In this work passive piezoelectric devices for vibration damping are studied. It is developed the basic idea of synthesizing electrical wave guides to obtain an optimal electro-mechanical energy exchange and therefore to dissipate the mechanical vibrational energy in the electric form. Modular PiezoElectroMechanical (PEM) structures are constituted by continuous elastic beams (or bars) coupled, by means of an array of PZT transducers, to lumped dissipative electric networks. Both refined and homogenized models of those periodic systems are derived by an energetic approach based on the principle of virtual powers. Weak and strong formulation of the dynamical problem are presented having in mind future studies involving the determination of numerical solutions. In this framework the effectiveness of the proposed devices for the suppression of mechanical vibrations is investigated by a wave approach, considering both the extensional and flexural oscillations. The optimal values of the electric parameters for a fixed network topology are derived analytically by a pole placement technique. Their sensitivities on the dimensions of the basic cell of the periodic system and on the design frequency are studied. Moreover the dependence of damping performances on the frequency is analyzed. Comparing the performances of different network topological configurations, the advantages of controlling a mechanical structure with an electric analog are shown. As a consequence of those results, new interconnections of PZT transducers are proposed. An experimental setup for the validation of the analytical and numerical results is proposed and tested. A classical experience on resonant shunted PZT is reproduced. Future experimental work is programmed. / Master of Science

The purpose of this study was to determine if light detection and ranging (LiDAR) imagery could provide a significantly more accurate data set for modeling near line-of-sight (LOS) propagation at higher frequencies, specifically 27.810 GHz. than a USGS digital elevation model (DEM). In addition, the study tested for significant differences in LiDAR elevation data created at various resolutions ranging from 1 to 100 meters. Finally, this study examined the effects of various classification thresholds for transforming continuous signal strength measurements into LOS or non-LOS (NLOS) classifications used in determining prediction accuracy. The capability to transmit information via higher frequency wireless equipment requires a near LOS path between the transmitter and the antenna receiving the signal. USGS DEMs, commonly used in GIS programs to predict communication viewsheds (commsheds), represent the bare earth topography and do not reflect surface features such as vegetation and buildings. In actuality these surface features can significantly influence near LOS paths and therefore a data set that contains these features can greatly improve the ability to predict commshed areas. LiDAR is a form of active imagery that records both the bare-earth as well as these surface features, at a high resolution, making it well suited for wireless modeling applications. Results indicate that signal strength threshold classification has a direct influence on the accuracy of predicted commsheds across all resolutions. Secondly, LiDAR resolutions lower than 40m as well as bare-earth DEMs were unsuccessful in predicting an accurate commshed while LiDAR resolutions coarser than 15m provided significant predictions of equal accuracy. These results indicate that high resolution LiDAR is needed to accurately model commsheds but signal strength threshold classification determines which of these higher resolutions are significant. / Master of Science

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24584号 / 工博第5090号 / 新制||工||1975(附属図書館) / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 篠原 真毅, 教授 大村 善治, 教授 小嶋 浩嗣, 教授 引原 隆士 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

No description available.

No description available.

When a power system is subjected to a disturbance, the power flow changes, leading to deviations in the synchronous generator rotor angles. The rotor angle deviations propagate as electromechanical waves (EMWs) throughout the power system. These waves became observable since the development of synchrophasor measurement instruments. The speed of EMW propagation is hundreds of miles per second, much less than the electromagnetic wave propagation speed, which is the speed of light. Recently, with the development of renewable energy resources and a growth in using HVDC and FACTS devices, these waves are propagating slower, and their impacts are more considerable and complicated. The protection system needs a control system that can take suitable action based on local measurements to overcome the results of power system faults. Therefore, the dynamic behavior of power systems should be properly observed. The EMW propagation in the literature was studied using assumptions such as constant voltage throughout the entire power system and zero resistances and equal series reactances for the transmission lines. Although these assumptions help simplify the power system study model, the model cannot capture the entire power system's dynamic behaviors, since these assumptions are unrealistic. This research will develop an accurate model for EMW propagation when the system is facing a disturbance using a continuum model. The model includes a novel inertia distribution. It also investigates the impacts of voltage changes in the power system on EMW behaviors and when these impacts are negligible. Furthermore, the impacts of the internal reactances of synchronous generators and the resistances of transmission lines on EMW propagation are explored. / Doctor of Philosophy / Power systems, essential for electricity supply, undergo disturbances causing changes in power flow and synchronous generator behavior. These disturbances create electromechanical waves (EMWs) that influence system dynamics. Recent advancements, including renewable energy integration and new technologies, alter EMW behavior, posing challenges for control and protection systems. Existing studies simplify models, limiting their accuracy. This research aims to develop a realistic EMW propagation model considering factors like novel inertia distribution, voltage changes, and internal generator properties. This work addresses the evolving power landscape, enhancing our understanding of power system dynamics for improved control and reliability.

