This study involves the modeling and optimization of heterogeneous (HG) blankets for improved reduction of the sound transmission through double-panel systems at low frequencies. HG blankets consist of poro-elastic media with small, embedded masses, operating similar to a distributed mass-spring-damper system. Although most traditional poro-elastic materials have failed to effectively reduce low-frequency, radiated sound from structures, HG blankets show significant potential. A design tool predicting the response of a single-bay double panel system (DPS) with, acoustic cavity, HG blanket and radiated field, later a multi-bay DPS with frames, stringers, mounts, and four HG blankets, was developed and experimentally validated using impedance and mobility methods (IMM). A novel impedance matrix formulation for the HG blanket is derived and coupled to the DPS using an assembled matrix approach derived from the IMM. Genetic algorithms coupled with the previously described design tool of the DPS with the HG blanket treatment can optimize HG blanket design. This study presents a comparison of the performance obtained using the genetic algorithm optimization routine and a novel interactive optimization routine based on sequential addition of masses in the blanket. This research offers a detailed analysis of the behavior of the mass inclusions, highlighting controlled stiffness variation of the mass-spring-damper systems inside the HG blanket. A novel, empirical approach to predict the natural frequency of different mass shapes embedded in porous media was derived and experimentally verified for many different types of porous media. In addition, simplifying a model for poro-elastic materials for low frequencies that Biot and Allard originally proposed and implementing basic elastomechanical solutions produce a novel analytical approach to describe the interaction of the mass inclusions with a poro-elastic layer. A full-scale fuselage experiment performed on a Gulfstream section involves using the design tool for the positions of the mass inclusions, and the results of the previously described empirical approach facilitate tuning of the natural frequencies of the mass inclusions to the desired natural frequencies. The presented results indicate that proper tuning of the HG blankets can result in broadband noise reduction below 500Hz with less than 10% added mass. / Ph. D.

The objective of this thesis is to develop a dual-functional approach to effectively mitigate the earthquake induced vibrations of low- or mid-rise buildings, and at the same time to efficiently harvest utility-scale energy by using an optimally configured multi-resonant electromagnetic shunt in base isolation. In this research, two multi-resonant shunt configurations, parallel and series, were proposed and optimized based on the H2 criteria when the base isolation system is subjected to ground acceleration excitations. The performance of the proposed multi-resonant electromagnetic shunt was compared with traditional multiple tuned mass dampers (TMDs) applied to the base isolation system. It shows that, for multiple TMDs and multi-resonant electromagnetic shunt dampers with 5% total stiffness ratio, the parallel shunt electromagnetic shunt can achieve the best vibration mitigation performance among other types of multi-resonant dampers, including parallel TMDs, series TMDs and the series electromagnetic shunt damper. Case study of a base-isolated structure was analyzed to investigate the effectiveness of the proposed multi-resonant electromagnetic shunt. It shows that both multi-mode shunt circuits outperform single mode shunt circuit by suppressing the primary and the second vibration modes simultaneously. Comparatively, the parallel shunt circuit is more effective in vibration mitigation and energy harvesting, and is also more robust in parameter mistuning than the series shunt circuit. The time history response analysis shows that, under the recorded Northridge earthquake, the instant peak power and total average power capable to be harvested by the multi-resonant shunt can reach up to 1.18 MW and 203.37KW, respectively. This thesis further experimentally validated the effectiveness of the multi-resonant electromagnetic shunt on a scaled-down base-isolated building. The impact hammer test shows that the multi-resonant electromagnetic shunt can achieve enhanced vibration suppression by reducing the first resonant peak by 27.50dB and the second resonant peak by 22.57dB regarding the primary structure acceleration. The shake table test shows that under scaled Kobe and Northridge earthquake signals, the electromagnetic shunt can effectively reduce the vibration resonant peak value by 38.92% and 66.61%, respectively. The voltage simultaneously generated in the multi-mode shunt circuit was also obtained, which demonstrated the dual functions of the multi-resonant electromagnetic shunt in base isolation. / Master of Science

