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A frequency self-tuning energy harvesting methodology is proposed to achieve efficient energy harvesting. To simulate the self-tuning process, a theoretical model of the harvester made of an aluminum beam bonded with piezoelectric patches is developed for numerical simulation. The energy harvesting is realized by converting ambient vibration to electric charge through piezoelectric patches on the host beam. To accomplish the frequency self-tuning process, a control voltage is applied on a piezoelectric stack actuator to tune the natural frequency of the beam harvester matching the major excitation frequency of the ambient vibration with large power generation. Two tuning methods with different electric circuits are developed to find the efficient and feasible self-tuning process, which is then further verified by the finite element method. Research findings show that the optimal frequency self-tuning method significantly increases the power output from the harvester by more than 26 times compared with the one without tuning. / October 2016

Piezoelectric-based energy harvesting devices provide an attractive approach to powering remote devices as ambient mechanical energy from vibrations is converted to electrical energy. These devices have numerous potential applications, including actuation, sensing, structural health monitoring, and vibration control -- the latter of which is of particular interest here. This work seeks to develop an understanding of energy harvesting behavior within the framework of a semi-active technique for reducing turbomachinery blade vibrations, namely resonance frequency detuning. In contrast with the bulk of energy harvesting research, this effort is not focused on maximizing the power output of the system, but rather providing the low power levels required by resonance frequency detuning. The demands of this technique dictate that harvesting conditions will be far from optimal, requiring that many common assumptions in conventional energy harvesting research be relaxed. Resonance frequency detuning has been proposed as a result of recent advances in turbomachinery blade design that have, while improving their overall efficiency, led to significantly reduced damping and thus large vibratory stresses. This technique uses piezoelectric materials to control the stiffness, and thus resonance frequency, of a blade as the excitation frequency sweeps through resonance. By detuning a structure*s resonance frequency from that of the excitation, the overall peak response can be reduced, delaying high cycle fatigue and extending the lifetime of a blade. Additional benefits include reduced weight, drag, and noise levels as reduced vibratory stresses allow for increasingly light blade construction. As resonance frequency detuning is most effective when the stiffness states are well separated, it is necessary to harvested at nominally open- and short-circuit states, corresponding to the largest separation in stiffness states. This presents a problem from a harvesting standpoint however, as open- and short-circuit correspond to zero charge displacement and zero voltage, respectively, and thus there is no energy flow. It is, then, desirable to operate as near these conditions as possible while still harvesting sufficient energy to provide the power for state-switching. In this research a metric is developed to study the relationship between harvested power and structural stiffness, and a key result is that appreciable energy can be harvested far from the usual optimal conditions in a typical energy harvesting approach. Indeed, sufficient energy is available to power the on-blade control while essentially maintaining the desired stiffness states for detuning. Furthermore, it is shown that the optimal switch in the control law for resonance frequency detuning may be triggered by a threshold harvested power, requiring minimal on-blade processing. This is an attractive idea for implementing a vibration control system on-blade, as size limitations encourage removing the need for additional sensing and signal processing hardware.

Cette thèse s’inscrit dans le contexte des contrôles non destructifs du béton par ultrasons. Nous focalisons notre étude sur les mesures en acoustique non linéaire qui permettent d’ausculter le béton à l’échelle mésoscopique où les ondes interagissent avec les microfissures et le réseau de porosité. Les paramètres associés sont connus comme étant beaucoup plus sensibles que ceux issus des mesures linéaires. Le béton est un matériau hétérogène et complexe, ce qui présente un comportement fortement non linéaire croissant avec l’état endommagé.Nous développons dans cette thèse un type de mesure non linéaire : Dynamic Acousto-Elastic Testing (DAET). Cette technique fondée sur le principe d’une excitation dynamique du matériau, utilise les ondes ultrasonores pour suivre la variation du comportement élastique en fonction de l’amplitude d’excitation. Nous focalisons notre étude sur le problème du béton d’enrobage qui tient un rôle essentiel dans la durée de vie d’une structure de génie civil. Nous étudions l’interaction des mesures non linéaires par DAET avec les ondes de Rayleigh qui se propagent dans le béton d’enrobage. Nous montrons la sensibilité importante de l’évolution de paramètres non linéaires en fonction de l’endommagement thermique et de la carbonatation.Par la suite, nous proposons une nouvelle méthodologie de la mesure DAET, dans laquelle la vibration transitoire est générée par un impact et les ondes sont générées en continue. Nous présentons différentes applications de la méthode proposée, incluant le cas des éprouvettes de grandes dimensions. Cette approche ouvre de larges possibilités de transposer les mesures pour une application sur site. / This thesis is related to the field of nondestructive evaluation of concrete by ultrasound. We focus our study on nonlinear acoustic-based measurements that allow the concrete auscultation at mesoscopic scale where waves interact with microcracks and porosity network. The nonlinear parameters are known to be much more sensitive than those from linear measurements. Concrete is a heterogeneous and complex material. Its behavior is highly nonlinear with increasing damaged state.We develop in this thesis a type of nonlinear measurement: Dynamic Acousto-Elastic Testing (DAET). This technique is based on the principle of a dynamic excitation of the material, using ultrasounds to follow the variation of the elastic behavior depending on the excitation amplitude. We focus our study on the problem of concrete cover which holds a key role in the life of a civil engineering structure. We study the interaction of the DAET measurement with the Rayleigh waves which propagate in the concrete cover. We show the high sensitivity evolution of non-linear parameters in function of thermal damage and carbonation.Subsequently, we propose a new methodology of DAET measurement, in which the transient vibration is generated by an impact and ultrasounds are generated continuously. We present different applications of the proposed method including the case of large specimens. This approach opens broad possibilities of transposing measurements for on-site application.