This paper reports on a novel measurement method to characterize piezoelectric thin layer (~ 20 μm) through a four-point bending (4PB) setup. Polymer-based piezoelectric composite is directly screen printed onto the surface of host structure, which is assimilated to an instrumented steel substrate. Because of electromechanical coupling properties, a voltage signal can be generated through the piezoelectric sensor when subjected to mechanical strain in its film plane. As the typically sensor layer is very thin in comparison to the underlying substrate, the mechanical properties of the whole system are mainly determined by the characteristics of the steel-type material. Furthermore, the piezoelectric layer properties strongly depend on underlying substrate, electrodes, and the processing route of the layered sensor structure. Therefore, the method developed here enables to reliably extract characteristics such as the effective piezoelectric behavior of the thin composite. In combination with a strain gage coated onto the back side of the substrate, the developed setup allows the determination of the effective piezoelectric sensitivity. The homogeneous strain distribution in the sensing layer is verified by finite element simulations. Furthermore, analytical model is investigated to predict the mechanical behavior of the 4PB structure. The results demonstrate that the developed sensing setup is capable to provide a direct strain/stress measurement instead of traditional techniques through interpolation, and thus offers an efficient method for on-line and in situ structural health monitoring of bearing.
It is well know that ZnO based polymer composite attract enormous research interests, especially in applications of sensor/actuator devices as well as energy harvesting systems. In this study, a PDMS polymer filled with ZnO microparticles were synthesized and its piezoelectric behavior was observed while varying the ZnO concentration, the dielectrophoretic alignment process, and the poling condition. A real-time observation of particles movement under an alternating (AC) field was achieved in non-cured composites through a digital microscopy. Several processing parameters including amplitude and frequency of the applied electric field was investigated to determine the best configuration of dielectrophoretic manipulation. Simulation and experiment results revealed that the alignment process of the particles improved the poling effect caused by the directionality of the network structures. Owing to the high ZnO/ZnO connectivity, the aligned composites give raise to significantly enhanced dielectric and piezoelectric properties with respect to those randomly dispersed. The dielectric permittivity and piezoelectric response of composites increase with an increasing ZnO volume fraction as well. Finally, a 2D finite element model (FEM) was developed to predict the dielectric constant and the piezoelectric coefficient of the ZnO material. Simulation was in good agreement with the experiment results, confirming that a well-designed network structure of the composite would be a viable approach to improve the piezoelectric performance.
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