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Reflectivity Estimation using an Expectation Maximization Algorithm for Ultrasonic Testing of Adhesive Bonds

This paper discusses the use of the pulse-echo technique of ultrasonic testing (UT) for nondestructive testing (NDT) of adhesive bonds of thin metal sheets (less than 1 mm thick) in which the thickness of the adhesive layer and the metal sheets, as well as the acoustical properties of the adhesive, are unknown. The aim of UT of adhesive joints is to assess the integrity of the front (interface close to the interrogating transducer) and rear adherent/adhesive interfaces. To achieve this goal, accurate estimation of the reflectivity is required. In this study, a model-based approach was applied to signals obtained from adhesively bonded joints in order to estimate their reflectivity. The transducer wavelet was modeled by gaussian echoes and the reflectivity (the desired system response) was estimated by using the expectation maximization algorithm. The decay rate of reverberations in the front metal sheet calculated from the estimated amplitudes of the sequence was used as a basis for detection of disbond in the front interface. By applying the proposed technique, it was possible to detect void-disbonds and the presence of anomalies, such as grease and water in the front interface of a joint made by 0.5 mm thick aluminum plates. Disbonds in the rear interface of this joint were also detected by examining the phase inversion of the echo reflected from the rear interface.

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