Electroelastic properties of piezoelectric paint for ultrasonic guided wave sensing and damage detection

This research is concerned with the development and characterization of a new type of smart material - flexible piezoelectric paint, with a target application in low-profile broad-band ultrasonic guided wave sensors for cost-effective structural health monitoring. Piezoelectric paint is a composite piezoelectric material that is comprised of tiny piezoelectric particles randomly dispersed within a polymer matrix phase. Because its adjustable material properties that are not easily attainable in a single-phase material, the composite properties of piezoelectric paint can be tailored to meet the specific requirements of an application condition through judicious selection of its constituents. The advantages of piezoelectric paint for sensor application include low fabrication cost, ability to cover large area, as well as flexible cured paint film if properly selected polymer matrix material is used.;In the present research, a PZT-5A/Epoxy based flexible piezoelectric paint has been developed for ultrasonic based nondestructive evaluation (NDE) applications. With an emphasis on sensor application, an improved micromechanics model that considers the special characteristics of piezoelectric paint such as mixed connectivity and air void content has been formulated to predict the elastoelectric properties of piezoelectric paint with mixed connectivity. Percolation theory is used to model the rapid increase of piezoelectric activity after a threshold level of piezoelectric ceramic particle volume fraction is exceeded in the piezoelectric paint. Such an analytical model is very useful to sensor design since the insight gained from a parametric study of piezoelectric paint properties will provide input to the optimization of the paint formulation and guide the design of piezoelectric paint. Subsequently, the elastoelectric properties of lossy piezoelectric paint material are experimentally characterized with a resonance analysis. Specifically, the elastic, dielectric and piezoelectric constants of the piezoelectric paint were determined from measured electrical impedance data. The impedance measurements and associated data analysis provide the data for calibrating both the predictable and unpredictable properties of the piezoelectric paint. The impedance analysis also enables a deep understanding of the electromechanical behaviors of the piezoelectric paint over an ultrasonic frequency range from 100 kHz to 1 MHz.;On the sensor level, the capability and characteristics of piezoelectric paint in sensing ultrasonic guided waves and acoustic emission signal were evaluated using both experimental and numerical approaches. A series of ultrasonic tests including pitch catch and pencil break tests were carried out to validate the ultrasonic wave sensing capability of piezoelectric paint. The results of finite element simulation of ultrasonic tests, including acoustic emission source generated by a pencil lead break on a thin aluminum plate are presented along with corresponding experimental data. An experimental program on monitoring acoustic emission (AE) signals generated by the fiber breakage during tensile loading of a CFRP plate was carried out to evaluate the effectiveness of the piezoelectric paint sensor for such application. The piezoelectric paint sensor is found to be able to detect the AE signals as early as about two-thirds of the ultimate load. Waveform based AE analysis suggests that the piezoelectric paint sensor with a broader band tends to provide more faithful information in terms of frequency content, possible AE source mechanism and minimizing noise-induced false alarm.;Lastly, future work is outlined for this promising sensing material, which has a potential for use in real-time damage detection at hot spots of civil engineering structures.