APPLICATION OF ULTRASONIC RESONANCE TECHNIQUE AT 300 MHz TO THE DETECTION OF DISBONDS IN

This paper describes the results of work on the feasibility of using high frequency ultrasound to determine .the degree of bondedness of multilayer structures. A sandwich structure of two identical metal sheets of thickness on the order of 25 ~m, bonded together by a dissimilar third layer of thickness 10 ~m was examined. The primary disbond was expected to be in the region of the interface between the top two metal layers. Frequency domain analysis using a Wiener-filtered FFT was used to extract a signature related to the properties of the upper metal strip as an acoustic resonator, which is strongly dependent on the boundary conditions on its back surface. The frequency range of 200 to 300 MHz was determined to be the appropriate range at which to perform the inspections. The sandwich structure was modelled forvarying degrees of bondedness, and frequency domain measurements were made on a number of samples using a 300 MHz acoustic transducer and electronic setup. Experimental results have indicated strong correlation between the depth and width of nulls in the Wiener-filtered spectrum and the state of bondedness. The results correspond well to the acoustic resonance of the disbonded upper metal layer as predicted by the theory.