Laser ultrasonic imaging and damage detection for a rotating structure

Abstract

This study presents a laser ultrasonic imaging and damage detection technique that creates images of ultrasonic waves propagating on a rotating structure and identifies damage. Laser ultrasonics is attractive for nondestructive testing mainly because of two reasons: (1) ultrasonic waves can be generated and/or measured in a noncontact manner and (2) even a small defect can be detected when laser ultrasonic scanning produces ultrasonic images with high spatial resolution. However, when it comes to a moving target, it becomes challenging to create reliable ultrasonic images. In this study, ultrasonic wave propagation images are obtained from a rotating blade using a pulse laser beam for ultrasonic generation, a galvanometer for laser scanning, and an embedded piezoelectric sensor for ultrasonic measurement. To properly estimate the laser excitation points during the scanning process rather than to precisely control the excitation points, a simple but rather effective localization technique is developed so that ultrasonic images can be constructed even from a moving target. Once the ultrasonic wave propagation images are created, damage on the target structure is visualized using a specially designed standing wave filter.

Non-contact damage detection on a rotating blade by Lamb wave analysis

“…According to the US Federal Aviation Administration (FAA), aircraft propeller integrity can be compromised due to e.g. delamination, corrosion, lightning strikes, stone nicks, etc [1] [2] [3]. Northrop Inc., under a contract from the US Air Force, set several requirements for flaw detection techniques: the inspection should be carried out with reliable sensors capable of detecting cracks, 1.25mm in length, situated in complicated locations such as sharp edges, apertures, and rivets [4].…”

“…One illuminates the sample with a high power pulsed (100 mJ per pulse) laser beam to excite propagating ultrasonic waves and another laser, usually a laser Doppler vibrometer (LDV) detects the surface waves. LU can analyze moving samples in hazardous environments [3] [18] [17]. Furthermore, it permits fast and large area scans, and it provides high spatial resolution imaging and remote access [9] [2].…”

“…Park et.al successfully tested a non-contacting method to image cracks in propellers but the experiment they performed was on a static aluminium plate [2]. They also carried out experiments on a moving propeller but they embedded transducers in the sample which makes it a contacting method [3].In another article, Park et.al suggested and tested a laser ultrasound damage detection technique on a static wind turbine blade. Finally, in reference [13], Raišutis et.al imaged defects on a static wind turbine blade using air coupled transducers.…”

Random Sequence for Optimal Low-Power Laser Generated Ultrasound

“…The more utilized instruments for this kind of inspection are narrow band, piezoelectric probes which need the contact with the inspected object to be guaranteed. Non-contact applications use high-power pulsed lasers as a source of elastic waves [3][4][5][6], mainly in order to allow an on-line inspection [7]. These waves are generated by the material expansion and contraction due to the laser quick heating and the subsequent sudden cooling.…”

Study on the most influential parameters in low-power laser generated ultrasound

“…Application of a transient heating creates a sudden expansion and a consecutive contraction in the material, generating longitudinal, shear and surface ultrasounds [3]. The technique's fundamental advantage is the possibility of a non-contact inspection, therefore usable in operation too [7][8][9][10]. The thermo-elastic effect is generally obtained through pulsed lasers: a brief and single pulse (in ns) with a power of approximately 1 MW, creates a broad band oscillation with frequencies up to hundreds of MHz.…”