Diffusion, Waves, Phase and Eddy Current Imaging

Abstract

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.