Showing papers in "Journal of Nondestructive Evaluation in 1985"

The volume integral method of eddy current modeling

Abstract: The volume integral method is proposed as an alternative method for computing the impedance changes associated with eddy current measurements. In this method, the use of an appropriate Green's function for the host medium requires that integration be performed only over the volumes of defects. Although only rather simple host medium geometries are amenable to this type of modeling, the method can be linked to finite element models of more complicated geometries. In this way, the amount of finite element modeling may be minimized.

A surface acoustic wave technique for monitoring the growth behavior of small surface fatigue cracks

Abstract: The theory of Kino and Auld which relates the reflection coefficient of acoustic waves from a crack to its size is summarized. A scattering model is evaluated from this theory concerning the reflection of surface acoustic waves (SAW) from a small surface fatigue crack at a frequency such that the crack depth is much smaller than the acoustic wavelength. Acoustic predictions of crack depth are compared to postfracture measurements of depth for small surface cracks in Pyrex glass, 7075-T651 aluminum, and 4340 steel. Additionally, the minimum detectable crack depth as limited by the acoustic noise level is determined for several typical aluminum and steel alloys. The utility of SAW reflection coefficient measurements for inferring crack depth, crack growth, and crack opening behaviorin situ during fatigue cycling is discussed.

A new acoustic spectroscopy: Resonance spectroscopy by the MIIR

Abstract: Recent and remarkable advances in the experimental study of acoustic scattering from targets immersed in water are leading to a new spectroscopy: resonance acoustic spectroscopy. The discovery and improvement of an intriguing method, the Method of Isolation and Identification of Resonances (MIIR), has made possible experimental determination of the eigenfrequency spectra of aluminum-elastic cylinders and cylindrical shells. This method gives a quasilinear “resonance spectra.” In addition, it shows the importance of circumferential waves which generate standing waves. They allow us to explain the “reradiation” of targets after the end of insonification. The MIIR has numerous applications, especially in “underwater acoustics” and “nondestructive testing.”

Three dimensional strain measurements with x-ray energy dispersive spectroscopy

Abstract: A feasibility study was performed to show the ability of energy dispersive x-ray diffraction (EDXRD) to measure three dimensional strain distributions in thick industrial materials. Photon energies up to 130 keV were used to guarantee penetration through the sample and curve fitting techniques applied to peak position determination. This system was used to measure the strain gradient through the thickness of a 9.5 mm thick cantilevered steel bar.

Two-dimensional inverse born approximation in ultrasonic flaw characterization

Abstract: A method is developed to characterize flaws of arbitrary shape by using ultrasound pulse echoes at multiple coplanar incident directions. The three-dimensional image reconstruction problem is reduced to a series of two-dimensional image reconstructions, thereby avoiding the difficulties associated with three-dimensional image reconstructions, such as taking and processing a large amount of data, and the complications associated with three-dimensional image reconstructions, such as three-dimensional interpolation, long computing time, etc. The reconstructed two-dimensional images represent the two-dimensional projections or shadows of the three-dimensional flaw characteristic function. Each projection image is reconstructed independently using well-developed computerized tomography reconstruction techniques. If the shape of the flaw is not too irregular, or if the fine details of the shape are not of interest, only a few of these projection images suffice to characterize the flaw. The magnitude scaling problem and the alignment problem of the echoes at different incident directions can be handled easily in the algorithm. Simulation studies yielded encouraging results.

Characterization of flaws using the zeroes of the real and imaginary parts of the ultrasonic scattering amplitude

Abstract: A new method for determining the size of flaws, such as cracks and voids, in structural materials is reported The method is based on the observation that, for a wide class of flaws, the zeroes in the real and imaginary parts of the scattering amplitude occur at characteristic frequencies which depend on the flaw size The method of reconstruction using the zeroes is simpler than the inverse Born approximation because the flaw sizes are extracted directly from the characteristic frequencies The new method employs the same data input as the inverse Born algorithm and has certain common limitations Following a theoretical treatment, experimental results are presented for a number of different types of flaws

An acoustoelastic determination of the stress tensor in textured metal sheets using the birefringency of ultrasonic shear waves

Abstract: A theory is formulated that enables one to determine two-dimensional applied stresses in sheets of slightly orthotropic material by means of ultrasonic shear waves To relate experimental acoustic data to a stress tensor we introduce an acoustoelastic tensork. Our experimental technique can measure times of flight and polarization angles within 1/3 nsec and 1°, respectively. In the case of the aluminium 2024-T351 alloy we used, calibration using tensile specimens leads to a nonisotropick tensor. Measurements of the complete (plane) stress tensor are carried out in the center of a compressed disc.