Finite difference approximations are methods for solving differential equations by approximating derivatives. This work will begin with how to solve a partial differential equation (PDE) called the advection equation, ut + cux = 0. Both analytically, and approximately with three different finite difference methods for the spatial part of the equation: • Central in space, • First order upwind in space, • Beam-Warming in space, and forward Euler for the temporal part. We then use the theoretical approximations considered for the advection equation and apply it on Maxwell’s equations for electromagnetism in 1D. This is a system of advection equations that describes how electromagnetic waves propagate through a dielectric material. In the end of this work we will model this electromagnetic wave, or wave of light moving through materials with different refraction indexes.

<p dir="ltr">A key challenge in the assessment of the stability of fractures in rock is the identification of precursory geophysical signatures of shear failure. Accurate estimation and prediction of shear failure along rock discontinuities is crucial to prevent failure of geotechnical structures and potential natural hazards, such as landslides and earthquakes. Active seismic monitoring, such as compressional (P) and shear (S) waves, has been used to monitor the evolution of contact area and contact stress along rock discontinuities. Past laboratory experiments determined that changes in the amplitude of the transmitted, reflected, and converted P- and S-waves can be used to assess local changes in contact area and fracture specific stiffness, and to identify precursory events to shear failure of rock fractures. Those studies have identified the peaks (maxima or minima) in wave amplitudes as the seismic precursors to shear failure. Past studies were performed on dry artificial rock discontinuities with homogeneous and well-matched contact surfaces. However, in nature, rock discontinuities are not always homogeneous and well-matched, and are often found below the water table. In addition, at large depths, e.g. in enhanced geothermal systems (EGS), fractures are subjected to high temperatures.</p><p dir="ltr">The objectives of this research are to: (1) characterize the geophysical response of rock fractures during shear for dry and saturated conditions at room temperature, and saturated conditions at elevated temperatures; and (2) detect and identify seismic signatures of shear failure/slip for each of the three conditions. To achieve the goal of the research, a novel shear test apparatus was designed and built to test saturated jointed rock specimens under normal and shear loading, with a back pressure and at elevated temperatures, while also being capable of housing seismic transducers to monitor simultaneously the mechanical and geophysical response of the rock joints during shear. The system consisted of a sealed and heated pressure chamber and a biaxial compression frame. The pressure chamber was also used to perform B-value tests on cylindrical rock specimens to determine the minimum magnitude of back pressure required for fluid saturation.</p><p dir="ltr">Laboratory direct shear tests were performed on tension-induced fractures in Indiana limestone and Sierra White granite specimens with non-homogeneous rough contact surfaces. The contact surfaces were created by axial splitting of prismatic rock blocks. Shear tests were conducted on the rock fractures at a constant displacement rate in the pressure chamber, which enabled control of effective normal stress, pore water pressure, and temperature. During the tests, transmitted and converted P- and S-waves propagated across rock fractures and their changes in wave amplitude were monitored to assess the evolution of local contact areas during shear and detect precursory changes in wave amplitudes prior to shear failure.</p><p dir="ltr">Seismic precursors were observed in the wave amplitude data from all tests conducted under the three conditions. Precursors were most identifiable in the transmitted S-wave data. For all three conditions, the transmitted S-wave showed the same form of a seismic precursor; a peak (maximum) in wave amplitude was observed prior to the peak shear strength, as local contact surfaces interlocked and failed before macroscopic shear failure. However, the transmitted P-wave and converted waves (P-to-S and S-to-P) exhibited different behavior compared to the transmitted S-wave and depended on the test conditions. While, for dry conditions, the transmitted P-wave and converted waves still exhibited seismic precursors as peaks in their wave amplitudes, they did not display an observable peak for saturated fractures at room temperature, but rather either a very slight increase or a continuous reduction in amplitude. Instead of observable peaks, an abrupt change in the rate of reduction in the transmitted P-wave and converted amplitudes was observed that either coincided or occurred close to the peak in the transmitted S-wave amplitude. Thus, an onset of dramatic change in the reduction rate can be also taken as a seismic precursor to shear failure. This phenomenon can be explained by the large stiffness of the highly incompressible fluid, water, which leads to a decrease in P-wave sensitivity to changes in the normal fracture stiffness that arise from rock asperities under saturated conditions.</p><p dir="ltr">Even though the seismic wave amplitude generally contains a seismic precursor to shear failure, some exceptions exist: the wave amplitudes also depend on the local characteristics of the frictional area. No peak or seismic precursor in wave amplitude is observed prior to failure when the contact area between the fractures surfaces decreases because of dilation/opening. In addition, a delay peak in amplitude after shear failure may be observed when the fracture surfaces contain an initial large void or aperture in the region probed by the sensor. These exceptions may occur at a relatively low effective normal stress (2 MPa) and may disappear when a better contact has been established between the fracture surfaces by increasing the effective stress. Direct shear tests under an effective stress of 6 MPa, but at 50^oC, showed that both the transmitted P-waves and converted waves exhibited peaks in their amplitudes prior to the failure. However, these exceptions still require further exploration for the systematic identification and detection of seismic precursors.</p><p dir="ltr">The research shows that seismic monitoring is an effective tool to monitor the shear behavior of discontinuities, to provide an assessment of the local behavior of the frictional surface under the transducer, and to predict failure of the discontinuity. It can be used for dry, saturated discontinuities and for a wide range of pore pressures and temperatures. Other potential applications include fault monitoring, and even possibly earthquake prediction with additional research.</p>

