Interest in the interaction of acoustic waves with particulate mixtures has a long historydating back to the work of Rayleigh in the 19th century. This interest has intensified over the last fifteen years as advances in electronics and instrumentation science have brought the possibility of using ultrasound to characterize colloidal mixtures both in the laboratory and in-process, and in both of these contexts a small number of instruments are currently in use. The characterization of colloidal mixtures by ultrasound requires a formal theoretical basis which relates the properties of the mixture, particularly the dispersed phase particle size distribution (PSD), to the complex wavenumber governing propagation. The number of theoretical treatments is vast, having evolved over more than a century. This paper is intended to provide a review of these developments in a form which will enable new researchers in the field to climb a very steep learning curve in a relatively short time. We discuss definitions and production techniques for colloidal mixtures and the basic physical phenomena underlying wave propagation through them. We identify two approaches to the propagation problemscattering and coupled-phase; these are treated both separately and comparatively, particularly in relation to limitations that arise when the concentration of particles is high and the basic theories break down. We introduce the basic method for the measurement of PSD and show how dynamic effects such as flocculation and crystallization can be observed and modelled. The core of all ultrasonic characterization procedures is the physical measurement of the ultrasonic wave attenuation coefficient and phase velocity as functions of frequency; here we discuss these techniques on the basis that what is observable or measurable about a colloid depends on both its physical properties and the frequency bandwidth available for measurement. This paper concludes with our view on future developments of measurement technique and theoretical treatments.

The transmission of Lamb waves across adhesively bonded lap joints is investigated using finite element analysis. The studies consider three modes for excitation and reception, s0, a0, and a1, applied to lap joints consisting of parallel aluminum sheets bonded with an epoxy adhesive. Transmission coefficient results for a two-dimensional range of bond thicknesses and bond overlap lengths are presented for all three modes. The transmission coefficients are calculated from the spectra of the received and transmitted signals using an approach which is insensitive to the presence of multimode signals and reverberated signals, and which approximates to a power transmission coefficient. Detailed analysis is then performed for one of the modes in order to investigate the nature of the mode conversion in the overlap region of the joint. It is found that the relative amplitudes of the different modes which propagate in the overlap region can be estimated reliably and simply from the properties of the incident wave mode. As well as demonstrating the physics of the mode conversion behavior, the study provides a basis for the selection of modes for nondestructive evaluation (NDE) of the bond region and for measuring the bond dimensions.

The aim of this paper is to present a unified approach to the calculation of the complex wavenumber for a randomly distributed ensemble of homogeneous isotropic spheres suspended in a homogeneous isotropic continuum. Three classical formulations of the diffraction problem for a compression wave incident on a single particle are reviewed; the first is for liquid particles in a liquid continuum (Epstein and Carhart), the second for solid or liquid particles in a liquid continuum (Allegra and Hawley), and the third for solid particles in a solid continuum (Ying and Truell). Equivalences between these formulations are demonstrated and it is shown that the Allegra and Hawley formulation can be adapted to provide a basis for calculation in all three regimes. The complex wavenumber that results from an ensemble of such scatterers is treated using the formulations of Foldy (simple forward scattering), Waterman and Truell, and Lloyd and Berry (multiple scattering). The analysis is extended to provide an approximation for the case of a distribution of particle sizes in the mixture. A number of experimental measurements using a broadband spectrometric technique (reported elsewhere) to obtain the attenuation coefficient and phase velocity as functions of frequency are presented for various mixtures of differing contrasts in physical properties between phases in order to provide a comparison with theory. The materials used were aqueous suspensions of polystyrene spheres, silica spheres, iron spheres, pigment (AHR), droplets of 1-bromohexadecane, and a suspension of talc particles in a cured epoxy resin.

An investigation is made into the errors in estimated position that are caused by noise and drift effects in stationary accelerometers. An analytical study is made into the effects of biases in the accelerometer data and the effects of changing the cut-off frequency in the anti-aliasing filter. The root mean square errors in position are calculated as a function of time and sampling frequency. A comparison is made between the theoretical results and experimental data taken from two commercial accelerometers. Recommendations are made regarding the calibration of accelerometers prior to their use in practical situations.

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.

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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.

With the rapidly escalating usage of composite materials, not only in military aircraft but in civil airliners as well, production NDT throughput is already stretched to its limit internationally. NDT data analysis is set to become the bottleneck preventing the required rise in production rates of composite civil aircraft in the next few years. Thus there is an urgent requirement for rapid, automated analysis of up to a Terabyte of data per airliner, escalating to over 200 Terabytes per year-worldwide. The primary aim of automated analysis is to release operators from the time-consuming analysis of all scans and focus operator attention on non-compliant structures. A secondary aim is to provide threedimensional quantitative information that lightens the operator's decision-making burden. Through advanced characterisation methods, NDT also has the potential to provide crucial feedback to control the composite production process, increase production yield and decrease costs. Current analysis methods for ultrasonic scans produce through-thickness average parameters, which provide little useful information to assist the stress analysis for defects, or the production process. Three-dimensional characterisation of defects can increase yield by informing the concession/disposition process for defects. For future process control, information is required about the 3D distribution of material properties in the structures on the production line, providing comprehensive long-term trend analysis. 'ANDSCAN' is a Registered Trademark of QinetiQ Ltd. 'StackScan', 'Ply Fingerprinting' and 'PinPoint' are Trademarks of QinetiQ Ltd. 'MLM-PropMat' is a Trademark of the University of Nottingham. Patents have been filed by QinetiQ Ltd covering the technology described in this paper.