Laser ultrasonic inspection for crack detection in a rotating tube under dynamic load

Laser-based ultrasonic inspection is an emerging versatile non-contact technique for defect detection in a variety of critical components. This article reports on the adaptation of this method for the inspection of rotating cylindrical components undergoing varying stress, by detecting the dynamically changing mechanical response of cracks, which goes along with modulation of the guided acoustic waves that encounter the cracks. Optical interferometry was used for the remote detection of photo-acoustically generated guided waves. An application of laser ultrasonic to investigate guided waves and their interaction with cracks detected by a Laser Doppler Velocimetry (LDV) on cylindrical structures under dynamically changing stress is presented. Mode conversion, which occurs when guided waves are encountering a crack, is also investigated. The experimentally assessed effects of defects on the envelope, spectrum and other receiving wave characteristics are analysed and exploited for their detection and characterization, giving a basis for the use of laser ultrasonic for Non Destructive Testing ( NDT).