Marion E. Unwin

Models of ultrasonic wave propagation in epoxy materials

This paper is concerned with modeling ultrasonic wave propagation in epoxy materials to better understand NDE procedures and to provide reliable input to more complex models of guided wave propagation in layered structures. Different physical models are considered in the context of how well they simulate the (known) linear relationship between bulk wave attenuation coefficients and frequency. The identified models are then extended to simulate wave propagation in materials with mechanical properties, which vary gradually in the spatial dimension. This is achieved using electric circuit transmission line analogs to the viscoelastic mechanical system. Verifying experimental results are included.

The use of ultrasonic imaging to evaluate the effect of protozoan grazing and movement on the topography of bacterial biofilms.

Aims:  This study evaluated the effect of protozoan movement and grazing on the topography of a dual‐bacterial biofilm using both conventional light microscopy and a new ultrasonic technique.

Effects of frame jitter in data acquisition systems

This paper focuses on the analysis of frame jitter and the impact of data acquisition architecture on the associated disturbances to the acquired record. Frame jitter leads to the same random shift of all samples in an acquired record. It results in errors in the estimates of time intervals, and makes consecutive records slightly incoherent - compromising data averaging procedures. Two complementary algorithms are developed for the quantification of frame jitter, and their performance has been simulated and verified by experiment. They allow the estimation of the standard deviation of the frame jitter using a low-resolution instrument even in noisy environments. An expression for the minimum value of the standard deviation of the frame jitter has been obtained and verified experimentally for typical architectures of data acquisition systems. It is shown that this value could be reduced by specific improvements in the design of data acquisition system architectures.

Super-resolution in situ ultrasonic monitoring of chemical reactions

This paper describes experiments to compare the sensitivity and robustness of ultrasound measurements with pH measurements when monitoring chemical reactions under laboratory conditions, the aim being to determine the relative suitability of the two techniques for process monitoring and control. It is shown that ultrasonic time-of-flight measurements, based on the centre of the area of an ultrasonic pulse, provides for super-resolution with respect to the sampling frequency. In comparison to reaction monitoring based on pH measurements, ultrasound was found to be superior in terms of its response time and resolution.

Ultrasonic imaging of biofilms utilizing echoes from the biofilm/air interface

Ultrasonic imaging of biofilms in water is difficult due to the very low contrast in acoustic impedance between the biofilm and water. In this paper, biofilms exposed to moist air are scanned through the substrate in order to obtain echoes from the biofilm/air interface. A 50 MHz scanning system was used to scan 1 mm /spl times/ 1 mm areas of biofilms in a 10 /spl mu/m grid pattern. Two fast Fourier transform (FFT) based methods for enhancement of the film thickness measurement resolution are compared. Using these techniques, the surface topography of biofilms with thickness less than the acoustic wavelength can be imaged.

Quantification of frame jitter in data acquisition systems

The paper develops an algorithm for the estimation of the standard deviation of the frame jitter associated with digital oscilloscopes and other data capture equipment. Frame jitter is considered as a special case of the more general timing jitter expressed in the frequency domain. An approximation is used that maps the standard deviation /spl sigma//sub f/ of the time domain jitter disturbance into the quadrature component of the recorded signal consisting of required information plus noise. /spl sigma//sub f/ is estimated from the statistical moments of the phase and quadrature components of the disturbed signal in a manner that minimizes the influence of additive noise on the estimation. The method has been demonstrated on an ultrasonic pulse-echo spectrometer that features relatively narrow bandwidth, low time domain resolution and significant additive noise. The obtained estimate of the standard deviation of the frame jitter was about four times lower than the sampling interval.

On the Modelling of Ultrasonic Bulk Wave Propagation in Epoxies

Bulk wave attenuation in epoxies has been found experimentally to be a linear function of frequency. Anecdotal evidence suggests that the formulations used for bulk wave attenuation to be input to computational propagation models, including models for guided waves, can take many forms: Either the simple linear relationship between attenuation and frequency can be input directly, or any of a number of analytic models could, in principle, be used – examples being the Maxwell, Kelvin-Voigt and Zener anelastic solid models, or the hysteretic damping model sometimes employed for studies of structural vibrations. This paper considers the physical bases of these various models, as well as multiple degree of freedom models that have their origins in dielectric theory. It is shown that a simple two degree of freedom adaptation of the Zener model provides a good simulation of experimentally observed data for both attenuation and phase speed over a wide frequency band.

Bulk shear wave propagation in an epoxy: attenuation and phase velocity over five decades of frequency

Proportionality between the ultrasonic wave attenuation coefficient in epoxies and other polymers and frequency is a commonly observed but little understood phenomenon. How it is ultimately explained will depend on the breadth of the frequency range over which it is significant. This paper presents results of experiments to measure loss in a single epoxy material over 5 decades of frequency using 4 complementary techniques - dynamic mechanical analysis, microwave excited low-frequency resonances, a novel guidedwave technique based on a metal-epoxy-metal sandwich, and a conventional pulse mode ultrasonic spectrometer. The results are confined to bulk shear waves in the epoxy. They confirm the linear relationship between attenuation and frequency, and it is shown that the broadband behavior of the attenuation and shear wave phase velocity is consistent with the Kramers- Kronig relationships.

The Consistency of Phenomenological Models of Ultrasonic Wave Propagation in a Curing Thermoset

This paper investigates the use of phenomenological models of viscoelasticity to simulate ultrasonic compression wave propagation in a curing thermoset. Kelvin‐Voigt and Maxwell models do not give physical results whilst the anelastic solid (Debye) formulation and its Cole‐Davidson (CD) extension provide a qualitative match to experiment, although with some inconsistencies. The CD model when fitted to experimental phase velocity data gave parameters that could be used to track molecular polydispersity in the curing thermoset.

Models of Ultrasonic Bulk Wave Propagation in Thermosets with Spatially Varying Viscoelastic Properties

The degree of cure in thick thermoset sections is likely to vary across their thickness due to local exotherm. The development of ultrasonic NDE techniques for such sections requires a computational model of wave propagation in materials with spatially varying viscoelastic properties. This paper presents a means by which the viscoelastic wave equation can be solved in spatially non‐uniform materials; it is based on an electric circuit analog for the mechanical behavior of viscoelastic materials, implemented using a SPICE circuit simulator package.