R.E. Challis

Near-plane-wave acoustic propagation measurements in thin layers of adhesive polymer

The authors present a novel wide bandwidth pulse transmission technique for the study of the interactions between near-plane-wave ultrasound and thin films (down to 50 mu m) 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. Preliminary 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 for an anelastic solid provides a mechanistic equivalent to the observed relaxation.

Ultrasonic wave propagation in colloidal suspensions and emulsions: a comparison of four models

Four models that describe the propagation of ultrasound through two-phase media are compared. The models differ in the complexity of their mathematical formulations, and the applicability of any model to real colloidal systems is limited by the assumptions implicit within that derivation. Simulated comparisons of ultrasonic phase velocity and attenuation are presented for three suspensions that are physically diverse. The Allegra and Hawley model is shown to give the most thorough interpretation of acoustic propagation through a two-phase system.

Ultrasonic wave propagation in colloid suspensions and emulsions: recent experimental results

Early theories of acoustic propagation such as those by Urick, and Urick and Ament, are attractive because of their computational simplicity, but limited in application. The more complex models of Allegra and Hawley, and Harker and Temple, have since gained application. A wideband ultrasonic attenuation and phase velocity spectrometer have been used to determine the validity of these models for a range of colloidal materials. Measurements taken validate the Allegra and Hawley model for colloids with spherical base particles. The Harker and Temple model is used to infer changes in the flocculation state of kaolin slurries which result from chemical intervention.

Errors and uncertainties in the ultrasonic pulse-echo reflectometry method for measuring acoustic impedance

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.

Digital signal processing applied to ultrasonic absorption measurements

The signal pathway in a short pulse (< 10 ns) transmission system is described for two circular thick piezoelectric transducers aligned coaxially on either side of a small (30 ml) liquid filled test cell. Discrete models are used to describe transducer insertion and transient radiation coupling. Approximate inverse models are developed and used to deconvolve transducer and coupling effects so that acoustic absorption in the medium between the transducers can be estimated. The methods are applied to a prototype near plane wave absorption spectrometer for liquid materials. The instrument is controlled by a small computer that also carries out the signal corrections and other processing. The system is used to estimate absorption versus frequency (α, αλ, or α/f2) over bandwidths approaching 60 MHz. It has many advantages over conventional methods in that measurement is rapid (< 100 ms) and only a single pair of transducers is required. Preliminary results show excellent agreement with spot frequency absorption measurements. The apparatus has potential applications to absorption studies in non-stationary chemical systems.

Ultrasonic NDE of adhered T-joints using Lamb waves and intelligent signal processing

Abstract This paper examines the application of artificial neural networks to the estimation of geometrical parameters of an adhered aluminium T-joint using ultrasonic Lamb waves (s0 + a1). Modulus FFTs of received signals were applied as inputs to conventional feed-forward networks, which were trained using the delta rule with momentum. The success rate of various network structures in recognising bond categories was studied as a function of the density of information applied to the network inputs and the number of hidden nodes in the network. An optimum network structure appears to exist that will solve a number of problems of this type.

Compression wave NDE of adhered metal lap joints: uncertainties and echo feature extraction

Abstract This paper deals with the use of compression wave pulse-echo ultrasound for the NDE of simple adhered metal lap joints in which both the bondline and the adherend thicknesses may be variable and unknown—a situation commonly found in automotive superstructures. Of specific interest are the detection of void disbonds at near and far adherend/adhesive boundaries, and the estimation of the characteristic acoustic impedance of the adhesive as an indicator of cure. Modelling of the wave system indicates how absorption in the adhesive, reverberations in the adherends, and the transducer response affect the signal detections and the robustness of signal calculations required for NDE.

ULTRASONIC ABSORPTION AND VELOCITY DISPERSION MEASUREMENTS IN THIN ADHESIVE LAYERS

Abstract This paper presents a new technique for the measurement of plane wave absorption and propagation velocity as functions of frequency in thin layers of adhesive polymer. The adhesive layer thickness can be in the range 0.05–2 mm and the frequency range of measurement from d.c. up to 30 MHz, although 100 MHz is achievable in principle. The technique is based on the use of acoustically thick transducers to generate and receive very wide bandwidth pulses, and signal processing of received data to calculate absorption and velocity as functions of frequency. Preliminary results are presented.

Ultrasonic compression wave propagation in adhesive polymers described as a signal filtering process

The ability to test adhesively bonded metal structures non-destructively requires an understanding of ultrasound wave propagation in adhesive materials. This paper presents the results of an experimental study of wave absorption and phase velocity dispersion in a number of structural adhesives in terms of the frequency response and the time domain impulse response of propagation in a thin layer of material. A simple single relaxation time model provides a phenomenological basis on which to consolidate the measured data into a form that can be used to model wave propagation in more complex structures containing an adhesive. It is shown that high rates of attenuation, increasing with frequency, will seriously limit the frequency bandwidth available for ultrasonic non-destructive testing of adhered structures. Extensions to the model are appended that will provide the basis for its implementation by means of linear electric network simulation packages.

A model fitting approach to the broad band measurement of ultrasonic wave velocities in thin samples of engineering material

This paper presents a novel broad band short-pulse ultrasonic system for the simultaneous measurement of compression and shear acoustic wave velocities in thin solid slabs. Using a goniometer stage a test sample is immersed in a water cell between two axially aligned thick ultrasonic transducers. Compression and shear wave reverberations are generated in the test material and the resulting compression waves in the water on the receiver side are received and digitized. Each wave reverberation can be separately time resolved and time-of-flight (TOF) measurements can be made. A time domain model of the wave system in the solid is presented with comparisons with existing frequency domain models. The measured arrival times of the successive reverberations are input to a computer algorithm derived from the time domain model which, using a parameter fitting routine, calculates the acoustic wave velocity for both the compression and shear reverberations in the sample. The problem of interference between successive reverberations in the time domain is greatly reduced due to the wide bandwidth/short pulse operating mode possible with acoustically thick transducers.