Donald O. Thompson

Review of progress in quantitative nondestructive evaluation

This work is part of a continuing effort to develop a capability for quantifying the scattering of ultrasonic waves by arbitrarily shaped flaws. The general problem of elastodynamic behavior of a homogeneous, isotropic defect in an otherwise homogeneous, isotropic fullspace is cast as a Boundary Integral Equation (BIE). A general scattering model is needed to provide information for probability of detection (POD) models and inversion schemes for cases when low or high frequency approximations are not appropriate. Previously the Boundary Element Method (BEM), a method for solving the BIE, was adapted to NDE and the void problem was investigated [1]. Here we focus on the inclusion problem, experimental verification, and to overall extensions of the capability.

Bessel beam ultrasonic transducer: Fabrication method and experimental results

We report experimental results from a first‐of‐a‐kind ultrasonic transducer that generates a beam with a Bessel function profile. Using a technique of nonuniform poling, an axially symmetric Bessel function pattern is ‘‘polarized into’’ a piezoelectric ceramic element. The resulting circular‐disk transducer has the usual full‐plating electrode configuration, but produces an ultrasonic beam with a radial displacement profile approximating that of the Bessel function J0 (r), both in amplitude and in phase. The radiation field of a 1‐in.‐diam, 2.25 MHz Bessel transducer mapped out with a point probe shows good agreement with calculated results using a Gauss‐Hermite model. Bessel transducers are of particular interest in attempts to achieve ‘‘diffractionless’’ beams.

Reconstruction of inclusions in solids using ultrasonic Born inversion

Voids and inclusions in elastic solids are characterized experimentally using scattered ultrasonic waves. The flaws are reconstructed using a one‐dimensional elastic wave inverse scattering algorithm based on the Born–Neuman expansion. This method emphasizes the role of low and intermediate frequency longitudinal waves. The utility of the inverse Born approximation is tested for several new circumstances. First the algorithm is tested for pitch‐catch (bistatic) geometries. Secondly the effects of resonant excitation of the scatterer on flaw characterization are measured for several spherical flaws. The third and major result shows that the one‐dimensional algorithm can be used to determine the size, shape and orientation of nearly ellipsoidal flaws when access angle is limited. The effects of varying access aperture on the reconstruction are reported. Another common experimental limitation in flaw characterization arises from interferences of the flaw signal with nearby surfaces. We briefly report that th...

Higher harmonics of finite amplitude ultrasonic waves in solids

Absolute measurements of the amplitude of the first four harmonicof 30‐MHz finite amplitude ultrasonic waves are reported for fused silica samples of various lengths and for aluminum single crystals with different amounts of cold work. The harmonically distorted motion of the end of the sample is detected by a capacity microphone and the individual harmonics are then selected and amplified by a heterodyne receiver with flat frequency response from 30 to 250 MHz. The fused silica data are found to be in excellent agreement with a model of Fubini, originally developed for gases, which for solids depends on the second‐ and third‐order elastic constants but none of the higher constants. A discussion is presented of the reasons for the insensitivity of the measurements to the values of the fourth‐ and higher‐order elastic constants. Agreement with the model of Fubinin is not observed in single crystal aluminum. The quantitative differences are not fully consistent with existing models for dislocation contribut...

The elastic moduli of a thick composite as measured by ultrasonic bulk wave pulse velocity

One thick filament-wound composite in the form of a large thick-walled cylinder with locally orthorhombic symmetry has been measured by ultrasonic velocity to calculate its elastic moduli. The basic assumption was that small sections of the composite could be treated as a homogeneous body analogous to a crystal for ultrasonic propagation. The experimental work and the results as best expressing homogeneous body theory are presented. Because of the high anisotropy with the normal to the layers (the three-direction) much different from the axial and hoop directions, it was necessary to calculate slowness surfaces with approximate values of c13 and c23 in order to find the directions of the Poynting vectors to use in making actual measurements on c12 and c13.

Apparatus and technique for reconstruction of flaws using model‐based elastic wave inverse ultrasonic scattering

An automated multiviewing ultrasonic apparatus and signal processing routine have been developed for utilization in nondestructive evaluation (NDE) of materials. The instrument has been developed to take advantage of recent advances in long and intermediate wavelength inverse scattering of elastic waves, and provides a 3‐D reconstruction of a flaw. Although the reconstruction obtained does not contain fine details of the flaw’s structure, it provides sufficient information about the flaw (size, orientation, and selected materials properties) so that failure‐predictive decisions can be made.

The Measurement and Analysis of Acoustic Noise as a Random Variable

In ultrasonic nondestructive evaluation, experimental measurements of the scattered wave field resulting from sonification of a flaw are corrupted with acoustic noise. Acoustic noise results from non-flaw related scattering or reflection of the incident waves. In many probabilistic approaches to flaw detection, classification, and characterization, a stochastic model for a noise-corrupted flaw signal is utilized where acoustic noise is assumed to be an uncorrelated, Gaussian random variable with zero mean. In addition, it is assumed that an estimate of the average power spectra of the noise is available [1–3]. The goal of the work presented here was to measure and analyze acoustic noise as a random variable. Emphasis was placed on evaluating these assumptions and on estimating the average power spectra of the noise.

A prior knowledge based optimal Wiener filtering approach to ultrasonic scattering amplitude estimation

In order to interpret measurements of the scattered wave field resulting from sonification of an object, the effects of the measurement system must be removed from the measured data. In ultrasonic nondestructive evaluation, estimation of the farfield scattering amplitude for a flaw in a material, e.g., a crack, void, or inclusion, involves removing the measurement system effects in the presence of electronic noise and noise due to the scattering of sound from inherent material features such as grain boundaries. The purpose of the work reported here was to evaluate an optimal Wiener filtering approach to scattering amplitude estimation. The filter was shown to determine an optimal estimate as the weighted average of the information derived from measurement of the scattered wave field and prior information about the flaw distribution. The optimal Wiener filter was evaluated and compared to a nonoptimal form of the Wiener filter. The optimal Wiener filter was shown to provide improved scattering amplitude es...

Reliability of Reconstruction of Arbitrarily Oriented Flaws Using Multiview Transducers

Some basic problems associated with the reconstruction of arbitrarily oriented flaws using an automated multiviewing ultrasonic transducer system and its associated signal-processing algorithms are addressed. The effects of aperture size on the reconstruction reliability are investigated by computer simulation, and experimental results are presented for the reconstruction of arbitrarily oriented oblate and prolate spheroidlike flaws. The multiviewing capability of the system is used to determine a spatial data-acquisition pattern which improves the reliability of reconstruction by optimizing the illumination of the flaw surface by the multiviewing transducer and the signal-to-noise ratio of the scattering signal.

Elastic Wave Scattering from Multiple Voids (Porosity)

The purpose of the work described in this paper is the development of an ultrasonic measurement technique which provides a convenient way to detect dilute porosity conditions in materials and to extract certain properties of the flaw distribution which are important in failure prediction. Use has been made entirely of ultrasonic backscatter measurements; thus, the technique differs considerably from other investigations which lead to porosity determinations in that no reliance is placed upon either attenuation measurements or precise ultrasonic velocity measurements [1,2]. The technique thus possesses a distinct advantage for practical implementation, i.e., it is a “one-sided” measurement which does not require ultrasonic echo returns from an opposite face of the sample in order to be useful. At present, the work is limited to dilute porosity concentrations. Reasons for this limitation will become clear in the paper. With additional effort it is expected that this limitation can be removed and the work extended to larger concentrations.