W. Lord

A finite element study of the remote field eddy current phenomenon

The difference in operation of conventional and remote field eddy-current nondestructive evaluation procedures is described. A finite-element study conducted to provide physical understanding of the remote-field eddy-current phenomenon is reported. Finite-element predictions of probe performance are shown to agree very well with experimentally observations. They also indicate the presence of an unusual potential valley and corresponding phase knot in the pipeline wall that play a significant role in explaining the underlying physics of the method. >

Applications of numerical field modeling to electromagnetic methods of nondestructive testing

Electromagnetic field interactions with metals can be used to determine both material properties and the presence of defects. Such techniques are used widely in the nondestructive testing of critical components for aerospace, transportation, energy and metals industries where reliability, safety and product quality considerations are important. This paper describes the common physical basis for active dc, residual and ac methods of specimen magnetization and shows how finite element analysis techniques originally developed for the study of fields in electrical machinery can be extended to predict electromagnetic NDT transducer signals for all three forms of specimen excitation.

Speech Pitch Frequency as an Emotional State Indicator

Computer-based education systems utilizing adaptive teaching strategies could benefit greatly from a knowledge of the student-users emotional state. Comparative studies of speech analysis techniques suitable for a two-way audio communications system between student and computer led to the hypothesis that voice pitch frequency is related to emotional state. Experimental results are given based on the analysis of words spoken by drama students, which validate this hypothesis, and recommendations are made for incorporating a two-way audio component into future computer-based learning systems.

A finite element model for three-dimensional Eddy current NDT phenomena

The defect detection mechanism for eddy current nondestructive testing (NDT) probes is related to the interaction of induced eddy currents in the metal test specimen with flaws and the coupling of these interaction effects with the moving test probe. To date, numerical modeling of these phenomena has been limited to two-dimensional and axisymmetric geometries. A three-dimensional magnetic vector potential finite element formulation for the modeling of eddy current NDT phenomena is described, and the technique is illustrated by predicting differential eddy current probe impedance plane trajectories for flaws in PWR steam generator tubing.

3-D finite element predictions of magnetostatic leakage fields

Traditionally electromagnetic leakage fields have largely been of interest to the designers of electrical machinery and magnetic tape heads. An increasingly important application of such leakage fields, however, relates to their use as a mechanism for the detection of defects in ferromagnetic materials. The finite element simulation of three-dimensional active leakage fields is described, and the theoretical predictions are compared with experimentally obtained leakage field profiles for a rectangular slot in a carbon steel bar. Particular emphasis is placed on techniques for determining boundary conditions and the appropriate excitation current distribution in the bar.

Finite element modeling of absolute eddy current probe signals

Finite element analysis techniques are applied to the problem of predicting signals from an absolute eddy current probe in the tube sheet region of a PWR steam generator for the purpose of optimizing the probe coil geometry and determining the feasibility of using such a probe to characterize the condition of the tube and tube sheet crevice.

DC Servomotors-Modeling and Parameter Determination

Present methods of defining and measuring the parameters of separately excited dc servomotors are inadequate. Eight practical servomotor models are identified, and the parameters of each model are clearly defined. Testing procedures are given for determining a servomotors model type and all the model parameters based only on the current response of the machine to a step input of armature voltage. Excellent agreement is obtained between the actual current responses of several servomotors and their predicted responses obtained from the corresponding mathematical models, indicating the efficacy of the technique.

Diffusion, Waves, Phase and Eddy Current Imaging

The discovery of electromagnetic induction by Faraday and Henry in 1831 not only served as the catalyst needed for the very creation of electrical engineering but also provided the physical basis for eddy current nondestructive testing (NDT) as we know it today and as first realized in the classical experiments of Hughes1. As this fundamental work preceded Maxwell’s prediction of electromagnetic wave phenomena by over half a century, it may seem somewhat surprising to the casual reader that there should be any need to explain why eddy current NDT phenomena can be classified as quasi-static in nature with none of the attributes of classical electromagnetic waves. Unfortunately, there are many misconceptions concerning the wave-like nature of eddy current NDT phenomena which have even led to the suggestion2 that conventional eddy current NDT probe signals can be treated holographically. There are several reasons for the existence of these misconceptions: 1. Many papers in the field (see for example Hochschild3) describe the propagation of an electromagnetic plane wave in a medium as being analogous to eddy current NDT phenomena. Although the analog itself has some limited validity, it is rarely if ever mentioned that a conventional eddy current NDT probe does not launch an electromagnetic wave (as does say an antenna). 2. Solution of the quasi-static skin effect equation for current density does have the same form as would a damped electromagnetic wave. However, this is more a statement of the consistency of Maxwell’s equations across different regimes (see Figure 1) than support for eddy current waves. A number of authors address this seemingly anomalous situation (see for example, Stoll4, Ferrari5, and Melcher6) and clearly differentiate between electromagnetic diffusion and electromagnetic wave phenomena. 3. Much of the terminology associated with eddy current NDT phenomena (phase, for example) has a direct counterpart in electromagnetic wave parlance.

Simulating Electromagnetic NDT Probe Fields

Using computer graphics and film technologies, engineers can now numerically model the nondestructive testing technique that detects flaws in the pressurized steam pipes of nuclear power plants.

A Survey of Electromagnetic Methods of Nondestructive Testing

Electromagnetic methods of nondestructive testing (NDT) can be classified according to their mode of specimen excitation: Direct current excitation results in active leakage fields around defects in ferromagnetic materials. Residual leakage fields around defects in ferromagnetic materials occur after removal of the dc excitation current. Alternating current excitation of conducting materials results in induced eddy currents which are sensitive to a wide variety of specimen properties.