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2008. / Suspension seats are commonly used for earth moving machinery to isolate vehicle operators from vibrations transmitted to the vehicle body. To provide the required stiffness and damping for these seats, air-springs are typically used in conjunction with dampers. However, to eliminate the need for additional dampers, air-springs can be used in conjunction with auxiliary air volumes to provide both spring stiffness and damping. The damping is introduced through the flow restriction connecting the two air volumes. In this study, simplified models of an air-spring were derived followed by a model including the addition of an auxiliary volume. Subsequent to simulations, tests were performed on an experimental apparatus to validate the models. The air-spring models were shown to predict the behaviour of the experimental apparatus. The air-spring and auxiliary volume model followed the trend predicted by literature but showed approximately 27 % lower transmissibility amplitude and 21 % lower system natural frequency than obtained by tests when using large flow restriction diameters. This inaccuracy was assumed to be introduced by the simplified mass transfer equations defining the flow restriction between air-spring and auxiliary volume. The models however showed correlation when the auxiliary volume size was decreased by two thirds of the volume actually used for the experiment. This design of a prototype air-spring and auxiliary volume is presented for a suspension seat used in articulated or rigid frame dump trucks. The goal of this study was to design a suspension seat for this application and to obtain a SEAT value below 1,1. The design was optimised by varying auxiliary volume size, flow diameter and load. A SEAT value of less than 0,9 was achieved.

This study investigated the vibration serviceability of timber structures with dowel-type connections. It addressed the use of such connections in cutting-edge timber structures such as multi-storey buildings and long-span bridges, in which the light weight and flexibility of the structure make it possible that vibration induced by dynamic forces such as wind or footfall may cause discomfort to occupants or users of the structure, or otherwise impair its intended use. The nature of the oscillating force imposed on connections by this form of vibration was defined based on literature review and the use of established mathematical models. This allowed the appropriate cyclic load to be applied in experimental work on the most basic component of a dowel-type connection: a steel dowel embedding into a block of timber. A model for the stiffness of the timber in embedment under this cyclic load was developed based on an elastic stress function, which could then be used as the basis of a model for a complete connector. Nonlinear and time-dependent behaviour was also observed in embedment, and a simple rheological model incorporating elastic, viscoelastic and plastic elements was fitted to the measured response to cyclic load. Observations of the embedment response of the timber were then used to explain features of the behaviour of complete single- and multiple-dowel connections under cyclic load representative of in-service vibration. Complete portal frames and cantilever beams were tested under cyclic load, and a design method was derived for predicting the stiffness of such structures, using analytical equations based on the model for embedment behaviour. In each cyclic load test the energy dissipation in the specimen, which contributes to the damping in a complete structure, was measured. The analytical model was used to predict frictional energy dissipation in embedment, which was shown to make a significant contribution to damping in single-dowel connections. Based on the experimental results and analysis, several defining aspects of the dynamic response of the complete structures, such as a reduction of natural frequency with increased amplitude of applied load, were related to the observed and modelled embedment behaviour of the connections.

The use of sandwich structures in automobile body panels is investigated in this thesis. The applications on vehicles such as trains, aeroplanes and automobiles, advantages, isadvantages and modelling of sandwich structures are discussed and studies about static, vibrational and acoustic benefits of sandwich structures by several authors are presented. The floor, luggage, firewall and rear wheel panels in sheet metal form is replaced with panel made from sandwich materials in order to reduce the weight obtained by a trial and error based optimization method by keeping the same bending stiffness performance. In addition to these, the use of sandwich structures over free layer surface damping treatments glued on floor panel to decrease the vibration levels and air-borne noise inside the cabin is investigated. It has been proven that, the same vibration performance of both flat beam and floor panel can be obtained using sandwich structures instead of free layer surface damping treatments with a less weight addition. Furthermore, the damping effect of sandwich structures on sound transmission loss of complex shaped panels like floor panel is investigated. A 2D flat and curved panel representing the floor panel of FIAT Car model are analysed in a very large frequency range. Four different loss factors are applied on these panels and it is seen that, until it reaches damping controlled region, damping has a very little effect on TL of flat panels but has an obvious damping effect on TL of curved panels. However in that region, damping has an increasing effect on TL of both flat and curved panels.

