Phase change measurement of ultrasonic shear waves on reflection from a curing epoxy system

Simple modelling shows that an ultrasonic shear wave with a frequency in the megahertz range, travelling in an elastic medium, that is subsequently reflected at normal incidence from an interface with a viscous medium will undergo a phase change equivalent to a temporal shift of the order of nanoseconds for realistic material properties. This suggests a new technique to observe and characterize curing epoxy adhesives in a reflection geometry that is preferable in many industrial situations. It has been shown by previous workers that in the case of reflection from a classic viscous liquid only the phase or the amplitude need to be measured in order to return the viscosity of the liquid. This paper explores the more general case for a viscoelastic substance where both amplitude and phase must be measured in order to return the viscosity and elastic modulus of the material under investigation. Non-contact electromagnetic acoustic transducers, generating wideband shear wave pulses in the 0.5-8 MHz frequency range, are used to perform ultrasonic measurements of a shear wave reflected from the interface of an aluminium-curing epoxy resin sample. The viscosity and the shear elastic modulus of the epoxy resin sample are calculated over the cure cycle. As expected the accuracy is limited by significant measurement errors in the reflected amplitude