can take into account complex loadings such as multi-level strain-controlled tests.In the last chapter of the thesis, a vast biaxial campaign is presented, along with developments to make biaxial strain-controlled tests. The biaxial campaign was performed on cross-shaped samples using LASER sensors, mono and stereo full-field measurements using Digital Image Correlation (DIC). In order to analyze biaxial mean stress relaxation, a reliable measurement and control method had to be developed for the high plastic strains occurring in the region of interest of the sample. By using an Integrated-DIC (I-DIC) algorithm with adequate shape functions on one element and GPU computations we were able to obtain measurement frequencies of 100 Hz. Moreover, with its precision and speed, I-DIC proved to be a suitable technique for controlling a biaxial hydraulic machine. An important result obtained when performing equi-biaxial I-DIC strain-controlled tests was that there was very little biaxial mean stress relaxation, with respect to the uniaxial case. This result will have to be taken into account in future studies when performing finite element computations of the whole sample.

Test anxiety can be debilitating to test performance, thus the abilities of test anxious individuals may not be measured accurately. This study examined the efficacy of relaxation training, guided imagery, positive self-talk, and study skills training on anxiety reduction and academic self - esteem in test-anxious , learning disabled high school and college students. A randomized treatment control group design with pretest and post-test measures was used to assess the efficacy of treatment. Participants in the treatment group showed a greater reduction in test anxiety, and improvements in academic self-esteem from pretest to post-test relative to participants in the control group. Implications of the results are discussed.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第17828号 / 工博第3771号 / 新制||工||1576(附属図書館) / 30643 / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 渡辺 宏, 教授 金谷 利治, 教授 山本 量一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No description available.

Currently, forensic scientists are only able to determine time since death (or postmortem interval) up to the first 60 hours. This is based largely on insect activity. Herein, it is proposed to use the degradation of the intervertebral disc (IVD) after death to determine a relationship between the mechanical properties of cadaveric tissue and time since death in order to extend the 60-hour window. To that end, 1 fresh human spine and 6 pig spines were each separated into sections (6 human and 48 pig), with each section having one intact disc. The sections were buried, unearthed, and cleaned, leaving only the disc and bone. To determine the mechanical properties, each disc underwent three different tests: cyclic conditioning, compression, and stress relaxation testing. The Schapery collocation method was used to create a theoretical curve from the data for the experimental curve. Observations were made involving the corresponding k values of the curve. Although there are trends in the data for k values that approximate the experimental stress relaxation curve, a correlation could not be determined.

No description available.

The Thin Slab Cast Direct Rolling (TSCDR) process offers several economic and environmental advantages. The elimination of slab reheating and roughing deformation, however, leave fewer opportunities for grain refinement and some large grains persist in the microstructure. To solve this problem, a new chemistry which leads to a two-phase mixture of ferrite and austenite over a wide temperature range was introduced by Zhou et al. The two phase mixture is highly resistant to grain coarsening leading to a small initial grain size compared with the grain size of conventional TSCDR slab. In addition, ferrite and austenite co-exist over wide range of temperature in many third generation steels, making it extremely important to understand the hot deformation behavior of these materials, which have traditionally received less attention in the literature. In order to investigate the microstructure evolution of ferrite-austenite mixtures during thermomechanical processing, an Al containing model alloy, for which the two phases co-exist over a wide temperature range, was designed. Two types of experiments were carried out: the first involved single hit hot compression tests; and the second involved stress relaxation tests. According to the microstructure observation the main change of austenite microstructure under deformation conditions was a decrease in the spacing of the austenite particles within the ferrite matrix. In other words the austenite phase behaved as hard particles inside a soft ferrite matrix. Hot deformation led to the static recrystallization of the ferrite matrix. The most favourable nucleation sites were in the vicinity of the old grain boundaries and the around austenite particles. The recovery and recrystallization kinetics of ferrite were analyzed using the stress relaxation test. Based on analysis of the stress relaxation tests, more than 95% of stored energy was consumed by recovery, while static recrystallization consumed less than 5% of the stored energy. The retardation of recrystallization in the model alloy is attributed to both the high rate of recovery in BCC materials and texture effects. / Thesis / Master of Applied Science (MASc) / The Thin Slab Cast Direct Rolling (TSCDR) process offers several economic and environmental advantages. The elimination of slab reheating and roughing deformation, however, leave fewer opportunities for grain refinement and some large grains persist in the microstructure. To solve this problem, a new chemistry which leads to a two-phase mixture of ferrite and austenite over a wide temperature range was introduced by Zhou et al. The two phase mixture is highly resistant to grain coarsening leading to a small initial grain size compared with the grain size of conventional TSCDR slab. In addition, ferrite and austenite co-exist over wide range of temperature in many third generation steels, making it extremely important to understand the hot deformation behavior of these materials, which have traditionally received less attention in the literature. In order to investigate the microstructure evolution of ferrite-austenite mixtures during thermomechanical processing, an Al containing model alloy, for which the two phases co-exist over a wide temperature range, was designed. Two types of experiments were carried out: the first involved single hit hot compression tests; and the second involved stress relaxation tests. According to the microstructure observation the main change of austenite microstructure under deformation conditions was a decrease in the spacing of the austenite particles within the ferrite matrix. In other words the austenite phase behaved as hard particles inside a soft ferrite matrix. Hot deformation led to the static recrystallization of the ferrite matrix. The most favourable nucleation sites were in the vicinity of the old grain boundaries and the around austenite particles. The recovery and recrystallization kinetics of ferrite were analyzed using the stress relaxation test. Based on analysis of the stress relaxation tests, more than 95% of stored energy was consumed by recovery, while static recrystallization consumed less than 5% of the stored energy. The retardation of recrystallization in the model alloy is attributed to both the high rate of recovery in BCC materials and texture effects.

