The tracking of the cure of epoxy adhesives and the assessment of the cure state of adhesive bondlines joining engineering components are important for quality assurance during manufacture and for the safe functioning of manufactured assemblies in the field. Ultrasound can be used to give estimates of the compression modulus of curing and cured materials and thereby provide a means to assess non-destructively the cure state of adhered joints during manufacture and in service. These techniques are at present difficult to apply and are predominantly empirical in that little is known about the relationships between the measured ultrasound data and the evolving molecular structure of the adhesive as it cures. The present paper describes the application of a group of physical techniques that can be used to characterise the polymer structure during cure, with the aim of relating these to phenomena measurable by ultrasound. Wide angle X-ray scattering (WAXS) provides a basic measure of polymer chain formation, which is seen to correlate closely with the compression modulus as it develops during cure. Low resolution nuclear magnetic resonance (NMR) provides a means to observe the mobility of bound hydrogen nuclei and thereby to track the change in state of a resinhardener system from a viscous liquid to a crosslinked solid. The NMR data obtained during cure correlated well with compression modulus development. Ultrasonic shear wave spectrometry indicates when a curing material can first support shear motions and this agrees well with NMR data and with specifications of gel point given by manufacturers. Ultrasonic compression wave absorption data provide frequency dependent patterns that change during the cure cycle and that can be explained on the basis of the results of the WAXS, NMR, and shear wave experiments. These changing patterns have potential for tracking cure using low cost ultrasonic techniques, the results of which can be related to phenomena taking place on a molecular scale.PRC/1575
An acoustic near plane-wave absorption spectrometer has been developed for use in small samples of liquid (30 ml) over bandwidths of up to 60 MHz, using a two transducer short-pulse transmission technique. The instrument is controlled by computer, and on-line digital signal processing is used to correct for transducer insertion, radiation coupling, and the transient responses of transmitter and receiver electronics. The raw data acquisition time is short and the instrument in its present form could be used to estimate absorption at 100-ms intervals. The instrument can be used on both stationary chemical systems and systems undergoing chemical reaction. Experimental results are presented that show excellent agreement with absorption measurements by traditional (slower) techniques.
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 pm) 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 lor an anelastic Solid provides a mechanistic equivalent to the observed relaxation.
Absorption of ultrasound in adhesive materials is significant in forming ultrasonic signals obtained in nondestructive evaluation (NDE) procedures, and may also have diagnostic value in the evaluation of adhesive materials themselves. This paper investigates the effects of filler particles in adhesive polymers on ultrasonic compression wave absorption and phase velocity dispersion as functions of frequency. Wave propagation is affected by relaxations in the continuous polymer phase as well as by scattering at filler particles. The complex compression wave number for the composite material is derived on the basis of conventional theories of scattering in randomly distributed fields of particles, together with a formulation of the particle diffraction problem based on a combination of the cases for solid particles in solid continua and solid particles in liquid continua first proposed by Ying and Truell, and Allegra and Hawley, respectively. Experiments are described which demonstrate the effects of relaxations and scattering on absorption and phase velocity as functions of frequency in an epoxy material containing a mineral filler. Good agreement is obtained between theory and experiment.
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