Environmental Robustness and Resilience of Direct-Write Ultrasonic Transducers Made from P(VDF-TrFE) Piezoelectric Coating.

Conformability, lightweight, consistency and low cost due to batch fabrication in situ on host structures are the attractive advantages of ultrasonic transducers made of piezoelectric polymer coatings for structural health monitoring (SHM). However, knowledge about the environmental impacts of piezoelectric polymer ultrasonic transducers is lacking, limiting their widespread use for SHM in industries. The purpose of this work is to evaluate whether direct-write transducers (DWTs) fabricated from piezoelectric polymer coatings can withstand various natural environmental impacts. The ultrasonic signals of the DWTs and properties of the piezoelectric polymer coatings fabricated in situ on the test coupons were evaluated during and after exposure to various environmental conditions, including high and low temperatures, icing, rain, humidity, and the salt fog test. Our experimental results and analyses showed that it is promising for the DWTs made of piezoelectric P(VDF-TrFE) polymer coating with an appropriate protective layer to pass various operational conditions according to US standards.

Surface Crack Monitoring by Rayleigh Waves with a Piezoelectric-Polymer-Film Ultrasonic Transducer Array.

This paper presents a method for measuring surface cracks based on the analysis of Rayleigh waves in the frequency domain. The Rayleigh waves were detected by a Rayleigh wave receiver array made of a piezoelectric polyvinylidene fluoride (PVDF) film and enhanced by a delay-and-sum algorithm. This method employs the determined reflection factors of Rayleigh waves scattered at a surface fatigue crack to calculate the crack depth. In the frequency domain, the inverse scattering problem is solved by comparing the reflection factor of the Rayleigh waves between the measured and the theoretical curves. The experimental measurement results quantitatively matched the simulated surface crack depths. The advantages of using the low-profile Rayleigh wave receiver array made of a PVDF film for detecting the incident and reflected Rayleigh waves were analyzed in contrast with those of a Rayleigh wave receiver using a laser vibrometer and a conventional lead zirconate titanate (PZT) array. It was found that the Rayleigh waves propagating across the Rayleigh wave receiver array made of the PVDF film had a lower attenuation rate of 0.15 dB/mm compared to that of 0.30 dB/mm of the PZT array. Multiple Rayleigh wave receiver arrays made of the PVDF film were applied for monitoring surface fatigue crack initiation and propagation at welded joints under cyclic mechanical loading. Cracks with a depth range of 0.36-0.94 mm were successfully monitored.

Active Ultrasonic Structural Health Monitoring Enabled by Piezoelectric Direct-Write Transducers and Edge Computing Process.

While the active ultrasonic method is an attractive structural health monitoring (SHM) technology, many practical issues such as weight of transducers and cables, energy consumption, reliability and cost of implementation are restraining its application. To overcome these challenges, an active ultrasonic SHM technology enabled by a direct-write transducer (DWT) array and edge computing process is proposed in this work. The operation feasibility of the monitoring function is demonstrated with Lamb wave excited and detected by a linear DWT array fabricated in situ from piezoelectric P(VDF-TrFE) polymer coating on an aluminum alloy plate with a simulated defect. The DWT array features lightweight, small profile, high conformability, and implementation scalability, whilst the edge-computing circuit dedicatedly designed for the active ultrasonic SHM is able to perform signal processing at the sensor nodes before wirelessly transmitting the data to a remote host device. The successful implementation of edge-computing processes is able to greatly decrease the amount of data to be transferred by 331 times and decrease the total energy consumption for the wireless module by 224 times. The results and analyses show that the combination of the piezoelectric DWT and edge-computing process provides a promising technical solution for realizing practical wireless active ultrasonic SHM system.

Thermal Sprayed Lead-Free Piezoelectric Ceramic Coatings for Ultrasonic Structural Health Monitoring.

Structural health monitoring (SHM) is growing rapidly with strong demand from industrial automation, digital twins, and Internet of Things (IoT). In contrast to the manual installation of discrete devices, piezoelectric transducers by directly coating and patterning the piezoelectric materials on the engineering structures show the potential for achieving SHM function with improved benefits over cost. Until the recent years, high-performance lead-free piezoelectric ceramic coatings, including potassium-sodium niobate (KNN) and bismuth sodium titanate (BNT)-based coatings, are produced by thermal spray method. This article reviews the background and progresses of using thermal spray method for fabricating piezoelectric ceramic coatings and their values for SHM applications. The review shows the combination of environmentally friendly lead-free compositions, and the scalable thermal spray processing method opens substantial application opportunities. Ultrasonic SHM technology enabled by thermal-sprayed piezoelectric ceramic coatings is an important area where the lead-free piezoelectric ceramic materials can play with their technical competitiveness and commercial values over the lead-based compositions.

Direct-Write Piezoelectric Transducers on Carbon-Fiber-Reinforced Polymer Structures for Exciting and Receiving Guided Ultrasonic Waves.

Advancements in the structural health monitoring (SHM) technology of composite materials are of paramount importance for early detection of critical damage. In this work, direct-write transducers (DWTs) were designed for the excitation and reception of selective ultrasonic guided waves and fabricated by spraying 25- [Formula: see text]-thick piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TRFE)] coating with a comb-shaped electrode on carbon fiber-reinforced polymer (CFRP) plates. The characteristics and performance of the ultrasonic DWTs were benchmarked with the state-of-the-art devices, discrete lead zirconate titanate (PZT) ceramic transducers surface-mounted on the same CFRP plates. The DWTs exhibited improved Lamb wave mode excitation (A0 or S0 mode) relative to the discrete PZT transducers. Moreover, high signal-to-noise ratio was obtained by effectively canceling other modes and enhancing the directivity with the periodic comb-shaped electrode design of the DWTs, despite the smaller signal amplitudes. The enhanced directivity overcompensates for lower amplitude attenuation, making DWT a good candidate for locally monitoring critical stress hot spot regions in the CFRP structure prone to early damage initiation. It is shown that pairing a DWT sensor with a discrete PZT actuator could further achieve balanced performance in both wave mode selection and signal amplitudes, making this combination really attractive for ultrasonic SHM.