Narrowband ultrasonic spectroscopy for NDE of layered structures

NDE of airspace sandwich structures is often performed using a simple resonant transducer sensing the information in the frequency domain obtained due to the constructive and destructive interference of elastic waves. Application field of ultrasonic narrowband ultrasonic spectroscopy (NBS) is likely to increase rapidly with the growing application of layered structures in modern aircraft, for example, GLARE. The aim of this paper is to enlighten the potential and the limitations of the NBS techniques. Operation of the NBS technique based on the measurements performed using a specially designed resonance transducer with carefully selected narrow frequency band is explained in detail. A novel method, based on the use of impedance plane for sensing transducer resonance, is presented. Theoretical results illustrate the relation of the impedance plane indications to the adhesive strengths in aluminium sandwich structures. Finally, practical application of the proposed technique on mechanised inspection of carbon fibre reinforced structures is presented.