There are many active and passive vibration control techniques to reduce the effect of energy on structures which emerges during an earthquake and reduce the displacement of buildings that is caused by ground acceleration. Main advantage of passive vibration control techniques over active vibration control techniques is / no external power or a sensor is required for passive vibration control devices (PVCDs) and it results in lower installation and maintenance costs. However, PVCDs require a predefined optimum damping ratio and optimum damping distribution along the structure since they are not adaptive to changing ground acceleration values. During the design of the PVCDs numerous factors such as building properties and earthquake characteristics should be considered. Dry friction damper is an example of PVCD and has an extensive usage in many different fields due to its high energy damping capacity with low cost and ease of installation. In this thesis, damping of seismic energy at buildings with dry friction dampers is investigated and a new optimization method is developed in frequency domain by employing Describing Function Method (DFM) which reduces the computational effort compared to the time domain and finite element solutions drastically. The accuracy and verification of the presented method is investigated by comparing the frequency domain results with time marching solutions. Furthermore, damper placement and slip forces on the dampers are optimized for single and multi-story buildings equipped with dry friction dampers by utilizing the developed method.

Advanced materials such as carbon fiber, composite materials et al. are more and more used in modern industry. They make the structures lighter and stiffer. However, they bring vibration problems. Researchers studied numerous methods to eliminate the undesirable vibrations. These treatments are expected to be a compact, light, intellectual and modular system. Recently, a nonlinear technique which is known as Synchronized Switch Damping (SSD) technique was proposed. These techniques synchronously switched when structure got to its displacement extremes that leading to a nonlinear voltage on the piezoelectric elements. This resulting voltage showed a time lag with the piezoelectric strain thus causing energy dissipation. Based on the developed SSD techniques, a new synchronized switch damping e.g. Synchronized Switch Damping with Energy Transfer (SSDET) was proposed in this document. This method damped the vibration by using the energy from other vibrating form. The objectives of the work reported in this document were threefold. The first one consisted of introduction of SSDET principle and developing its control law. This part aimed at establishing the mathematical model and verifying the proposed method by mathematical tools. Then, the experimental validations were carried out. Three experiments with different configurations demonstrated that SSDET can be implemented not only between structures but also vibrating modes in one structure. A SSDET scheme with multi-patches was also investigated for improving the damping. Finally, a bidirectional SSDET concept was introduced based on the original SSDET technique. This technique be regarded as a multimode control SSDET. Since it privileged the target vibration while keeps a decent control effect on the source vibration.