Ligaments and tendons are rope-like structures in our body that possess time- and history-dependent material properties. Despite the many advances made in experimental and theoretical biomechanics, the material properties of these biological structures are still not fully characterized. This dissertation represents a step forward in the development of combined theoretical and experimental tools that capture the time- and history-dependent material properties of ligaments and tendons. The mechanical behavior of bundles of collagen fibers which form ligaments and tendons was investigated. Axial stress-stretch data and stress relaxation data at different axial stretches were collected by testing rat tail tendon fascicles. The experimental results demonstrated, for the first time, that the shape of the normalized axial stress relaxation curve depends on the axial stretch level thus suggesting that the fascicles are nonlinear viscoelastic. A constitutive model was then formulated within the nonlinear integral representation frame- work proposed by Pipkin and Rogers (1968). Unlike the well-known quasi-linear viscoelastic model, the proposed constitutive law was able to capture the observed nonlinearities in the stress relaxation response of rat tail tendon fascicles. By extending the constitutive model for collagen fiber bundles, a new nonlinear three- dimensional model for the stress relaxation of skeletal ligaments was formulated. The model accounts for the contribution of the collagen fibers and the group substance in which they are embedded. Published uniaxial experimental data on the stress relaxation of human medial collateral ligaments were used to determine the model parameters. The model predictions for simple shear in the fiber direction, simple shear transverse to the fiber direction, and equibiaxial extension were then examined and, for the case of simple shear in the fiber direction, such predictions were found to be in good agreement with published experimental data. The relationship between the mechanical response and structure of suspensory ligaments was examined by performing state-of-the-art small angle x-ray diffraction experiments in tandem with incremental stress relaxation tests. Specifically, small angle x-ray diffraction was used to measure changes in strain and orientation of collagen fibrils during the stress relaxation tests. Throughout the tests the collagen fibrils were found to gradually orient towards the loading direction. However, the collagen fibril strain did not change significantly suggesting that collagen fibers do not play a significant role in dissipating load during stress relaxation. / Ph. D.

Proton exchange membrane fuel cells typically consist of stacks of membrane electrode assemblies sandwiched between bipolar plates, effectively combining the individual cells in series to achieve the desired voltage levels. Elastomeric gaskets are commonly used between each cell to insure that the reactant gases are isolated; any failure of a fuel cell gasket can cause the reactants to mix, which may lead to failure of the fuel cell. An investigation of the durability of these fuel cell seals was performed by using accelerated characterization methods. A hydrocarbon sealant was tested in five different environments to simulate fuel cell conditions. Viscoelastic properties of these seals were analyzed using momentary and relaxation compressive stress tests. Material properties such as secant modulus at 100% strain, tensile strength, and strain at failure were determined using dog-bone samples aged at several different imposed strains and aging times in environments of interest. Tearing energy was evaluated using trouser test samples tested under different rates and temperatures after various environmental aging conditions. Additionally, tearing tests were conducted on samples tested in liquid environment. A viscoelastic and mechanical property characterization of these elastomeric seals under accelerated aging conditions could help understand the behavior and predict durability in the presence of mechanical and environmental loading. Additionally, the effects of confinement have been evaluated for a bonded joint with varying thickness along the bonded direction. The Dreaming project is a glass art project in Fredrick, MD which incorporates such a varying thickness joint where thermal expansion of the adhesive has caused the glass adherend to break and debonding of the sealant. To examine this joint design, finite element analysis has been used to determine the effects of thermal expansion on such a complex geometry. Nine different test geometries have been evaluated to determine the effect of confinement coupled with thermal expansion on joint design with an elastomeric adhesive. Once evaluated, design changes were performed to try to reduce the loading while maintaining the general joint design. Results of this analysis can be used to determine the effects of confinement on a complex elastomeric joint. / Master of Science

No description available.