Eddy current analysis of precipitation kinetics in aluminum alloys
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I. INTRODUCTION
THE eddy
current method is an efficient and reliable technique to nondestructively examine electrically conductive materials. In addition to the capability of detecting surface and subsurface flaws, eddy currents can be used to characterize physical, chemical, and mechanical properties of materials. The technique can be employed t o detect variations in electrical conductivity, magnetic permeability, bulk density, chemical composition, microstructure, and grain orientation, as well as changes in the physical dimensions of metallic objects as the material is subjected to thermomechanical treatment. The eddy current methodology has several advantages compared to other nondestructive (NDE) techniques. It is noncontact and nonintrusive; it can be applied to thin samples and can be used in hostile environments. Furthermore, the eddy current characterization techniques are adaptable for high speed inspection and for localized inspection with miniature sensors of varying geometry. The output is in the form of electric signals which can be easily digitized and, therefore, applied to in-process automatic monitoring and control. However, the eddy current method is rather difficult to apply to ferromagnetic materials. The depth of penetration, or probing depth, is shallow in most practical cases, and difficulties may be encountered in data analysis and interpretation, as is usually the case with electrical conductivity measurements. II. NATURE OF EDDY CURRENTS Jean Foucault (1825) is credited with the discovery of eddy currents. He demonstrated that, as a copper disc is moved in a spatially nonuniform magnetic field, an elecM. ROSEN is Professor and Chairman, Materials Science Department and Nondestructive Evaluation Center, Johns Hopkins University, Baltimore, MD 21218. This paper is based on a presentation made in the symposium "Nondestructive Evaluation--Predictive and Descriptive Probes of Mechanical Behavior of Metals" presented at the TMS-AIME fall meeting in Cincinnati, OH, October 13, 1987, under the auspices of the TMS Mechanical Metallurgy Committee. METALLURGICAL TRANSACTIONS A
tric current is induced. D.E. Hughes (1879) was the first to realize the importance of eddy currents in NDE. However, H.G. Doll (1949) was the first to use eddy currents to determine the electrical conductivity in geological strata. Generation of eddy current is based on the principles of electromagnetic induction (Faraday, 1831). In turn, the eddy currents create their own electromagnetic field, thus affecting the current in the primary excitation coil that generates the primary magnetic field. For electric current frequencies with wavelengths large compared with a metal sample, it can be assumed that the electric and magnetic fields are propagated instantaneously. Following Farada_y's law of induction stating that if a magnetic induction B in a conductor is changing, an electric field intensity/~ is produced that can be defined in magnitude and direction by means of Maxwell's equations. In their differential form the equ
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