Eddy Current testing can be described as an interaction between the fields of Theoretical Physics, Electrical Engineering, Electronics and Metallurgy.
Eddy currents are closed loops of alternating electromagnetic currents induced in a conductor (tube) by a varying magnetic field.
Eddy Current Testing is a comparative method of inspection only. It compares known depths of machined defects in a calibration tube to defects found in the tubes being tested.
The calibration tube is essential to all inspections and must be identical to the tubes being inspected, matching the tube O.D., I.D., wall thickness and tube material. In the calibration tube, there will be a variety of different machined defects with known varying defect depths including a through wall hole.
When an eddy current probe is inserted into the calibration tube, Eddy Currents are induced or created and exist only within the thickness of the tube wall.
The eddy currents must flow either under or over a defect and cannot pass through a defect. Only in the case of a sub surface defect will the Eddy Currents flow be around a defect.
The created eddy currents, the frequency generated by the instrument, the tube wall thickness, and the probe diameter are all constant when the probe is balanced in a good portion of the tube wall. When the probe is properly balanced the only variation to be expected will be in the tube wall.
When a probe balance is obtained, the probe is then drawn back through the calibration tube.
When the eddy currents sense a defect, the interaction is such that the same amount of eddy currents generated in a good part of the tube wall are now going through a narrower part of the tube wall thickness, hence setting up a resistive (impedance) change. This change in resistance (impedance) generates a signal on the screen that can be compared to the known depth of the defect in the calibration tube and now can be analyzed.
The signals angular deflection and or amplitude will determine the defects origin, whether it be I.D. or O.D. and the approximate depth.
Once all parameters are setup (i.e. Frequencies selected, Tube parameters, Calibration tube, Tube identification method, and signal response from calibration tube are established), the inspection can start.
The probe is fully inserted into each tube. The data is generally analyzed while the probe is being extracted, however each tube’s data can be stored to an optical disk and analyzed at a later time.
Prepared By
Allan S. Cherry
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