No description available.

Approved for Public Release; Distribution is Unlimited / Friction Stir Processing (FSP), although relatively simple in concept, results in an extremely complex thermomechanical treatment to the material being processed. Previous studies of FSP have shown that the process results in extremely high strain, strain rates and temperatures as well as gradients in strain, strain rate and temperature within a small volume of material. This thesis will study the effect of varying FSP parameters during the processing of Nickel-Aluminum-Bronze (NAB) propeller material. The modeling program CTH was used to define the relationship between tool rotation speed, traversing speed and the total power input to the material. The tool's mechanical power and the power generated by deformation of the material has been investigated. The modeling experiments were designed to gain an understanding of the relationship of process parameters, microstructure and mechanical properties, and to enhance our understanding of the flow patterns and thermal histories of the NAB material in the stir zone. / Lieutenant, United States Navy

No description available.

Depuis maintenant une dizaine d'années, la vision du cycle sismique en zone de subduction a beaucoup évolué. Des découvertes récentes ont mis en évidence une grande diversité des régimes de glissement dans ces zones, avec notamment des glissements asismiques transitoires appelés « séismes lents » (SSE) et des vibrations de faibles amplitudes, persistantes dans le temps, appelées « trémors non volcaniques » (NVT). Ce travail a pour objectif l'étude des trémors non volcaniques afin de caractériser ces nouvelles manifestations des zones de faille. Nous avons abordé ce problème avec deux approches distinctes :1. Observer les trémors dans le milieu naturel afin de déterminer leurs caractéristiques. La zone étudiée correspond à la lacune sismique de Guerrero le long de la subduction mexicaine. Nous avons développé une méthode de détection et de localisation des NVT au Mexique grâce à des analyses d'antennes par formation de voie sur les corrélations. Cette méthode permet de mettre en évidence cer taines caractéristiques des NVT : une complexité des sources pour un épisode de trémors, une corrélation entre les activités de NVT et les pics de vitesse des glissements lents à plus long terme. Par ailleurs, l'étude de l'impact du séisme de Maule (2010, Chili, Mw 8.8) au Mexique montre qu'il a déclenché le second sous évènement du séisme lent de 2009-2010. Ce déclenchement d'un SSE s'ac- compagne de fortes activités de trémors, modulées par les ondes du séisme de Maule dans un premier temps, puis simplement associées au SSE.2. Modéliser les trémors expérimentalement et numériquement pour mieux com- prendre leur origine physique et leurs évolutions sur le long terme. Nous avons en particulier utilisé une expérience de frottement à faible vitesse qui indique une corrélation systématique entre les accélérations d'un glissement et l'émission de signaux qui ressemblent à des NVT. Une modélisation numérique de la zone de subduction mexicaine est également présentée et montre la possibilité de reproduire des trémors en considérant une transition d'affaiblissement critique associée à un processus de décrochage. / The vision of the seismic cycle in subduction zones has considerably evolved over the last 10 years. New discoveries has pointed the diversity of slip behaviors in these zones with aseismic slow slip called « slow slip events » (SSE) and persistent low amplitudes vibrations called « non-volcanic tremors » (NVT). The goal of this thesis is to study the non-volcanic tremors in order to characterize these new manifestations of fault zones. We used two different approaches: 1. We first observed the non-volcanic tremors in the nature in order to characterize this phenomenon. The area of interest is the Guerrero seismic gap along the Mexican subduction zone. We develop a new detection and location method based on beamforming of correlations of seismic signals. This new method exhibits some characteristics of NVT: a complex source for a single tremor episode and a correlation between the NVT episodes and the long-term peak of movement velocity in southwards direction. Moreover, the study of the consequences of the Maule earthquake on the Mexican subduction zone showed that this earthquake triggered the 2009-2010 SSE in Guerrero. This triggering of slow slip is accompanied by strong seismic tremor actvity that are first modulated by the passing waves and then associated to the SSE. 2. We model numerically and experimentally the tremors in order to better understand their physical origin and their long-term evolution. We used a very slow friction experiment that indicates a systematic correlation between slip acceleration of a slider and emission of acoustic signals that are similar to NVT. A numerical modeling of the Mexican subduction zone is also presented and shows the possibility to reproduce NVT with a critical depinning transition.