This research reviews and extends the impedance-based structural health monitoring technique in order to detect and identify structural damage on various complex structures. The basic principle behind this technique is to apply high frequency structural excitations (typically higher than 30 kHz) through the surface-bonded piezoelectric transducers, and measure the impedance of structures by monitoring the current and voltage applied to the transducers. Changes in impedance indicate changes in the structure, which in turn can indicate that damage has occurred. Several case studies, including a pipeline structure, a composite reinforced aluminum plate, a precision part (gear), a quarter-scale bridge section, and a steel pipe header, demonstrate how this technique can be used to detect damage in real-time. A method to process impedance measurements to prevent significant temperature and boundary condition changes registering as damage has been developed and implemented. Furthermore, the feasibility of using the technique for high temperature structures and for condition monitoring of critical facilities subjected to a severe natural disaster has been investigated. While the impedance-based structural health monitoring technique indicates qualitatively that damage has occurred, more information on the nature of damage is necessary for remote structures. In this research, two different damage identification schemes have been combined with the impedance method in order to quantitatively assess the state of structures. One is based on a wave propagation modeling, and the other is the use of artificial neural networks. A newly developed wave propagation model has been developed and combined with the impedance method in order to estimate the severity of damage. Numerical and experimental investigations on 1-dimensional structures were presented to illustrate the effectiveness of the combined approach. Furthermore, to avoid the complexity introduced by conventional computational methods in high frequency ranges, multiple sets of artificial neural networks were integrated with the impedance-based health monitoring technique. By incorporating neural network features, the technique is able to detect damage in its early stage and to determine the severity of damage without prior knowledge of the model of structures. The dissertation concludes with experimental examples, investigations on a quarter-scale steel bridge section and a space truss structure, in order to verify the performance of the proposed methodology. / Ph. D.

Analytic methods of elastic wave scattering in fiber-reinforced composite materials are investigated in this study to calculate the effective static stiffness (axial shear modulus, m) and wave properties (axially shear wave speed, B and attenuation, Y) in composites. For simplicity only out-of-plane shear waves are modeled propagating in a plane transverse to the fiber axis. Statistical averaging of a spatially random distribution of fibers is performed and a simultaneous system of linear equations are obtained from which the effective global wave numbers are numerically calculated. The wave numbers, K=Re(K)+iIm(K), are complex numbers where the real parts are used to compute the effective axial shear static stiffness and wave speed; the imaginary parts are used to compute the effective axial shear wave attenuation in composites. Three major parts of this study are presented. The first part is the discussion of multiple scattering phenomena in a successive-events scattering approach. The successive-events scattering approach is proven to be mathematically exact by comparing the results obtained by the many-bodies-single-event approach. Scattering cross-section is computed and comparison of the first five scattering orders is made. Furthermore, the ubiquitous quasi-crystalline approximation theorem is given a justifiable foundation in the fiber-matrix composite context. The second part is to calculate m, B and Y for fiber-reinforced composites with interfacial layers between fibers and matrix. The material properties of the layers are assumed to be either linearly or exponentially distributed between the fibers and matrix. A concise formula is obtained where parameters can be computed using a computationally easy-to-program determinant of a square matrix. The numerical computations show, among other things, that the smoother (more divisional layers), or thinner, the interfacial region the less damped are the composite materials. Additionally composites with exponential order distribution of the interfacial region are more damped than the linear distribution ones. The third part is to calculate m, B and Y for fiber-reinforced composites with interfacial cracks. The procedures and computational techniques are similar to those in the second part except that the singularity near the crack tip needs the Chebychev function as a series expansion to be adopted in the computation. Both the interfacial layers and interfacial crack cases are analyzed in the low frequency range. The analytic results show that waves in both cases are attenuated and non-dispersive in the low frequency range. The composites with interfacial layers are transversely isotropic, while composites with interfacial cracks are generally transversely anisotropic. / Ph. D.