Ultrasonic evaluation of adhesively bonded sheet-metal joints is a challenging problem due to the large acoustic impedance mismatch between metal and adhesive, and variability in the thickness of metal and adhesive layers. In this paper, an ultrasonic non-destructive technique capable of robust evaluation of adhesive coverage in bonded metal sheets is presented. It is shown that an algorithm based on the energy of the reflected waveform weighted within a certain time gate is an efficient and robust parameter for discrimination between no/adhesive areas. To make the processing results robust, normalization of the calculated energy with respect to a reference value is used. The reference values can be obtained by processing the array responses recorded for a region with no adhesive. This algorithm was applied using a 52 element, 15 MHz matrix array of square transducers with 1.25 mm pitch for two-dimensional visualization of adhesive distribution under the metal sheet and estimation of bond width. The developed technique has been successfully tested in laboratory and industrial environments. The measurement accuracy of the width of the adhesive bead is about 1 mm.
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