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.

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.

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.

A New Highly Conductive Direct Gap p-Type Semiconductor La1-xYxCuOS for Dual Applications: Transparent Electronics and Thermoelectricity.

The absence of a high-performance p-type transparent semiconductor still remains as a roadblock to the development of future generation optoelectronics. Here, a highly conducting p-type transparent semiconductor based on Y incorporated LaCuOS oxychalcogenide is achieved for the first time, and the maximum Y substitution to obtain a single phase is found to be 25%. By enhancing both the hole mobility and concentration of the LaCuOS phase, Y-substituted oxychalcogenide single-phase La0.75Y0.25CuOS exhibits an outstanding p-type conductivity of 89.3 S·cm-1 with high optical transparency, which is the highest among all transparent oxychalcogenides with a band gap above 3 eV reported so far. The structural, electronic, and optical as well as the thermoelectric properties of La1-xYxCuOS with a different Y substitution level are investigated, and the power factor was greatly enhanced to 4.322 µW m-1 K-2 after Y substitution. The highly performing diode based on a p-type La0.75Y0.25CuOS thin film and n-type Al-doped ZnO heterojunction with a high rectifying ratio of 300 is demonstrated, indicating its promising aspect for the next generation of invisible electronics and optoelectronics.

Highly stretchable and transparent films based on cellulose.

Developing natural products to replace synthetic plastics is necessary due to the serious environmental problem of non-biodegradable plastic waste. Cellulose is the most abundant natural material that is from all plants. This paper reports our success in achieving the most stretchable and transparent cellulose-based films through a very green process. The films are highly transparent, with 90% transparency per 100 µm at 550 nm wavelength. The films are very flexible, able to be twisted and folded greatly without breaking. The film can reach an unprecedented maximum of 233% elongation at break. The success in such highly stretchable and transparent films sheds light on the great promise of cellulose for potential applications to replace synthetic plastics, such as transparent and stretchable substrates for flexible and stretchable electronics, transparent and stretchable films and various products. The mechanisms in achieving high transparency, flexibility and stretchability of the cellulose-based films are discussed.