ATTENUATION OF ULTRASONIC INTERFACE WAVES ON METAL-POLYMER-METAL BOUNDARIES

Abstract
Ultrasonic Stoneley and leaky interface wave techniques have been applied to the characterization of the bondline structure and the near-surface bulk properties of flat metal alloy samples separated by a thin layer of polymer adhesive. Interface wave energy is confined to the region containing the polymer and extending approximately one ultrasonic wavelength into each of the metal substrates. The amplitude and distribution of localized particle motions both normal and parallel to the interface are sensitive to point defects at the polymer-metal boundaries as well as to grain boundaries and dislocations near the metal surfaces and to microcracks in the polymer. In this paper, the measured sensitivity of interface wave attenuation to defects near the bondline and to variations in the viscosity of the adhesive layer is compared with theoretical changes predicted by the model developed by Rokhlin. Two experimental techniques utilized to obtain these measurements are presented. First, differential interferometric optical measurements of interface wave attenuation due to defects near glass-polymer-metal boundaries are discussed. This sensitive laser probe method allows the detection of averaged periodic surface displacements as small as 4 x 10^-3Å and localized reflections and scattering from individual buried defect sites. Next, pitch-catch and pulse-echo methods which use variable-angle wedge transducers to generate and receive modified interface waves and to measure large bondline defects and adhesive viscosity are described. Potential applications to the characterization of completed polymer adhesive bonds are suggested.