The development of a smart piezo-braid for composite applications

Abstract

The ever-increasing development of wearable sensors and self-powered microelectronic devices has become possible by the advent of new types of piezoelectric micropower generators. One important material that has enabled the possibility to seamlessly integrate such microgenerators into many emerging applications such as, smart textiles and fibre-reinforced polymer (FRP) composites, is Poly(vinylidene fluoride) (PVDF). A series of novel and efficient Piezo-Polymer based Energy Harvester (PPEH) concepts were first developed in this project [2]. For this aim, the textile braiding technology was employed as a new method for large-scale production of a number of piezoelectric PVDF braid microgenerators. The initial concept of microgenerators consists of a 0.4 mm solid core polyurethane (PUR)-enamelled copper wire (the inner electrode), which was over braided with twenty-four PVDF multifilament yarns (as the intermediate piezoelectric layer), and the whole structure was over braided again with sixteen strands of 0.1 mm PUR-enamelled copper wire (the outer electrode). The smart braid was then incorporated into a glass FRP composite panel as an embedded sensor/micropower harvester. The experimental results of a series of tensile tests, dynamic modal analysis, and three point-bend (3PB) cyclic loading tests showed that the tensile strength and strain-to-failure properties of the host composite were respectively improved by about seventy-four and sixty-two percent; the mechanical damping efficiency of the panel was also increased by about 125% via utilising a passive shunt damping circuit which simultaneously extracted and wasted the piezo generated power as heat over a 100 kW resistor. Furthermore, 3PB cyclic loading tests results (with an excitation amplitude of 1 mm between 1 Hz to 10 Hz) showed that the average power density of as-manufactured (unoptimized) smart composite prototypes is around 2.2 mW.cm−3. Further, in this work, two poling approaches were successfully implemented for improving the piezoelectric properties of as-received PVDF yarns. Verification on the degree of crystallinity and the β-phase content within the material was made by Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared (FTIR) spectroscopy techniques. The results showed that an in-house made radial poling apparatus was capable of improving the crystallinity level (%) of PVDF yarns by as much as nearly twenty-seven percent. Yet, the characterisation results of the second poling approach – the axial poling – where a high electric field (with an applied potential of ≈55 kV) was induced along the axial direction of the yarns, also showed that the overall melting temperature of the samples was decreased by about 3.5°C, indicating that a permanent piezoelectric effect has been induced/achieved by this method. Finally, strong positive correlations were found between the experimental results obtained from the tensile test procedures, modal analysis, three-point bending cyclic loading test, and the poling experiments and their corresponding finite element (FE) model counterparts simulated in Abaqus®/CAE 2017, Creo Parametric 3.0, COMSOL Multiphysics 5.2a software. These promising results were in addition to the successful development of a full-scale simulation of the braiding process which was performed using the Ls-Dyna® FE-code. This research project has successfully demonstrated the feasibility to design, manufacture, and characterise a unique type of “all-fibrous” piezoelectric polymer microgenerator that, for the first time, has proved to be functional and chemically sustainable in the harsh resin epoxy-involved environment at various stages of composite manufacturing process (during vacuum infusion and curing process). This is while no insulating layer/shield between the microgenerator and the resin/reinforcement components has been used. This work therefore offers a promising method for industrial-scale productions of flexible piezoelectric sensors (for Structural Health Monitoring (SHM) applications), and micropower energy harvesters (for the development of future sustainable energy harvesting technologies) in applications such as structural power composites, multifunctional composites, and wearable electronic devices from low-frequency, large-amplitude human body motions.