On the use of two emerging laser-based flaw-detection techniques - Considerations and practicalities

The pursuit of non-contact techniques for detecting ultrasonic waves propagating on the surface of solids is stimulated by the possibility of gaining some advantages over conventional contact methods. Non-contact methods are becoming a milestone in performing non-destructive evaluation in several industrial fields, where hazardous environments, narrow spaces, high temperatures, and complex geometries are encountered. Such methods exploit photo-acoustic interactions between light and sound waves. This work compares two non-contact laser-based methods for detecting artificial flaws of different sizes in metallic specimens. It uses multi-signal homodyne interferometry and single beam deflectometry to record the ultrasonic waves produced by a piezoelectric probe capable of generating surface Rayleigh waves. The results were analysed in both time and frequency domains. Moreover, the signal-to-noise ratio was evaluated for both measurement systems by changing the distance between the defect and the point where the signals were collected. The results show the main differences between the two techniques and the practicalities associated with their usage. The employed refractometric method has a lower sensitivity to surface roughness than the interferometric method. On the other hand, the interferometric technique allows for higher spatial resolution, wider frequency bandwidth and a higher detectable frequency (i.e., detecting low-sized defects).