Full Description This manual covers the principles of the eddy current method of nondestructive testing including relevant electromagnetic theory, instrumentation, testing techniques and signal analysis. It is intended to act as; - a source of educational material to persons who are seeking certification according to the requirements of GPM -- Certification of Nondestructive Testing Personnel Eddy Current Method , and - as a guide and reference text for educational organizations and training centres that are providing or planning courses of instruction in Eddy Current Testing.
Note that the degree of scientific detail in this manual is primarily directed towards Level II and Level III certification applicants. It is assumed that the reader is familiar with basic electrical theory and the elements of algebra and calculus. Variations in the changing phases and magnitude of these eddy currents is then monitored through the use of receiver-coil s , or by measuring changes to the alternate current flowing in the primary excitation-coil.
The electrical conductivity variations or magnetic permeability of the test-part, or the presence of any discontinuities, will cause a change in eddy current and a corresponding change in phases and amplitude of the measured current. The changes are shown on a screen for easy interpretation. Another method of using the eddy current principle is pulsed eddy current testing.
Methods are being developed to investigate surfaces through protective coatings, weather sheetings, corrosion products and even the insulation materials. Normally carried out with pencil probes or 'pancake' type probes on ferrous or non-ferrous metals. Frequencies from kHz to a few MHz are commonly used. Depending on surface condition it is usually possible to find cracks as small as 0. Differential probes are sometimes used, particularly in automated applications, care must be taken to ensure that the orientation of flaws is correct for detection.
This is essentially conductivity testing and for dedicated applications a conductivity meter may be a better choice. From the impedance plane diagram it will be seen that the indication from a conductivity change is essentially the same as from a crack, and both meter and impedance plane type crack detectors can be successfully used to sort similar metals using a suitable absolute probe. It should be remembered that widely different metals may have similar conductivity and that the allowable values for similar alloys my overlap, so conductivity measurement should only be used as an indication that a metal is of correct composition or heat-treatment.
Primarily used in Airframe inspection. By using a low frequency and a suitable probe eddy currents can penetrate aluminium or similar structures to a depth of 10mm or so, allowing the detection of second and third layer cracking, which is invisible from the surface, or thinning of any of the different layers making up the structure. Heat exchangers used for petrochemical or power generation applications may have many thousands of tubes, each up to 20m long.
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