Ultrasonic Spectroscopy for the Identification of Defects and Environmental Disturbances

Ultrasonic health monitoring of structures under realistic operating conditions is very difficult because ultrasonic waves are influenced by not only structural defects but also environmental disturbances. We examined a method of ultrasonic spectroscopy in this investigation to classify signal changes as either structural or experimental. We used aluminum plates to investigate the ultrasonic propagation characteristics with respect to the growth of defects (fatigue cracks, slits, and drilled holes) and the change in environmental disturbances (water drops, temperature, and load). The frequency dependency of the propagation delays and amplitudes of tone-burst waves differed for defects and environmental disturbances. Defects could be distinguished from environmental disturbances by monitoring the delay-amplitude plot lines of the tone-burst waves with increasing frequency.