Eddy current testing relies on the principle of electromagnetic induction to detect flaws in conductive metals. An eddy current tester sends an alternating current through a coil which
generates an oscillating magnetic field. When the probe is placed near the surface of a conductive metal, a circular flow of electrons, an eddy current, begins to move through the metal
and generate its own magnetic field which interacts with the magnetic field created by the coil. Flaws or other changes in the metal will cause changes in the amplitude of the eddy current
and the magnetic fields it creates. This, in turn, affects the movement of electrons in the coil by varying the electrical impedance of the coil. By noting the changes to the impedance
amplitude and phase angle, a trained technician can identify inconsistencies in the metal being tested which could indicate flaws.
Eddy current testers have a number of benefits, and limitations, as an inspection tool. One advantage of eddy current testers is that the instrument is capable of multiple measurements.
Under the right conditions, they can be used for crack detection, material thickness measurements, coating thickness measurements, as well as conductivity measurements for material
identification or heat damage detection. The instruments are sensitive to small cracks and imperfections and a minimum or preparation is needed prior to testing. Eddy current testers are
highly portable making them excellent for inspecting complex shapes in tight locations.
The primary limitation with eddy current testers is that they are only compatible with conductive metals. Additionally, they have a limited depth of penetration and the finish of the material
may cause bad readings. Some flaws, notably those parallel to the probe coil may be undetectable. Care should be taken to make sure the instrument is compatible with your application. Eddy
current testers also require more skill and training in interpreting the results than other inspection methods.
Calibration
Like most test and measurement equipment, eddy current testers need to be periodically calibrated to maintain accuracy. In the case of NDT equipment, calibrations are performed
using precision test blocks that simulate conditions and serve as reference standards. Different types of test blocks simulate different conditions. Test blocks can include steps of different
thicknesses for use with thickness gauges, holes and other “flaws” for use with flaw detectors, or predetermined hardness for hardness detectors.
Things to Consider When Selecting an Eddy Current Tester:
- What parameter (thickness, flaw detection, etc.) is being tested for?
- Does the equipment require specialized training?
- What accessories (transducers, couplant, calibration blocks) are required?
- Are there professional standards which must be adhered to?
If you have any questions regarding eddy current testers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
