Thursday, December 2, 2010

Eddy current method (ET)

Short research about the history of eddy current
The first person to observe current eddies was François Arago (1786-1853), the 25th president of France, Who was also a mathematician, physicist and astronomer.1824 He discovered what has been called rotatory magnetism, and the fact that most bodies could be magnetized; these discoveries were completed and explained by Michael Faraday (1791-1867).
In 1834, Heinrich Lenz stated the principle that defines how the properties of the test objects are communicated back to the test system. Lenz's law states that the direction of current flow in the test object will be such that its magnetic field will oppose the magnetic field that caused the current flow in the test object. This means, in practice, the eddy currents communicate with the test coil by developing secondary flux that cancels a portion of the coil's flux equivalent to the magnitude and phase of the flux developed by the eddy currents.
 
Léon Foucault (1819-1868)

However, the first person who really discovered eddy current (or Foucault current)
was the French physicist Léon Foucault (1819-1868). In September, 1855, he discovered that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy current induced in the metal.
The first use of eddy current for NDT occurred in 1879 when D. E. Hughes used the principles to conduct metallurgical sorting tests.
The development of the eddy current method progressed slowly until the late 1940s, when Dr. Friedreich Foester found the Institute Dr. Foerster, which made great strides in developing and marketing practical eddy current test instruments. by the late 1960s the institute had developed a product line covering virtually every application of the eddy current test method and worked with American manufacturers to firmly establish the method in the US. Two major contributions of Foerster were development of impedance plane display, which greatly aided in communication of test information to the user, and formulation of the law of similarity, which enables the practitioner to duplicate the same eddy current performance under a variety of test situations.
The next major contribution to the advancement of the method, multi frequency testing was also developed by an equipment manufacturer, Intercontrolle of France, in 1974. Driving the test coil at multiple frequencies helps to overcome what has traditionally been the major limitation of the eddy current method, the fact that the various conditions to which the method is sensitive can vector into a single displayed signal that is difficult to interpret. Originally developed to suppress the display of undesired test variables, multi frequency testing can also optimize an deddy current test for normally conflicting performance variable such as sensitivity and penetration as well as aid in identifying the nature of particular test response. Multi frequency testing is a very significant innovation that has markedly advanced the state of the art.
the development of microprocessor bases eddy current instruments since the mid 1980s has also enhanced the potential and user friendliness of the method. It has improved recording capability provided sophisticated post inspection signal analysis, and has allowed automatic mixing of multi frequency signals. Modern microprocessor based eddy current instruments offer a breadth of useful features virtually unimaginable in the days of analog equipment. Manufacturers such as Zetek, Hocking, Foerster, Nortec, ETC and Magnetic Analysis have been important contributors.
In addition to mainstream eddy current testing, more specialized techniques are employed for certain applications. These include flux leakage, remote field eddy current, and modulation analysis inspection. In classifying nondestructive test methods for the purpose of qualifying and certifying test personnel, the American society of Nondestructive Testing (ASNT) classifies all these techniques under umbrella of the electromagnetic testing method.