Optimization of shunt damped composite structures using negative capacitances

Vibrations in modern machines such as cars, airplanes and bridges constitute a real issue that can cause undesirable noise, damages and even catastrophic failures. In order to reduce this harmful effect, passive vibration attenuation measures have been extensively used, but can no longer cope with the increasing complexity of engineering systems. In this sense, novel smart structures have been created to efficiently suppress vibration without notable adverse effects. A well-known technique consists in coupling a piezoceramic transducer to a mechanical structure and then connecting it to a shunt circuit. In this thesis, a novel approach to deal with the application of shunted piezoceramics in lightweight composite structures for vibration attenuation is proposed. It is based on the simultaneous optimization of different sub-systems of the smart structure, i.e. host structure, transducers and electronics, so that a set of technical requirements can be met. Instead of being considered as an add-on solution, the shunted piezoceramics are regarded as additional design variables. In this sense, passive structural mass is substituted by active material in an intelligent way, which can potentially reduce overall weight and at the same time improve the dynamic response of the smart structure. To show the potential of this approach, a carbon fiber control arm will be considered as a realistic case study after the development steps described below. In the first part of this work, a study on the physical integration of piezoceramic transducers within laminate composites is carried out. The maximization of the generalized electromechanical coupling coefficient is proposed, since it dictates the damping effectiveness in shunt applications. The influences of the stacking sequence and the geometric integration pattern are numerically and experimentally analyzed using glass and carbon fiber test coupons. In the second part, the optimization process is taken from the coupon level to the sub component level. Numerical and experimental analyses are carried out using a scale model of the control arm. It consists of a cantilever carbon fiber beam with I-shaped cross-section, controlled by the use of piezoceramics. Vibration attenuation is achieved through an RL-shunt circuit connected in series with a negative capacitance, which is built through a synthetic circuit based on an operational amplifier. The classical sequential approach is first introduced, in which the piezoceramics are applied onto the surface of the beam. Then, it is compared to the novel approach, in which the beam, now with integrated piezoceramics, is fully optimized taking into account its geometry, the stacking sequence, the transducer dimensions and the shunt circuit components. Thanks to the simultaneous approach, not only the mechanical requirements of the structure, such as mass, global stiffness and dynamic behavior can be respected, but also the electrical characteristics of the shunt circuit. In the third part, a novel tuning technique for shunt damping with a negative capacitance is proposed. It is based on the measured electromechanical impedance of a smart structure, which is represented through an equivalent electrical circuit. A numerical optimization permits for the first time the correct choice of all electric components in the shunt circuit, since all mechanical quantities are analyzed in a purely electrical form. In the last part, the acquired knowledge is applied to the control arm. It is redesigned according to the methodology validated with the sub component to show that high vibration attenuation using shunt damping and high mass saving can be simultaneously attained.