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.

This article presents a comparison of data obtained from a low-temperature cure of an epoxy/amine system by three independent cure monitoring techniques: ultrasonic wave propagation, dielectric permittivity, and nuclear magnetic resonance. The sizes and thermal histories of the samples studied by the three techniques were controlled for comparability between the methods. The three techniques gave consistent information on the progress of cure and were complementary, in that each was particularly sensitive to different stages of the cure process.

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

Ultrasonic pulse-echo reflectometry provides a convenient method to measure the acoustic impedance of a layer of unknown material bonded to a substrate of another material whose acoustic properties are known. The amplitudes of echoes from the interface are used to calculate the interface reflection coefficient and, from this and a knowledge of the properties of the substrate material, the unknown impedance is obtained. The technique has potential for assessment of adhesive cure and measurement of mechanical moduli. The calculation is poorly conditioned in that small errors in raw echo data can result in unacceptably large errors in the estimate of the required impedance. This paper gives an analysis which demonstrates how bias and variance errors in raw data lead to bias and variance errors in the calculated reflection coefficient and how these lead to errors in impedance estimation which can be very much greater and which depend on the value of the coefficient of reflection at the interface. Experimental results are given which verify the analytical predictions of the error generation process. The effects of interface conditions such as transducer coupling film and paint coatings are considered quantitatively by a simulation of the wave system. The effects of substrate surface roughness are considered briefly in a series of experiments.

Abstract:There has been an increase in the use of composite materials in the aerospace industry which is driving a need for new NDT techniques that can rapidly scan large structures and provide quantitative data on the material integrity. In many applications there are common requirements for the ultrasonic inspection of composites for porosity, delaminations, foreign body contamination and fibre wrinkling. Traditional methods of ultrasonic inspection require the use of a single point probe or a multiplexed group of probes. NDT Solutions have developed new inspection techniques based on ultrasonic arrays housed within a rubber coupled wheel sensor. The wheel can be manually applied or scanned over the structure with an automated scanning system.Applications for the technology include real time imaging and reporting of impact damage, porosity sizing, delaminations and fibre wrinkles. Examples and case studies from a range of aerospace applications are given including manual and automated inspection of small and large area composites. Introduction:In recent years the number of applications for phased array inspection has increased dramatically, with many applications for weld and crack detection. Most of the applications utilise a sector scan or a B-scan raster mode to enable imaging of the defect as shown in figure 1.