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Eddy Current Inspection

Level III Training

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Eddy Current Inspection Training Course

          Non-destructive testing can perform measurements or tests on materials without damaging or altering those materials. There are several types of non-destructive testing methods, including x-ray, ultrasonic testing methods, magnetic particle testing, dye penetrant testing, acoustic emission testing and Eddy Current Testing.
          In this course we shall examine the Eddy Current principle. Eddy Currents are circulating electrical currents and they can only be generated in materials that will conduct electricity. Consequently, eddy current testing is used exclusively for tests and measurements on metals and for thickness measurements of non-conductive materials backed by metals.

Applications
          Eddy Current Testing is used by several different industries to perform various types of inspection. The two largest applications for Eddy Current are tube testing and aircraft inspection.
         Tube testing application include the inspection of tubing in power generating systems, air conditioning systems and nuclear submarines for hazardous conditions suck as cracks, pitting and tube wall thinning. The tests are performed by passing an Eddy Current probe through the inside of the tubing. As the result, this type of inspection is referred to as Inner Diameter Inspection or simply, I.D. Inspection.
          Aircraft applications include numerous different types or aircraft structures and engines. Typical examples are inspection of aircraft engine and airframe components for very small surface cracks, inspections on aircraft wings and fuselages for cracks located underneath rivet heads.
Inspection for defects through the multiple layers or aircraft surfaces and inspecting for cracks on critical areas of aircraft wheel head seats-that portion of the wheel rim where the tire makes contact with the wheel under tremendous pressure.
          Other applications include an enormous variety of tests that make use of the many capabilities of the Eddy Current method. Example include: thickness measurement of non-metallic coatings, such as paint, on metal surfaces; inspection of welds for surface cracks; the sorting of metals according to characteristics such as alloy and hardness; and high resolution thickness, measurement.

Characteristics
          The Eddy Current method has both its advantages and its limitations.
          The advantages are:
1. The equipment is usually easy to operate and much of it is portable, lightweight and battery powered.
2. The method is virtually non-destructive. Non couplants, powders or other physical substances are applied to the test material. The only link between the probe and test material is a magnetic field. However, the operator is cautioned against bringing the test probe near wristwatches, measuring instruments or other devices which could be affected by an electromagnetic field.
3. The method is sensitive to a wide variety of test variables. Consequently, any number of test variables can be measured providing that the other variables are either separately identifiable or suppressed.
4. Test results are usually instantaneous. As soon as the test coil is applied to the test specimen, a qualified operator can interpret the results.
5. Eddy current testing is ideal for go/no-go test situations. Audible and visual alarms, triggered by threshold gates or box gates are available to provide high speed test results.
6. There is no danger from radiation or other such hazards.

          The limitations of Eddy Current Testing are:
1. The test material must be electrically conductive. However, it is possible to measure the thickness of non-conductive coatings on conductive materials.
2. Eddy Currents normally cannot penetrate ferromagnetic materials. Consequently, testing on ferromagnetic materials is limited to inspection of surface defects only, unless the material has been magnetically saturated. Moreover, magnetic saturation is limited to certain test geometrics.
3. Even on non-ferromagnetic materials the Eddy Current method has less penetrating power than some of the other NDT methods.
4. In some cases, such as the interpretation of impedance plane displays, Eddy Current Testing requires a trained operator.

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