Estimation of mixing parameters for cancellation of discretized eddy current signals using time and frequency domain techniques

Abstract: In this paper, we present two methods for estimating the mixing parameters used in scaling, rotating, and time shifting discrete time eddy current impedance plane trajectories in order to suppress unwanted support plate signals. One method operates directly on the time signals. The other method operates on the frequency domain representation of these signals and consequently is computationally more involved. Both methods require the minimization of a functional which measures the energy difference between the horizontal and vertical components of the high and low frequency data. Three illustrative examples are presented, and it is shown that the use of the frequency domain method is justified if the data are corrupted with random noise as well as with unknown multisample time shifts.

Surface electromagnetic fields around surface flaws in metals

Abstract: In an earlier paper(3) it was shown mathematically and confirmed by experiment that when a uniform surface alternating conduction current is incident on a spherical cap indentation at the surface of a metal there is no perturbation to the current flow on the plane surface outside the cap. It is shown here that this result is true of all axisymmetric surface cavities which are interrogated by any form of surface Laplacian current flow. The result applies, for example, to surface cavities in the form of right cylindrical stopped holes or to conical pits. The paper describes experiments confirming the result in those two cases for uniform incident currents. It also describes the case of an annular cylindrical hole where the theory predicts that a uniform surface current of lower strength than the incident uniform current is established on the central part of the surface plane. This gives rise to a linear variation in surface probe readings in a center line traverse which is also confirmed by experiment. These results are in marked contrast to the perturbations to the surface currents produced by surface cracks, which have been described in earlier work(2) by the use of unfolding algorithms.

Testing steel-cord belt splices with a magnetic conveyor belt monitor

Abstract: Steel-cord belt splices fail for a variety of reasons, including corrosion, poor vulcanising, and incorrect construction. The latter often leads to early failure. A conveyor belt monitor (CBM) has been used to evaluate the splice lay-up. The mass of the overlapping cords and their magnetic signature are used to rapidly locate suspect splices in the belt. The general shape of the magnetic signature for ideal splices will be discussed.

Ultrasonic Crack Characterization: A Constrained Inversion Algorithm

Abstract: Obtaining flaw geometry and orientation information from ultrasonic measurements often involves time consuming scanning and data processing functions. One possible way of reducing the amount of information needed for flaw characterization purposes is to require that the fine details of the flaw not be resolved but instead to obtain the best “equivalent” flaw geometry and orientation that fits a predefined simple flaw shape of unknown size and orientation. Hsu et al. [1] have implemented such a procedure for isolated voids and inclusions by using the Born approximation and obtaining a non-linear least squares estimation of the best ellipsoid that fits the data. Here, we will demonstrate that a similar constrained inversion procedure can be developed using the Kirchhoff approximation [2-4] for a flat elliptical crack. Using the results of our previous paper [5], an explicit coordinate-invariant expression that relates the time difference, At, between the arrival of the waves diffracted from the flashpoints of the crack and the crack shape and orientation is obtained. This expression, together with At measurements in different scattering directions is placed into a regression analysis to obtain a set of equivalent flat elliptical crack parameters. A series of tests of this method using noisy synthetic data are considered and the sensitivity of the results to number of measurements and the viewing aperture of the transducer set-up is discussed.

Impedance of single-turn coil surrounding a conducting cylinder with a flaw

Abstract: The impedance of a single-turn coil which surrounds a conducting cylinder with a flaw is calculated employing Green's function technique. The Born approximation is used in order to know how the impedance change due to the presence of a flaw depends on the conductivity and size of a flaw.