A Matrix Array Technique for Evaluation of Adhesively Bonded Joints

Adhesive bonding technology is playing an increasingly important role in automotive industry. Ultrasonic evaluation of adhesive joints of metal sheets is a challenging problem in Non-Destructive Testing due to the large acoustic impedance mismatch between metal and adhesive, variability in the thickness of metal and adhesive layers, as well as variability in joint geometry. In this paper, we present the results from a matrix array of small flat ultrasonic transducers for evaluation of adhesively bonded joints in both laboratory and production environments. The reverberating waveforms recorded by the array elements are processed to obtain an informative parameter, whose two-dimensional distribution can be presented as a C-scan. Energy of the reflected waveform, normalized with respect to the energy obtained from an area with no adhesive, is a robust parameter for discriminating "adhesive/no-adhesive" regions. A 52-element, 15-MHz matrix array of square transducers was used in the experiments. The pitch of the array was 1.25 mm, and the size of the tested area was about 10 mm x 10 mm. Statistical analysis of the experimental data shows that the two-sigma intervals of the output parameter are approximately 0.55±0.2 and 1±0.2 for "adhesive" and "no adhesive" regions, respectively, providing satisfactory differentiation. The instrument and technique were subsequently tested in automotive assembly plants on various closure and body components. Comparison of the adhesive width obtained ultrasonically with that measured after teardown displayed an accuracy of 96% for obtaining the same bead width within 1 mm.