Application of piezoelectric transducers in multifunctional engineering structures

Abstract

This thesis studied the fabrication of piezoelectric transducers and the engineering structural design in pursuit of enhanced piezoelectric behaviour. The research investigated sandwich-based piezoelectric energy harvesters, and a macro fibre composite (MFC) patch as a piezoelectric element. Finite element models of the sandwich structures were established in COMSOL Multiphysics, where the natural frequencies of the first bending modes and the open-circuit voltage at the natural frequencies were predicted. The difference between the numerical voltage prediction and the experimental measurement is within 10%, so that the experimental measurement is able to validate the numerical calculation. It was found that the sandwich with optimised core cellular design, namely the re-entrant honeycomb, could harvest 24.4 μW under base excitation of 1 g p-p magnitude at a first bending mode resonant frequency. Further optimisation on the re-entrant honeycomb core was conducted by introducing functional grading, in pursuit of further piezoelectric energy harvesting performance. Grading was conducted on the re-entrant honeycomb, by varying the cellular wall angle and the optimum grading scheme was found to have a linear variation from 60 ° at the bottom surface, to 45 ° at the top surface. Similarly, the finite element models of sandwiches with functional graded design were established, and it has been found that the experimental result can validate the numerical prediction. It also consisted of a spin-coated P(VDF-TrFE) transducer and sandwich with the optimised graded cellular core producing a maximum 158.7 nW at 2g p-p acceleration. Moreover, a screen-printing system was built, with fully functioning screen exposure, washing, and substrate clamping units. A screen of 15-200 mesh was selected for P(VDF-TrFE) due to a high paste deposition efficiency. The screen-printed piezoelectric devices were assessed for the structural health monitoring of carbon fibre, and glass fibre reinforced composite panels. When the printed piezoelectric transducers were used as both an emitter and a sensor, the dynamic modes could not be captured by the frequency response function spectrum. When a printed transducer was replaced by a macro fibre composite, the modal information was obtained, and the variation of critical modal peaks was observed after the composite panels were impacted at 5 J for once and twice. It was also observed that the printed transducer behaved more effectively as a sensor than an emitter. These observations demonstrated the feasibility of printed piezoelectric transducers for structural health monitoring applications. To enhance the piezoelectric property of the printed transducer, BaTiO3 particles were added into the P(VDF-TrFE) matrix. The optimised concentration for the best piezoelectric coefficient was found to be 5 wt% BaTiO3 in relation to P(VDF-TrFE) polymers. At the optimised concentration, the screen-printed piezoelectric generator exhibited a piezoelectric constant d33 of -33.90 pC/N, dielectric constant of 17.05, and the Young’s modulus of 1.35 GPa. The measurement was used as the material properties of the piezoelectric elements in the finite element models of the smart sandwich energy harvesters. The numerically calculated natural frequencies of the first bending modes and the open-circuit voltage were well validated by the experimental result. Combining the optimised screen-printed piezoelectric harvester with carbon and glass fibre reinforced composite panels, resulted in a measured maximum power output between 2.9 μW and 3.8 μW. This power was generated under 1 g peak-peak (p-p) acceleration at the first bending mode resonant frequency. To demonstrate real applications of harvested energy, a commercial accelerometer was powered by a smart composite energy harvester. The smart composite energy harvester was excited at 5 g peak-peak acceleration and connected to a 100 μF capacitor via a full-bridge rectifier. The energy stored in the capacitor was used to power the accelerometer, and successful measurement was demonstrated. A further development included a screen-printed nonlinear harvester for energy harvesting, where the printed generator was combined with a buckled Duffing nonlinear oscillator with clamp-clamp configuration. The buckled beams were prototyped into rectangular and cellular configurations. The rectangular and cellular configurations provided a maximum power of 0.66 and 0.93 μW, respectively.