Mechanical Engineering PhD Thesis Defense by Amirreza Aghakhani

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

PHD THESIS DEFENSE BY AMIRREZA AGHAKHANI

Title: Advanced concepts of piezoelectric patch-based energy harvesting and locally resonant bandgap formation in thin plates.

Speaker: Amirreza Aghakhani

Time: January 17, 2018, 16:00

Place: ENG B15

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof. Dr. Ipek Basdogan (Advisor, Koç University)

Prof. Dr. Yaman Arkun (Koc University)

Assoc. Prof. Dr. B. Erdem Alaca (Koç University)

Prof. Dr. Kenan Yüce Şanlıtürk (Istanbul Technical University)

Assoc. Prof. Dr. Cetin Yilmaz (Boğaziçi University)

Abstract:

Vibrational energy harvesting using piezoelectric transducers has received significant attention over the past decade for enabling self-powered embedded devices in health monitoring applications, and thus reducing the maintenance costs of discarded batteries. In parallel, researchers have extensively investigated the piezoelectric shunt damping technique for efficiently attenuating the vibrations of flexible structures. More recently, in conjunction with shunt damping, locally resonant piezoelectric structures have been employed in the concept of metamaterial-based electromechanical structures to create bandgaps. This work is centered on the electromechanical modeling of piezoelectric patches integrated to thin plate-like structures, which are widely available in aerospace, space, and automotive industries, for the applications of vibration-based energy harvesting and locally resonant bandgap formation.

Vibration-based energy harvesters in the literature are typically made of cantilevered beams with piezoceramic layers which generate power under the base excitations. As alternative to these beam-based harvesters, piezoelectric patches can be integrated to plate structures in order to directly exploit the vibration modes of the host system without adding significant mass and volumetric occupancy of the cantilevers, thus enabling a compact two-dimensional energy harvesting system. Meanwhile, in order to charge the storage components such as a battery or a supercapacitor, a stable DC electrical signal is required. However, majority of the studies on beam and plate-like harvesters have used a simple resistance as an external electrical load. Generally, existing studies on the electrical part have oversimplified the mechanical models, while the works on the mechanical domain have overlooked the practical electrical circuits. Considering these needs, the first part of this thesis aims to address those problems by developing unified analytical and numerical frameworks to fully bridge the electrical and mechanical domains, and hence formulating a complete electromechanically coupled system. In chapter 2, a multi-modal equivalent circuit model (ECM) for a single piezo-patch energy harvester on a thin plate is developed using the analogy between the second-order circuitry equation and the governing electromechanical equation. The equivalent circuit parameters are obtained from the analytical modal analysis solution of a rectangular plate with a piezo-patch, and account for multiple vibration modes of the host structure as well as the two-way electromechanical coupling. In chapter 3, an equivalent impedance electroelastic model is developed by integrating the equivalent impedance load of the practical harvesting circuit into the electroelastic equations of the plate with multiple piezo-patch energy harvesters (MPEHs). Moreover, a generalized ECM for the MPEHs is established which is compatible with different interconnections of the piezo-patches (series, parallel, or mixed series-parallel configurations). Finally, a respective rectification technique as a solution for avoiding charge cancellations, due to opposite current signs of piezo-patches at higher vibration modes, is proposed.

In the second part of the thesis, a type of two-dimensional electromechanical metastructures composed of locally resonant piezo-patches integrated to a thin plate is introduced and investigated. Analogous to metamaterial-based mechanical structures where an array of purely mechanical resonators is integrated to the structure, many piezoelectric elements with resonant shunt circuits can be integrated to a thin plate in order to generate bandgaps in the host structure, enabling a compact and tunable shunt damping mechanism. In the literature of metamaterials, most studies assume an infinite structure made of repeated assembly of unit-cells and use Bloch theorem to model the bandgaps of structure. However, for practical implementation of locally resonant metamaterials where finite structures with distributed array of resonators are employed, a high-fidelity model is required to bridge the modal interaction of the host structure with the electromechanical behavior of the resonant piezoelectric elements. To this end, chapter 4 presents an electroelastic modeling of a thin plate with array of piezo-patches connected to resonant shunt circuits for investigating the bandgap formation. Displacement FRF surface plots are presented for a set of piezoelectric materials such as ceramics and single crystals, where the bandgaps are observed at frequency bandwidth with a very low vibration amplitude. It is shown that bandgaps are formed at different target frequencies and that their size is widened by the increase in the thickness of the piezo-patches.