Showing papers in "Journal of Nondestructive Evaluation in 1999"

Review of Advances in Quantitative Eddy Current Nondestructive Evaluation

Abstract: A comprehensive review of advancements in eddy current (EC) modeling is presented. This paper contains three main sections: a general treatise of EC theory, the thin skin EC forward modeling, and the EC inverse problem. (1) The general treatise of eddy current theory begins with an exposition of the reciprocity formulas for evaluating probe impedance changes, which are derivable from first principles. Two versions of the reciprocity formulas, one with a surface integral and the other with a volume integral, are given. Any particular type of defect, as well as both one-port and two-port probes, can be treated. Second, a brief account of analytical and numerical methods for calculating the field distributions is presented. Third, theory of probe/material interactions with various defect types is described. (2) The paper then proceeds to the forward modeling section, which contains a detailed treatment of the eddy current forward problem for surface breaking cracks and EDM notches in the thin skin approximation. (3) The inverse problem section begins with a general review of commonly used inversion methods, exemplified by selected references from the literature, followed by more detailed examinations of EC inversions for surface breaking cracks and slots. The last part of this section is devoted to the inverse problem for layered structures. Although being a review in nature, the paper contains a number of new accounts for time-domain eddy current interactions. In particular, a modification is proposed to the reciprocity formula in order to take a better account of pulsed eddy current signals.

Finite Element and Plate Theory Modeling of Acoustic Emission Waveforms

Abstract: A comparison was made between two approaches to predict acoustic emission waveforms in thin plates. A normal mode solution method for Mindlin plate theory was used to predict the response of the flexural plate mode to a point source, step-function load, applied on the plate surface. The second approach used a dynamic finite element method to model the problem using equations of motion based on exact linear elasticity. Calculations were made using properties for both isotropic (aluminum) and anisotropic (unidirectional graphite/epoxy composite) materials. For simulations of anisotropic plates, propagation along multiple directions was evaluated. In general, agreement between the two theoretical approaches was good. Discrepancies in the waveforms at longer times were caused by differences in reflections from the lateral plate boundaries. These differences resulted from the fact that the two methods used different boundary conditions. At shorter times in the signals, before reflections, the slight discrepancies in the waveforms were attributed to limitations of Mindlin plate theory, which is an approximate plate theory. The advantages of the finite element method are that it used the exact linear elasticity solutions, and that it can be used to model real source conditions and complicated, finite specimen geometries as well as thick plates. These advantages come at a cost of increased computational difficulty, requiring lengthy calculations on workstations or supercomputers. The Mindlin plat theory solutions, meanwhile, can be quickly generated on personal computers. Specimens with finite geometry can also be modeled. However, only limited simple geometries such as circular or rectangular plates can easily be accommodated with the normal mode solution technique. Likewise, very limited source configurations can be modeled and plate theory is applicable only to thin plates.

A Simulation Study of the Beam Steering Characteristics for Linear Phased Arrays

Abstract: Ultrasonic beam steering characteristics for linear phased array transducers are simulated numerically by visualizing the full-field acoustic pressure field of the waves radiated from a linear phased array transducer. The influences of various transducer parameters on the beam steering properties are studied, including number of elements, inter-element spacing, element size, frequency of the transducer and the steering angle. In addition, the effects of these parameters on the near field characteristics are investigated by analyzing the acoustic pressure profile in the steering direction. The simulation results agree well with the analytical solutions which are valid only in the far field. A suggested scheme for optimal transducer design is presented.

Experimental Study of Phased Array Beam Steering Characteristics

Abstract: The influence of several geometric parameters of linear phased arrays was studied. A systematic approach using an automated testing assembly was used to assess the steering performance of the array in a solid medium. In addition to calibrating the transducer with respect to its steering accuracy, this arrangement provided a detailed study of the effects of steering angle, number of elements, inter-element spacing and array aperture on the beam directivity. The experimental results show good agreement quantitatively with the predicted steering characteristics.

Reconstruction of Cracks with Multiple Eddy Current Coils Using a Database Approach

Abstract: This paper presents some recent studies dealing with flaw reconstruction using eddy current testing. The locations of parallel planar cracks in an Inconel 600 plate specimen are determined and the shapes of them are reconstructed at the same time using the signals from a multi-pancake coil probe. The reconstruction strategy, consisting of a data base used fast forward solver and a first order optimization algorithm, relies on the minimization of the nonlinear least square residual function. Validity of the inverse analysis method is investigated with respect to the influence of the arrangement of scanning paths. The efficiency of the computation is also discussed.

Analysis of X-Ray and Gamma Ray Scattering Through Computational Experiments

Abstract: A series of computational experiments are performed for gaining insight into scattering phenomena in industrial radiography. First of all, simulation code results are benchmarked against MCNP Monte Carlo code for verification purpose. Following this, various aspects of scattering phenomena in industrial radiography are studied through simulations. Impact of X-ray tube voltage, energy of gamma photons, volume of the scattering object, object material, scattering object to detector distance are among the parameters observed in these computational experiments. Impact of scattering on energy spectrum of the total flux is observed as well. Spatial distribution of scattered photons are displayed for plates and geometrically complicated objects.

