| Acoustic sensors which are capable of detecting acoustic waves possess numerous uin the field of monitoring structural health.Piezoelectric acoustic sensors possess the advantages of being compact,having a straightforward structure,not needing an external power source to generate electrical signals,and efficiently acoustic signals,as well as comprehensive equipment structure health detection.The traditional piezoelectric acoustic sensor,which employs piezoelectric ceramics or a single crystal as the sensitive element,has low structural flexibility and limited installation capabilities in situations with complex curved surfaces despite having high sensitivity and a wide temperature range.It is of great significance to explore high-performance flexible piezoelectric materials and acoustic sensors,in order to achieve lightweight,miniaturization,and high flexibility of devices.In this study,P(VDF-TrFE)piezoelectric polymer material served as the research object.To create an effective piezoelectric film,post-treatment method were used.The flexible acoustic sensors were designed by COMSOL Multiphysics finite element simulation method and experiment.A test platform was built to test the basic properties of acoustic pressure sensing and ultrasonic guided wave sensing.The research work mainly includes:(1)Regulation and multi-scale characterization of piezoelectric and ferroelectric properties of P(VDF-TrFE)piezoelectric polymerThe ferroelectric properties of poly(vinylidene fluoride)copolymer were regulated by heat treatment,mechanical drawing and electric polarization.The polymer chains were optimized by heat treatment to rearrange from amorphous region into crystalline region.With the help of microscopy and spectroscopic analysis,the content of polar phase increases with the increase of crystallinity.Thin film materials with excellent piezoelectric properties were obtained through mechanical force orientation,the piezoelectric constant was significantly increased by about 54%.By using microscopic and spectroscopic techniques,it was found that the significant increase in the piezoelectric constant of P(VDF-TrFE)thin films is closely related to grain orientation,polarization orientation,and conformational transformation.(2)Experimental study on P(VDF-TrFE)piezoelectric polymer film sound pressure sensorFocusing on the advantages of large flexibility,easy defonnation,and high piezoelectric voltage constant of piezoelectric polymer thin films,piezoelectric thin films are applied to sound pressure sensors with the synergistic effect of piezoelectric effect and Fabry-Perot interference.Weak acoustic signals are detected at resonance frequency,and the detection sensitivity based on piezoelectric effect can reach 157.25 mV/Pa,far exceeding the reference sensor(B&K 4189,sensitivity of about 50 mV/Pa).Furthermore,the signal-to-noise ratio and anti-interference capability of the sensor are improved by convolution of two signals.The development of the device provides a new idea for the application of acoustic detection at a specific frequency.(3)Experimental study of P(VDF-TrFE)piezoelectric polymer film by ultrasonic guided wave nondestructive testingThe experiment for ultrasonic guided wave sensing was conducted after the construction of the ultrasonic test platform.Initially,P(VDF-TrFE)piezoelectric polymer film is utilized as the sensing element and PZT piezoelectric ceramic is employed as the excitation element.The PZT’s operating frequency defines the magnitude of the sensor’s received signal,and the sensor can achieve high sensitivity at its resonance frequency.The sensor has a good ability to receive So in the flat plate thin-wall structure.The sensor initially picked up a signal with a 34.5 mV amplitude and a 48 dB signal-to-noise ratio.Due to the guided wave’s simultaneous axial and radial propagation properties,the received signal’s amplitude in the hollow cylindrical shell is only 7.9 mV,which is quite low.A patterned fork electrode is intended to improve the excitation and reception of a particular mode of guided wave,and the piezoelectric polymer is intended to serve as both a driver and a sensor.The received signal was analyzed in the time domain and frequency domain,with the analysis of the main received signal A0 guided wave accounting for the main component.The transducer’s signal-to-noise ratio was 35 dB,and this,along with the ultrasonic guided wave’s characteristics for propagation.It proved that piezoelectric polymer material as ultrasonic guided wave transducer has great application potential in the field of solid structure health monitoring. |
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