Ultrasound nondestructive Testing (NDT) of materials, processes, and structures has been broadly applied to space, defense, aerospace, civil and energy industries. Especially, industrial facilities such as nuclear power plants or chemical plants contain numerous high-temperature (< 700°C) pipes and pressure vessels that require continuous health monitoring for instant detection of structural flaws.
In this work, a new ultrasound NDT method based on photoacoustic Lamb wave detection was proposed for high temperature NDT. AlN single crystal is chosen as the sensor material primarily due to its temperature-robust piezoelectric properties and high Curie temperature (> 2000 °C). The fundamental symmetric (S0) and antisymmetric (A0) mode Lamb waves generated by the pulsed laser were successfully detected by the AlN sensor on a stainless steel plate. The time-of-flights (ToFs) of the S0 and A0 mode waves proportionally increased with the distance (D) between the laser source and the sensor, and almost no attenuation of the amplitude was observed as the distance increases. At the HT NDT experiment, the combination of non-contact, portable laser source as a Lamb wave generator and temperature-robust NDT sensor made of AIN has shown its great capability to detect the Lamb waves at elevated temperatures up to 800 °C. Based on the ToFs analysis of S0 and A0 mode Lamb waves propagated in a specimen with defects, the capability of this NDT method to localize the defect even at high temperature (~800 °C) has been verified.