Combined compression and shear wave ultrasonic measurements on curing adhesive

Abstract

This paper describes an ultrasonic technique to study the propagation of wide bandwidth compression and shear wave pulses in a curing adhesive. A temperature controlled water filled test cell with transducers placed at either end is used to couple ultrasound into a thin sample of adhesive. A novel sample holder is employed to contain the uncured liquid adhesive between thin polymer films to stop water ingress and a high-precision goniometer is used to align the sample with respect to the transducers. Consecutive normal and oblique incidence measurements are made at intervals during the adhesive cure. The oblique angle is selected to enable a shear wave to be excited in the adhesive sample by mode conversion. This occurs as soon as the adhesive is able to support shear displacements and hence the detection of the transition from liquid to solid state is possible. The compression and shear wave pulses are analysed in the frequency domain using Fourier analysis and this facilitates calculation of the frequency-dependent compression and shear wave absorption coefficients and phase velocities. From these measurements it is possible to calculate the complex bulk and shear moduli. Results are presented for a number of commercially available adhesives, and it is shown that ultrasound data signatures can be related to aspects of cure such as its rate and `gel point', as well as providing quantitative measurement of the elastic moduli.