No Abstract

Compte tenu de leurs propriétés mécaniques spécifiques extrêmement intéressantes, les alliages de titane sont largement utilisés notamment pour des applications aéronautiques. Une des limitations concerne certains composants critiques des turbomachines pour lesquels un effet « dwell » est parfois constaté. Ce processus survient dans des conditions particulières combinant un chargement cyclique (fatigue) et statique (fluage) à une température proche de l’ambiante et peut conduire à une rupture prématurée. Bien que de nombreuses études aient permis de cerner le problème, son origine physique/mécanique demeure mal comprise.Les travaux réalisés, essentiellement expérimentaux, visent à analyser les mécanismes de déformation et d’endommagement, sous chargement dwell, à différentes échelles et en lien avec la microstructure métallurgique de l’alliage Ti6242 lamellaire considéré. Des configurations particulières de colonies de lamelles que l’on a nommé « plumes » ont été identifiées comme étant à l’origine de la rupture en dwell. Les caractéristiques morphologiques et cristallographiques correspondantes ont été analysées par différents moyens de microscopie optique et électronique. Un outil d’analyse d’images basé sur la transformée de Fourier rapide a été spécifiquement développé afin de repérer et de quantifier ces éléments à l’échelle de la pièce. Au-delà des campagnes d’essais mécaniques mettant en évidence les conséquences de l’effet dwell en termes de durée de vie, des essais in situ sous microscope réalisés en traction, fluage et dwell sur des « micro-éprouvettes » instrumentées ont permis de mieux comprendre les processus élémentaires de déformation associés aux plumes et conduisant à des hétérogénéités des champs de déformation favorables à l’amorçage de fissures. / Thanks to their extremely attractive specific mechanical properties, titanium alloys are widely used, especially for aerospace applications. However, a “dwell-effect” may sometimes be observed, reducing the lifetime of some critical jet engine components. This phenomenon occurs under specific conditions combining cyclic (fatigue) and static (creep) loading at near ambient temperature and can lead to an early failure. Though many studies have investigated this problem, its physical and mechanical origin remains poorly understood.The aim of the present work was to analyse the deformation and damage mechanisms under dwell loading at different scales, and in relation with the microstructure of a fully lamellar Ti6242 alloy. Particular arrangements of colonies, so-called “feather-like structures”, have been identified as the origin of the failure under dwell conditions. Their morphological and crystallographic characteristics have been analysed by various optical and scanning electron microscopy means. A dedicated image analysis tool based on the fast Fourier transform has been developed to localize and quantify the feather-like structures at the scale of a jet engine component. A mechanical testing campaign demonstrated the dwell-effects consequence on the lifetime until failure. Furthermore, tensile, creep and dwell tests have been carried out in situ under the microscope on “microsamples” bearing gauges. They enabled a better understanding of the elementary deformation processes associated to the feather-like structures and how these structures lead to strain field heterogeneities favouring crack initiation.

Three experiments compared wrist and arm performance in a cyclical Fitts’ target task. The purpose of Experiment I was to determine if movement kinematics differed for wrist/elbow flexion/extension movements to targets of varying difficulty. Participants were asked to flex/extend a manipulandum in the horizontal plane at the wrist and elbow joint in an attempt to move back and forth between two targets. Online knowledge of effector position was displayed as a visual trace on a projector screen. Target widths were manipulated with amplitude constant (16 degrees) in order to create Indexes of Difficulty of 1.5, 3, 4.5, and 6. Results failed to detect differences in elbow and wrist movements either in terms of movement time, movement accuracy, or kinematic characteristics of the movement. In studies that have reported difference in wrist and arm performance in Fitts’ target tasks, experimenters have typically utilized visual amplification to counterbalance the small resulting wrist movements. The purpose of Experiments II and III was to investigate how changes in task parameters and visual gain play a role in providing a performance advantage for the wrist. In these experiments arm movement amplitude was increased to 32 degrees and wrist amplitude was decreased to 8 degrees. Results found similar overall movement times for arm and wrist movements. However, kinematic analysis of the movement revealed relatively large dwell times for wrist movements at IDs of 4.5 and 6. Removal of dwell time resulted in faster movement times for the wrist compared to arm. The results of these three experiments add to the limited literature examining how different effectors perform a Fitts’ target task. These findings suggest that performance differences in past literature may be due to the visual amplification often used when arm, wrist, and finger movements are studied.