Heat transfer and fluid mechanical behavior of water flowing horizontally in vertically-narrow rectangular channels was studied in this work. Friction factor and Nusselt number variations were determined experimentally for four different wall heating conditions, and for aspect ratios(height to width) of 5, 4, 3, 2, and 1. Wall heating conditions examined were: all surfaces heated; three surfaces heated, the top adiabatic; one side surface heated the others adiabatic; and the bottom surface heated, the others adiabatic. Friction factors, in laminar flow, varied as predicted from analysis. The critical Reynolds number varied linearly with 1n(D[subscript]). Local Nusselt numbers were determined, at each aspect ratio and heating condition, as functions of Reynolds and Rayleigh numbers. Mixed convection was the result of buoyancy-induced secondary flows. Local Nusselt numbers decreased in a manner common to pure forced convection, reaching minimum values some distance from the entrance, then increased due to the presence of the secondary flows. For given aspect ratio, local Nusselt numbers were found to increase and the thermal entrance lengths decreased, with increasing Rayleigh numbers. In the thermal entry region, for all heating conditions except the bottom-heating case, local Nusselt number behavior showed minor dependence on aspect ratio. / Graduation date: 1999

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 209-217).

Lead screw drives are used in various motion delivery systems ranging from manufacturing to high precision medical devices. Lead screws come in many different shapes and sizes; they may be big enough to move a 140 tons theatre stage or small enough to be used in a 10ml liquid dispensing micro-pump. Disproportionate to the popularity of lead screws and their wide range of applications, very little attention has been paid to their dynamical behavior. Only a few works can be found in the literature that touch on the subject of lead screw dynamics and the instabilities caused by friction. The current work aims to fill this gap by presenting a comprehensive study of lead screw dynamics focusing on the friction-induced instability in such systems. In this thesis, a number of mathematical models are developed for lead screw drive systems. Starting from the basic kinematic model of lead screw and nut, dynamic models are developed with varying number of degrees of freedom to reflect different components of a real lead screw drive from the rotary driver (motor) to the translating payload. In these models, velocity-dependent friction between meshing lead screw and nut threads constitute the main source nonlinearity. A practical case study is presented where friction-induced vibration in a lead screw drive is the cause of excessive audible noise. Using a complete dynamical model of this drive, a two-stage system parameter identification and fine-tuning method is developed to estimate parameters of the velocity-dependent coefficient of friction. In this approach the coupling stiffness and damping in the lead screw supports are also estimated. The numerical simulation results using the identified parameters show the applicability of the developed method in reproducing the actual systems behavior when compared with the measurements. The verified mathematical model is then used to study the role of various system parameters on the stability of the system and the amplitude of vibrations. These studies lead to possible design modifications that solve the system’s excessive noise problem. Friction can cause instability in a dynamical system through different mechanisms. In this work, the three mechanisms relevant to the lead screw systems are considered. These mechanisms are: 1. negative damping; 2. kinematic constraint, and; 3. mode coupling. The negative damping instability, which is caused by the negative gradient of friction with respect to sliding velocity, is studied thorough linear eigenvalue analysis of a 1-DOF lead screw drive model. The first order averaging method is applied to this model to gain deeper insight into the role of velocity-dependent coefficient of friction and to analyze the stability of possible periodic solutions. This analysis also is extended to a 2-DOF model. It is also shown that higher order averaging methods can be used to predict the amplitude of vibrations with improved accuracy. Unlike the negative damping instability mechanism, kinematic constraint and mode coupling instability mechanisms can affect a system even when the coefficient of friction is constant. Parametric conditions for these instability mechanisms are found through linear eigenvalue analysis. It is shown that kinematic constraint and mode coupling instability mechanisms can only occur in self-locking lead screws. The experimental case study presented in this work demonstrates the need for active vibration control when eliminating vibration by design fails or when it is not feasible. Using the sliding mode control method, two speed regulators are developed for 1-DOF and 2-DOF lead screw drive system models where torque generated by the motor is the controlled input. In these robust controllers, no knowledge of the actual value of any of the system parameters is required and only the upper and lower bounds of parameters are assumed to be available. Simulation results show the applicability and performance of these controllers. The current work provides a detailed treatment of the dynamics of lead screw drives and the topic of friction-induced vibration in such systems. The reported findings regarding the three instability mechanisms and the friction parameters identification approach can improve the design process of lead screw drives. Furthermore, the developed robust vibration controllers can be used to extend the applicability of lead screws to cases where persistent vibrations caused by negative damping cannot be eliminated by design modifications due to constraints.

No description available.

