NEAR-PLANE-WAVE ACOUSTIC PROPAGATION MEASUREMENTS IN THIN-LAYERS OF ADHESIVE POLYMER

This paper presents a novel wide bandwidth pulse transmission technique for the study of the interactions between near-plane-wave ultrasound and thin films (down to 50-mu-m) of adhesive polymer set between glass substrates. Acoustically thick transducers are clamped in coaxial alignment on either side of the glass substrates and short (less than 10 ns) acoustic transients are made to reverberate to and fro in the test bond. The signal received consists of time-resolvable and successively dispersed reverberations from the bond layer. It is digitized at 1 GHz and approximately corrected for the effects of transducer insertion and transient radiation coupling between the transducers. Frequency domain methods are then applied to estimate absorption coefficient, propagation velocity and the real and imaginary parts of the plane-wave elastic modulus, all as functions of frequency. Prelimary data obtained by this technique indicate that a number of adhesives display plane-wave velocity dispersion and absorption as a function of frequency that can be modelled by a relaxation process with a single time constant. A simple spring-dashpot model for an anelastic solid provides a mechanistic equivalent to the observed relaxation.