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The Eddy Current method is employed in two aspects of Non-Destructive Testing. Firstly as a means of finding surface and subsurface flaws, and secondly as a means of determining different metallurgical characteristics in place of destructive methods.
Eddy currents are produced in any electrical conducting material within which an alternating magnetic field (frequency range from 2KHz to 10MHz) has been generated. This alternating magnetic field can be generated in the workpiece to be tested by means of an encircling coil which is fed with an A.C. voltage.
Eddy currents possess the property of generating a voltage in a second coil which also surrounds the subject. The principle is somewhat similar to that of a transformer with primary winding, core and secondary winding. The value in terms of amplitude, phase, etc. of the current so generated in the secondary winding by the eddy currents is dependent on the characteristics of the material in which the eddy currents circulate. So there would be a difference in the value of the current from the secondary winding between two pieces of material which were identical in size, shape and composition, but in one of which there was a crack - the effect of the discontinuity being to disturb the circulation of the eddy currents. The difference in phase by a cathode ray tube. In this manner not only can cracks be detected but, to a certain degree their size can be determined, i.e. the absence of metal.
The value of the current in the secondary can also be altered in comparison with a standard by differences in composition, hardness, texture, shape, conductivity and size - and in theory there should be only one variable. For instance, if one is searching for cracks in, say, bar material, then the other characteristics mentioned above should be constant. It frequently happens, though, that a standard is set for a particular item and any deviation, for whatever reason, results in rejection.
Normally equipments have two separate test coils, each of which houses a primary and a secondary winding. In one coil the known standard is placed, the unknown subject being placed in the second coil. The differences are displayed on the instrumentation. It is not strictly necessary that the coils encircle the subject. Recent developments use a single coil which uses impedance generated by the eddy currents. This means that probe systems have evolved which can be scanned over the surface of the part to be tested.
The depth of penetration of eddy currents depends on the material. At a given composition the lower the frequency of the A.C. current fed to the primary the deeper the penetration. However, the lower the frequency the less sensitive the method is to small changes.
Being a purely electrical method eddy current testing is easily automated, since the encircling coil or probe need not come into contact with the test piece. The mechanical handling makes it a requirement that the pieces are of a simple geometric shape.
Advantages of Eddy Current Testing:
- Suitable for the determination of a wide range of conditions of conducting material, such as defect detection, composition, hardness, conductivity, etc. in a wide variety of engineering metals.
- Information is provided in simple terms: often go/no go.
- Can be portable.
- No consumables.
- Suitable for total automation.
Disadvantages of Eddy Curent Testing:
- The wide range of characteristics which have an effect on an eddy curent system tend to militte against it, e.g. whilst carbon variation can be tolerated in particular steels, cracks would not, and the instrument might not be able to discriminate.
- Generally tests restricted to surface conditions.
This
information is taken from the Insight NDT technical paper entitled
'A Brief Explanation of Non-Destructive Testing Methods'.
A copy of the full paper in Adobe Acrobat format is available by
clicking Here.
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