Comparative Study of Laser Doppler Vibrometer and Capacitive Air-coupled Transducer for Ultrasonic Propagation Imager and the New Development of an Efficient Ultrasonic Wavenumber Imaging Algorithm

Damage detection techniques using guided waves have been studied for decades with very few successful real-world applications. The recent development with the full wavefield technique using the Ultrasonic Propagation Imager (UPI) is one of those few exceptions. In this paper, we study two non-contact sensors: the laser Doppler vibrometer and the capacitive air-coupled transducer in the context as the sensing modules for the UPI. The aim of this paper is to provide a comprehensive study for optimisation of the two sensors, as well as a comparison between them for use in the UPI. First, the parameters for laser ultrasonic measurement of each sensor were studied: surface treatment, measurement angle and stand-off distance in the case of the laser Doppler vibrometer and measurement angle, lift-off distance and bias voltage in the case of the capacitive air-coupled transducer. Two optimised sensors were then compared in terms of their ability to detect damages using the UPI. Also, in this paper, we presented the ultrasonic wavenumber imaging (UWI) algorithm with the new development towards an efficient implementation. The uniqueness of the UWI algorithm with the capability of damage size estimation makes this algorithm very attractive for the future study with full wavefield signal processing.