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 paper 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.

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.