<div>The development of novel passive techniques for vibration attenuation and control of broadband energy propagation through structural systems have been a major challenge in various complex engineering applications. These passive attenuation and control methodologies are necessary for the efficient performance of advanced lightweight aerospace and mechanical systems operating under extreme working conditions.</div><div><br></div><div>Acoustic Black Holes (ABH) have rapidly emerged as an effective approach to either dissipate or harvest mechanical energy in vibrating thin-walled structures. The characteristic dimension of an ABH, typically its diameter, is strictly connected to the occurrence of a cut-on frequency value below which the ABH is ineffective in absorbing the incoming wave. From a general perspective, lower the cut-on frequency, larger the ABH diameter needed to absorb the incoming wave. Design and manufacturing constraints of the host structure impose stringent limitations on the maximum ABH diameter and hence, limiting the lowest achievable cut-on frequency. The combination of these factors typically result in the poor energy extraction performance at low frequencies.</div><div><br></div><div>This thesis proposes the concept and explores the performance of an intentional nonlocal design for periodic grids of ABHs embedded in thin plates (referred to as ABH metastructures). The nonlocal design is conceived with the twofold objective of lowering the cut-on frequency of the ABH grids and extending the operating frequency range so as to achieve broadband performance. Different nonlocal designs are presented and their dynamic performances are investigated using numerical models. As opposed to the traditional material nonlocality, this thesis introduces nonlocal effects using an intentionally tailored geometric approach. A secondary layer is connected to the load-bearing ABH metastructure base, whose dynamic properties are sought to be controlled.</div><div><br></div><div>A semi-analytical model is also presented in order to characterize the role of nonlocality on the dispersion behavior and its effect on the broadband dynamic response. In linear elasticity, material nonlocality is mathematically represented by a spatially varying attenuation function. As the nonlocal model developed in this thesis follows geometric nonlocality approach, the required nonlocal attenuation factor is found to have a spatial as well as a temporal dependence. The analytical nonlocal constitutive relations in conjunction with the numerically obtained stress-strain parameters are used to identify the dynamic attenuation factor for the nonlocal ABH metastructure. The results provide substantial theoretical and numerical evidence of the potential of engineered nonlocal ABH design as an efficient ultra-low frequency passive attenuation technique for lightweight structures.</div>