Lead screw drives are used in various motion delivery systems ranging from manufacturing to high precision medical devices. Lead screws come in many different shapes and sizes; they may be big enough to move a 140 tons theatre stage or small enough to be used in a 10ml liquid dispensing micro-pump. Disproportionate to the popularity of lead screws and their wide range of applications, very little attention has been paid to their dynamical behavior. Only a few works can be found in the literature that touch on the subject of lead screw dynamics and the instabilities caused by friction. The current work aims to fill this gap by presenting a comprehensive study of lead screw dynamics focusing on the friction-induced instability in such systems. In this thesis, a number of mathematical models are developed for lead screw drive systems. Starting from the basic kinematic model of lead screw and nut, dynamic models are developed with varying number of degrees of freedom to reflect different components of a real lead screw drive from the rotary driver (motor) to the translating payload. In these models, velocity-dependent friction between meshing lead screw and nut threads constitute the main source nonlinearity. A practical case study is presented where friction-induced vibration in a lead screw drive is the cause of excessive audible noise. Using a complete dynamical model of this drive, a two-stage system parameter identification and fine-tuning method is developed to estimate parameters of the velocity-dependent coefficient of friction. In this approach the coupling stiffness and damping in the lead screw supports are also estimated. The numerical simulation results using the identified parameters show the applicability of the developed method in reproducing the actual systems behavior when compared with the measurements. The verified mathematical model is then used to study the role of various system parameters on the stability of the system and the amplitude of vibrations. These studies lead to possible design modifications that solve the system’s excessive noise problem. Friction can cause instability in a dynamical system through different mechanisms. In this work, the three mechanisms relevant to the lead screw systems are considered. These mechanisms are: 1. negative damping; 2. kinematic constraint, and; 3. mode coupling. The negative damping instability, which is caused by the negative gradient of friction with respect to sliding velocity, is studied thorough linear eigenvalue analysis of a 1-DOF lead screw drive model. The first order averaging method is applied to this model to gain deeper insight into the role of velocity-dependent coefficient of friction and to analyze the stability of possible periodic solutions. This analysis also is extended to a 2-DOF model. It is also shown that higher order averaging methods can be used to predict the amplitude of vibrations with improved accuracy. Unlike the negative damping instability mechanism, kinematic constraint and mode coupling instability mechanisms can affect a system even when the coefficient of friction is constant. Parametric conditions for these instability mechanisms are found through linear eigenvalue analysis. It is shown that kinematic constraint and mode coupling instability mechanisms can only occur in self-locking lead screws. The experimental case study presented in this work demonstrates the need for active vibration control when eliminating vibration by design fails or when it is not feasible. Using the sliding mode control method, two speed regulators are developed for 1-DOF and 2-DOF lead screw drive system models where torque generated by the motor is the controlled input. In these robust controllers, no knowledge of the actual value of any of the system parameters is required and only the upper and lower bounds of parameters are assumed to be available. Simulation results show the applicability and performance of these controllers. The current work provides a detailed treatment of the dynamics of lead screw drives and the topic of friction-induced vibration in such systems. The reported findings regarding the three instability mechanisms and the friction parameters identification approach can improve the design process of lead screw drives. Furthermore, the developed robust vibration controllers can be used to extend the applicability of lead screws to cases where persistent vibrations caused by negative damping cannot be eliminated by design modifications due to constraints.

Precision surface finishes are used in a wide variety of applications. From bearing races and rolling elements to parallel slide ways, the frictional characteristics of these surfaces are critical to the performance of the products. Experimental trial and error has shown that certain surfaces outperform others in certain applications, but the specific surface characteristics that make this true have yet to be fully understood. The research goal was to develop an apparatus that can test the coefficient of rolling/sliding friction of different precision machined surfaces and to combine this data with topographic analysis of the surfaces to correlate specific 3-D parameters with the frictional performance of a surface. The sample treatments consisted of four different surface textures (hard-turned, ground, honed and isotropic finish) and four different relative surface speeds. By monitoring the torque in the sample-mounting shaft under lubricated conditions the coefficient of rolling/sliding friction of each surface was found. Utilizing white light interferometry measurement of the surfaces, a highly detailed map of each surface was obtained. Using different characteristic values of each machined surface (RMS roughness, asperity density, lay direction, etc.), the frictional behavior of the surfaces were compared to the surface characteristics yielding insight into the relationship between surface finish and friction in rolling/sliding contact. Friction coefficient was found to correlate most strongly with RMS roughness (Sq) and density of surface summits (Sds). These parameters govern mechanical interference of asperities and surface adhesion respectively. These findings suggest that friction coefficients of surfaces could be optimized through manipulation of three-dimensional surface parameters.