This study proposes a theory of texture for Elliott Carter's song cycle A Mirror on Which to Dwell. Texture is an important structural aspect of much recent music, as is exemplified in Carter's music. The first chapter is introductory and discusses other textural theories, and the concept of auditory streams. It also provides background to Elizabeth Bishop and to the poems that Carter selected for the cycle, and introduces the song cycle as a whole. The second chapter outlines a textural theory based on streaming. Texture is defined as comprising those contextually defined aspects of sound which lead one to perceive music as consisting of distinct streams. Individual streams are defined by properties, behaviours processes. The ways in which streams are combined, and the changes that occur in the streams during the combinations constitute textural behaviours and processes. The third chapter presents the ways in which textural behaviours and processes can function. They can have symbolic functions by representing personae and actions in the text. They can also have musical functions and thus can play a role in the delineation of form. In the fourth chapter each song in the Mirror cycle is analyzed. Songs that are texturally clear are analyzed first, followed by those that are more texturally complex. Because of the close relationship between text and music in these songs, each analysis starts with a discussion of the text before turning to the music. The main textural features of each song are presented, and then the analytical discussion focuses on form and text-setting. In the conclusion an overview of the songs is presented. The songs, although formally diverse, are similar in many respects. In all of them Carter establishes the important streams, some of which have significant symbolic roles, in the opening measures. The songs are characterized by similar textural processes, which help to delineate the form of the songs and to portray musically the meaning of the text. An analysis of these songs using the textural theory presented in this study provides insight into their form and meaning.

This study proposes a theory of texture for Elliott Carter's song cycle A Mirror on Which to Dwell. Texture is an important structural aspect of much recent music, as is exemplified in Carter's music. The first chapter is introductory and discusses other textural theories, and the concept of auditory streams. It also provides background to Elizabeth Bishop and to the poems that Carter selected for the cycle, and introduces the song cycle as a whole. The second chapter outlines a textural theory based on streaming. Texture is defined as comprising those contextually defined aspects of sound which lead one to perceive music as consisting of distinct streams. Individual streams are defined by properties, behaviours processes. The ways in which streams are combined, and the changes that occur in the streams during the combinations constitute textural behaviours and processes. The third chapter presents the ways in which textural behaviours and processes can function. They can have symbolic functions by representing personae and actions in the text. They can also have musical functions and thus can play a role in the delineation of form. In the fourth chapter each song in the Mirror cycle is analyzed. Songs that are texturally clear are analyzed first, followed by those that are more texturally complex. Because of the close relationship between text and music in these songs, each analysis starts with a discussion of the text before turning to the music. The main textural features of each song are presented, and then the analytical discussion focuses on form and text-setting. In the conclusion an overview of the songs is presented. The songs, although formally diverse, are similar in many respects. In all of them Carter establishes the important streams, some of which have significant symbolic roles, in the opening measures. The songs are characterized by similar textural processes, which help to delineate the form of the songs and to portray musically the meaning of the text. An analysis of these songs using the textural theory presented in this study provides insight into their form and meaning. / Arts, Faculty of / Music, School of / Graduate

Boarding, alighting and seating maneuvers were monitored on selected Arlington Transit (ART) bus routes in order to determine the link between passenger demographics, seating preferences and boarding and alighting times within the system. The data collection methodology employed digital stopwatch timings of boardings and alightings in conjunction with a coordinate-based spreadsheet seating chart tracking system in order to document passenger movements and seating occupancies. Passengers were visually profiled according to their ethnicity, sex, general age group and bulkiness. Multiple linear regression analysis was used to develop boarding and alighting models, and t-tests were used to isolate statistically significant differences between profiled groups in terms of their seating preferences and inter-group separation distances aboard the buses. It was observed that female passengers tend to sit closer to other passengers than males do, and that older female passengers have a preference for sitting in aisle seats and towards the lower level front of the bus - each of these preferences is linked with shorter boarding times. Males, in general, tend to prefer window seating to aisle seating, and this preference is linked with longer boarding and alighting times. It was also observed that younger passengers prefer less efficient upper level seating to lower level seating and that white passengers, on average, tend to sit closer to other white passengers on routes where whites are a minority in terms of passenger composition. Monetary fare payment was observed to contribute to longer boarding times than the use of a swipe card (e.g., SmarTrip® card). / Master of Science

No description available.

Bus Signal Priority (BSP), which has been deployed in many cities around the world, is a traffic signal enhancement strategy that facilitates efficient movement of buses through signalized intersections. Most BSP systems do not work well in transit networks with nearside bus stop because of the uncertainty in dwell time. Unfortunately, most bus stops on arterial roadways are of this type in the U.S. This dissertation showed that dwell time at nearside bus stops could be modeled using weighted least squares regression. More importantly, the prediction intervals associated with the estimate dwell time were calculated. These prediction intervals were subsequently used in the improved BSP algorithm that attempted to reduce the negative effects of nearside bus stops on BSP operations. The improved BSP algorithm was tested on urban arterial section of Bellaire Boulevard in Houston, Texas. VISSIM, a micro simulation model was used to evaluate the performance of the BSP operations. Prior to evaluating the algorithm, the parameters of the micro simulation model were calibrated using an automated Genetic Algorithm based methodology in order to make the model accurately represent the traffic conditions observed in the field. It was shown that the improved BSP algorithm significantly improved the bus operations in terms of bus delay. In addition, it was found that the delay to other vehicles on the network was not statistically different from other BSP algorithms currently being deployed. It is hypothesized that the new approach would be particularly useful in North America where there are many transit systems that utilize nearside bus stops in their networks.