Das Ziel dieser Arbeit war die Aufklärung der Dämpfungsmechanismen partikelgefüllter Hohlkugelstrukturen und die Charakterisierung ihres Dämpfungsvermögens, um werkstoffgerechte Konstruktionen zur Schwingungsdämpfung zu ermöglichen. Seit die Reduktion der Masse bewegter Baugruppen zur Reduktion des Energieverbrauchs von Maschinen in den Fokus der Entwicklung im Maschinenbau gerückt ist, und gleichzeitig der Anspruch an die Genauigkeit und Oberflächengüte der zu fertigenden Bauteile steigt, hat sich die Dämpfung mechanischer Schwingungen von einem dem Komfort dienenden Thema zu einer essentiellen Aufgabe vor allem in der Konstruktion von Werkzeugmaschinen gewandelt. In der Literatur werden viele Verfahren zur aktiven, semiaktiven und passiven Dämpfung mechanischer Schwingungen diskutiert. Industriell etabliert haben sich bisher hauptsächlich Verfahren, die auf der Dämpfung im Gefüge der Werkstoffe beruhen. Zur Aktivierung der Dämpfungsmechanismen im Gefüge ist immer eine Verformung des Werkstoffs notwendig. Die in der Arbeit untersuchten partikelgefüllten Hohlkugeln dagegen ermöglichen eine Starrkörperdämpfung. Ausgehend von der Erfahrung, dass eine fallen gelassene partikelgefüllte Hohlkugel kaum oder gar nicht hüpft, wurde ein Verfahren zur Messung der Dämpfungsfähigkeit von Einzelkugeln entwickelt. Es wurden die Parameter der partikelgefüllten Hohlkugeln identifiziert, die durch ihre Veränderung Rückschlüsse auf deren Dämpfungswirkung erwarten ließen. Von den Eigenschaften der Partikel, über technologische Messungen an den Pulvern, die Messung des Dämpfungsvermögens der Einzelkugel und der Dämpfung von Probekörpern bis hin zum Einsatz im Frässchlitten einer Beispielmaschine wurde das Dämpfungsverhalten des realen Werkstoffs untersucht. Durch die Abschätzungen anhand eines Modellsystems wurde eine vertiefte Vorstellung der in den partikelgefüllten Hohlkugeln ablaufenden Vorgänge erreicht. Anhand eines Frässchlittens einer Beispielmaschine konnte die Schwingungsdämpfung im Leichtbau nachgewiesen werden.:1 Einleitung 5 2 Literatur und Stand der Technik 7 2.1 Dämpfung mechanischer Schwingungen 7 2.1.1 Begriffsbestimmungen 7 2.1.2 Dämpfung im Gefüge von Werkstoffen 9 2.1.3 Besonderheiten der Dämpfung in zellularen metallischen Werkstoffen (ZMW) 13 2.1.4 Dämpfung durch Reibung zwischen Bauteilen 15 2.1.5 Modelluntersuchungen an Schlag- und Partikeldämpfern 15 2.1.6 Reibung in Pulvern und Schüttgütern 21 2.2 Kombination von Leichtbau und Schwingungsdämpfung im Maschinenbau 29 2.2.1 Notwendigkeit und Potential 29 2.2.2 Sandwichbauweise 29 2.3 Technologie 35 2.3.1 Herstellung ungefüllter Hohlkugeln und Hohlkugelstrukturen 35 2.3.2 Herstellung gefüllter Hohlkugeln im Labormaßstab 37 3 Ziel der vorliegenden Arbeit 39 4 Methoden 41 4.1 Probenherstellung 41 4.1.1 Ausgangsmaterialien 41 4.1.2 Beschichtung 42 4.1.3 Entbinderung und Sinterung 43 4.1.4 Partikelgefüllte keramische Hohlkugeln 44 4.1.5 Herstellung von Probekörpern 45 4.2 Charakterisierungsmethoden 47 4.2.1 Pulvercharakterisierung 47 4.2.2 Dämpfung von Einzelkugeln 52 4.2.3 Resonanzfrequenz – Dämpfungsanalyse (RFDA) 54 5 Ergebnisse 58 5.1 Partikelbewegung in gefüllten Hohlkugeln 58 5.2 Dämpfungspulver 61 5.2.1 Partikelform 61 5.2.2 Partikelgrößenverteilung 62 5.2.3 Pulverdichten 64 5.2.4 Veränderung der Partikeloberfläche 66 5.3 Dämpfung bei geringem Energieeintrag 67 5.4 Dämpfung der Partikel bei hohem Energieeintrag 69 5.4.1 Einfluss der Hausnerzahl 69 5.4.2 Einfluss der Fähigkeit zur Fluidisierung 71 5.4.3 Abhängigkeit vom Masseverhältnis 75 5.4.4 Abhängigkeit von der Partikelgröße 76 5.4.5 Abhängigkeit von der spezifischen Oberfläche 78 5.4.6 Abhängigkeit von der Kugelschale 79 5.5 Dämpfung von Bauteilen 81 5.5.1 Dämpfung von Probekörpern in Abhängigkeit von der Dämpfung der Einzelkugeln 81 5.5.2 Dämpfung von Probekörpern in Abhängigkeit vom Masseverhältnis Partikel – Probekörper 83 5.5.3 Volumenanteil der Einzelkugeln im Probekörper 84 5.5.4 Dämpfung in Abhängigkeit von der Anzahl der Dämpfungszentren 85 6 Diskussion 87 6.1 Analyse der Bewegungszustände der Partikel 87 6.2 Analyse der wirkenden Kräfte 89 6.3 Modellierung der Dämpfung 93 6.4 Folgerungen aus dem Modell 94 6.5 Vergleich mit experimentellen Ergebnissen 98 6.5.1 Energiedissipation im Pulverbett 99 6.5.2 Fluidisierung des Pulverbettes 100 6.5.3 Gasartige Partikelbewegung 102 6.5.4 Füllgrad 103 6.5.5 Dämpfung im Verbundwerkstoff 106 6.6 Abschätzungen zur Bauteildämpfung 108 6.7 Anwendungsbeispiel Frässchlitten 109 6.7.1 Technische Ergebnisse 109 6.7.2 Kostenabschätzung im Manufakturbetrieb 113 7 Zusammenfassung 117 8 Danksagung 120 9 Anhang 122 9.1 Literaturverzeichnis 122 9.2 Berechnungen 130 9.2.1 Berechnung der Stoßzahl 130 9.2.2 Partikeloberfläche pro Hohlkugel 130 9.2.3 Füllgrad von Einzelkugeln 132 9.3 Verzeichnis der Abkürzungen und Symbole 135 9.4 Verzeichnis der Abbildungen 139

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