Cut-off properties of leaky surface waves for the evaluation of adhesive-adherend interlayers by acoustic microscope

The standard surface pre-treatment used on aluminium adherends in aerospace applications produces a porous oxide structure on the adherend surface. The adhesive penetrates some distance into the pores to form a microcomposite. The thickness of these interlayers in a typical aluminium-epoxy joint is of the order of 1 /spl mu/m only. Surface acoustic wave measurements have been carried out with an acoustic microscope at frequencies between 220 and 980 MHz for different thicknesses of oxide layer. A good agreement has been found between the predictions and the acoustic microscope measurements. Attention has been focused on the anomalous behaviour of the attenuation around the cutoff.