Amplification of laser diode-induced photoacoustic signals for non-destructive testing of mechanical components

The present work highlights that the laser diode, acting as an ultrasonic source by photoacoustic effect, can be successfully employed for defect detection in mechanical components; the performed investigation specifically involves a railway axle with cracks on the body and fillets, respectively 13 mm-deep and 2 mm-deep. To ease the inspection process, a methodology for amplification of the ultrasound is introduced which is based on the appropriate choice of the TTL signal modulating the diode; amplification is achieved by constructive interference between two ultrasonic signals: the first is induced by the dilation resulting from the laser ignition, the second conversely derives from the contraction obtained when the laser is powered off. The proposed amplification methodology allows ultrasonic energy to be focused in a narrow range of frequencies, promoting the use of traditional detection devices like narrowband probes. The developed inspection system, which takes advantage of a 20 W source, enables the identification of the ultrasonic pattern and defect detection regardless of the application of post-processing techniques on the acquired signals: allowing for excitation of high-amplitude ultrasound without directly contacting the component, the laser diode represents a suitable source for the non-destructive inspection of the axle while it rotates (i.e., during operation). Thanks to the cost reduction achieved in comparison with the use of more traditional pulsed lasers, the diode lends itself to large-scale application in the field of non-destructive testing on mechanical structures.