Ultrasonic spectroscopy for the inspection of airspace structures

Nondestructive evaluation of airspace structures can be performed using ultrasonic spectroscopy utilizing the information in the frequency domain obtained due to the constructive and destructive interference of elastic waves. The application field of ultrasonic resonance spectroscopy (URS) is likely to increase rapidly with the growing application of layered structures in modem aircraft. The aim of this paper is to enlighten the potential and the limitations of the URS techniques. We start from explaining principles of URS applied to multi-layered structures and then we present a theoretical model that enables predicting the modal shapes and resonance frequencies of the thickness mode resonances occurring in multi-layered structures. The model also includes the piezoelectric transducer used for sensing the structure resonances. Presentation of the narrowband URS technique is illustrated with results of measurements performed using specially designed resonance transducers in carefully selected narrow frequency bands. We also present a novel method for sensing transducer's resonance based on the use of a phase locked loop (PLL).