Nous avons caractérisé de manière détaillée les  fortes hétérogénéités de texture et de microstructure (macrozones) par EBSD dans des zones déformées différemment d’un disque en IMI834 de structure bimodale αP/αS, utilisé en motorisation aéronautique. Des échantillons spécifiques ont permis de montrer que l’amplitude des signaux ultrasonores rétrodiffusés dans ces échantillons dépendait de la présence et de l’organisation des macrozones. Partant de ces observations, une analyse ultrasonore des signaux rétrodiffusés dans les parties du disque soumis à de fortes déformations lors du forgeage a permis de sélectionner un jeu d’éprouvettes ayant des caractéristiques microstructurales différentes et destinées à des essais en fatigue-dwell. Une forte dispersion des durées de vie liée aux microstructures présentes a été observée. L’analyse fractographique des surfaces de rupture et leurs études par EBSD ont permis de localiser les sites d’amorçage et les chemins de propagation des fissures ainsi que leurs caractéristiques cristallographiques. Les résultats des essais en fatigue-dwell montrent que la réduction de la tenue en service est fortement influencée par la présence de macrozones formées d’agglomérats de grains αP allongés associés à des lamelles αS plus épaisses et organisées en colonies et d’autre part présentant majoritairement des axes c à moins de 30° de l’axe de sollicitation / We have characterized in detail the strong heterogeneity of texture and microstructure (macrozones) by EBSD in areas, differently deformed, of a disk in IMI834 with a bimodal structure αP/αS, used in aircraft engines. Specific samples have shown that the amplitude of back-scattered ultrasonic signals depended on the presence and organization of macrozones. Based on these observations, an analysis of back-scattered ultrasonic signals in parts of the disc subject to strong deformation during forging led to select a set of specimens with different microstructural features for fatigue-dwell tests. A high dispersion of lifetimes associated with these microstructures was observed. The fractographic analysis of fracture surfaces and their EBSD studies were used to locate crack initiation sites and propagation paths and their crystallographic characteristics. The results of fatigue-dwell tests showed that the lifetime reduction is strongly influenced by the presence of macrozones formed by agglomerates of grains elongated αP associated with thicker αS lamellae, organized in colonies and with c-axis predominantly at less than 30 ° of the solicitation axis

Micro Combined Heat and Power systems or microCHP systems generate heat and electricity for a home. Stirling engines are widely used as prime movers in microCHP applications. Stirling engine is an external combustion engine having an enclosed working fluid (as helium) that is alternately compressed and expanded to operate a piston. The displacer shuttles the working fluid between the hot and cold ends. The piston is coupled to a transmission and to an electrical machine to generate power. Conventional Stirling engines are not controllable to a great degree. The piston and displacer are connected to the same crank and they maintain the same phase difference throughout the cycle. Also the piston and displacer are normally constructed to move in a sinusoidal fashion. The Active Stirling Engine is a new concept introduced in this thesis which has a free displacer. The displacer is driven separately compared to a coupled drive in conventional Stirling engines. The displacer motion can be non-linear with dwell at each ends of the stroke, opening up the possibilities to increase the pressure volume diagram which indicates the work done by the engine. A separately driven displacer also allows introducing phase control and stroke control to improve the controllability of a Stirling engine. This thesis examines the effect of non-linear displacer motion and phase control of the displacer on Stirling engine performance. Simulations are performed in Sage, the leading Stirling engine simulation software, to understand the effect of displacer phase control. A test rig is constructed with the actively controlled displacer connected to a linear machine controlled by a programmable servo. Heat is applied to the test rig though an electric heating coil. The test rig is charged with nitrogen at 20Bar pressure. The power piston is connected to a rotating electrical machine via the transmission. The rotating electrical machine is used to start the engine and to act as the generator. The test rig is instrumented to determine the linear position of the displacer and piston, angular position of the rotating electrical machine shaft, temperatures, pressures and flow. A LabVIEW™ based data acquisition system is set up to capture data from the test rig. Data is collected at various test cases. The simulation result is compared against post processed data. An efficiency improvement of 15% is achieved using this method and is demonstrated experimentally. Applications in micro combined heat and power systems utilising the improved efficiency due to non linear motion and controllability due to phase control are explored in this thesis.