Nondestructive Evaluation of Fiberglass Reinforced Plastic Subjected to Combined Localized Heat Damage and Fatigue Damage Using Acoustic Emission

Abstract: The effect of fatigue damage to unidirectional fiberglass composite specimens with prior contact heat damage was investigated. After damaging the specimens by contacting them to a hot tip at 360°C, the specimens were subjected to fatigue loading at cyclic stress amplitude corresponding to 65% of the specimens' ultimate tensile strength. The fatigue experiments was halted after 3,000 cycles. The specimens were then subjected to tensile tests while monitoring their acoustic emission (AE) activity. In addition, acoustic emission activities of undamaged and contact heat-damaged specimens were monitored during tensile tests for comparison with specimens with combined fatigue and heat damage. AE activities of all specimens can be categorized into three regions: an early rise in activity, a relatively dormant period in activity, and a high exponential activity before failure. The early rise in activity did not appear on the specimens with combined contact heat and fatigue damage. For undamaged and contact heat-damaged specimens, the period of the dormant activity was independent of the contact heat duration of less than 15 minutes. However, the period was a function of the contact heat duration for combined contact heat and fatigue damaged specimens. Analyzing event duration distribution identified micro-mechanisms of the damage growth upon tensile loading. AE-stress delay concept was used to predict the state of the damage in the composite. A correlation between stress delay parameter and damage parameter was obtained for all of the specimens. Fatigue life of contact heat damaged specimen was also studied. It was found that localized heat damage reduced the fatigue life significantly. Loss of matrix to transfer the load to the fibers uniformly was believed to be responsible for the reduction in the fatigue life.

Laser Ultrasonics: Simultaneous Generation by Means of Thermoelastic Expansion and Material Ablation

Abstract: This paper describes a laser ultrasonic probe capable of generating ultrasound by both thermoelastic expansion and material ablation mechanisms simultaneously. Because bulk wave propagation is centered around a 67° line of sight for thermoelastically generated signals, and near normal (0°) for signals generated by means of ablation, the simultaneous generation by both mechanisms results in a wider range of useful observation angles. The system described uses a Nd:Yag laser with fiber optic delivery and a focusing objective, with an independent receiver, such as an EMAT, interferometer or contact transducer. The optical fiber delivery system allows the probe to be easily positioned (i.e. using a robot), and has the added benefit of allowing a single laser to service several test sites. The focusing objective provides the means for generating ultrasound by either thermoelastic expansion or ablation, or a combination of the two mechanism. The objective also serves to protect the fiber from ablated material and manufacturing contaminants (dust, welding gas, etc.). The resulting system is both physically robust and highly adaptable for a wide range of industrial ultrasonic inspection applications.

Ultrasonic Measurement of Stress in Pin and Hanger Connections

Abstract: Pin and hanger connections are used in bridges to suspend an interior span from the outer spans. The connections can sometimes lock up due to corrosion. If lockup occurs the stresses in the connection are cycled due to thermal expansion and contraction of the bridge; fatigue cracking and failure may occur. We constructed an apparatus to simulate a locked-up pin and hanger connection. We performed proof-of-concept tests of a method to detect stresses in pin and hanger connections. The method uses the stress-induced changes in sound velocity of shear waves polarized parallel and perpendicular to the hanger axis. The birefringence is the normalized difference in these shear wave velocities. We measured the birefringence at opposite sides of the hangers, at midsection. We simulated three scenarios: continuous monitoring of hanger status; intermittent monitoring from a known initial state; measurement with no a priori knowledge of hanger status. Good agreement with strain gauge data was obtained for all three scenarios.

The Effect of a Short Pulse of Current on Small Particles in a Conducting Fluid

Abstract: This letter describes the effect of a heavy current pulse, of approximately 10 kA, produced by a charge capacitor bank, on a small volume of liquid metal, confined in a cylindrical container (tube diameter is 1 cm and tube length is about 10 cm). The liquid metal contamination implies the presence of microscopic nonconducting particles. Theoretical consideration shows that a typical heavy current pulse of 15 msec duration leads to extremely large Lorentz force, due to the appearance of a self-induced magnetic field. Even though the particle contaminants are of micron size they can be moved by this force to macroscopic distances. Such a motion phenomenon can be used in the development of a cleanliness control system.

Experimental Investigation of the Grain Noise in Interferometric Detection of Ultrasonic Waves

Abstract: Besides their other obvious advantages over conventional ultrasonic sensors, laser interferometers offer optical diffraction limited apertures that are far smaller than the acoustic wavelength in the specimen under inspection. This unique feature can be exploited for the purposes of super-resolution near-field acoustic microscopy, which detects the rapidly decaying evanescent vibrations produced by surface and near-surface scatterers such as small fatigue cracks, pores, anomalous grains, etc. However, higher resolution also means higher sensitivity to inherent microscopic material inhomoge-neities. In this paper, experimental results are presented for the incoherent material noise in 2024 aluminum and Ti-6Al-4V titanium alloys at two different nominal frequencies of 5 and 10 MHz. It is shown that the incoherent grain noise significantly increases as the illuminated spot size decreases. Above the acoustic wavelength, the observed phenomenon is mainly due to the increasing sensitivity of the receiver to propagating scattered waves generated in the interior of the specimen. Below the acoustic wavelength, the further increasing material noise is mainly due to evanescent vibrations caused by nearby scatterers.

Characterization of Layered Cylindrical Structures Using Axially Symmetric Waves

Abstract: For the characterization of the unknown material properties of a layered cylindrical structure, axially symmetric wave signals transmitted and reflected by the structure have been used. Since only a single wave mode propagates in the structure, the measurement and analysis of the transmitted and reflected signals can be simplified significantly. The evaluation of the material properties of the layers can be achieved with great accuracy. In this paper, we first derive the transmission and reflection coefficients for the layered cylindrical structure sonified axisymmetrically by an incident cylindrical wave. We then relate the spectra of the transmitted and reflected wave signals to the transmission and reflection coefficients as ratio functions. The time-domain signals transmitted and reflected by the structure can then be reconstructed from a routine application of the Fourier integrals. A three-layered aluminum/epoxy/aluminum tube is used to illustrate the application of the expressions for both the forward and inverse problems. The results show that the technique developed in this study can be used very effectively for the characterization of layered cylindrical structures.