Inspecting multilayered airspace structures using ultrasonic narrowband spectroscopy

Abstract

Recently, an increased need for effective NDE tools for layered structures has been observed with the increased use of bonded structures (e.g. GLARE) and carbon fiber reinforced panels (CFRP) in modern aircraft constructions. NDE of multilayered airspace structures can be performed using ultrasonic spectroscopy which makes use of the information in the frequency domain generated due to the constructive and destructive interference of elastic waves. The paper presents a novel narrowband ultrasonic spectroscopy (NBS) technique which utilizes specially designed resonance transducers with carefully selected narrow frequency bands. The NBS is based on sensing electrical impedance of the piezoelectric transducer in the vicinity of its resonance. The theoretical model which enables predicting the resonance frequencies and modal shapes of the thickness mode resonances occurring in multi-layered structures is presented in the first part of the paper. The model includes the KLM equivalent circuit of a piezoelectric transducer used for sensing the resonances. Theoretical and experimental results are presented showing the relation of the impedance plane indications to the parameters of adhesive in aluminum sandwich structures. Results of the mechanized inspection CFRP specimens are presented in the second part of the paper.