Eddy Current inspection is one of several NDT methods that use the principle of electromagnetism as the basis for conducting examinations. Several other methods such as remote field testing (RFT), flux leakage and Barkhausen noise also use this principle.
The alternating current field measurement (ACFM) technique is an electromagnetic technique capable of both detecting and sizing (length and depth) surface-breaking cracks in metals. The basis of the technique is that an alternating constant current in a tangential solenoid, remote from the test surface, induces electric currents in the sample surface which are uni-directional and of uniform strength over a localised area under the solenoid. When no defects are present in this area, these electric currents will be undisturbed. If a crack is present, the uniform current is disturbed and the current flows around the ends and down the face of the crack. A standard PC is used to control the equipment and display results. ACFM is unique in the way data is displayed.
RTD INCOTEST (INsulated COmponent TESTing) is based on the pulsed eddy current (PEC) principle and is a reliable way to survey ferrous pipes and vessels through their thermal insulation and protective coatings. The in-depth PEC technology is an excellent tool for prioritising further inspections. In this technique, the sending coil generates eddy currents at the surface of the material. As they diffuse, they generate a magnetic field that is detected by the receiving coil in the probe. The average remaining wall thickness within the enclosed magnetic field is proportional to the decay time of the received signal.
Magnetic flux leakage (MFL) is a tube-testing technique primarily designed for the rapid testing of ferromagnetic tubes with non-ferromagnetic fins wrapped around them, such as in air fin coolers. Two strong magnets generate a static magnetic field that saturates the tube wall (Fig. 1 ). When a flaw (pitting, wall loss, etc.) is located between the two magnets, the magnetic flux in the tube wall is disturbed and a small amount of flux will leak into the inner tube. This leakage of flux is detected by the coils placed between the magnets. The variation of the flux leakage induces current in the coils, thereby causing a signal output. This signal output can be used to provide information on any wall-thickness reduction in the tube. Magnetic flux leakage (MFL) is mainly applied in the inspection of air fin coolers, but it can also be used for inspecting bare tubes with diameters of one inch (2.5cm) and above.
Remote field testing or RFT is one of several electromagnetic testing methods commonly employed in the field of non-destructive testing. Other electromagnetic inspection methods include magnetic flux leakage, conventional eddy current and alternating current field measurement testing. Remote field testing is associated with eddy current testing and the term 'remote field eddy current testing' is often used when describing remote field testing. However, there are several major differences between eddy current testing and remote field testing. RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials. For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm thick (such as what might be found in heat exchangers) would require frequencies around 30 Hz to achieve adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency results in a very low sensitivity of flaw detection. The degree of penetration can, in principle, be increased using partial-saturation eddy current probes, magnetically biased probes and pulsed saturation probes. However, because of the large volume of metal present as well as potential permeability variations within the product, these specialist eddy current probes are still limited in their inspection capabilities. The difficulties encountered in the testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field testing method. The RFT method has the advantage of allowing nearly equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. The method is highly sensitive to variations in wall thickness and tends to be less sensitive to fill-factor changes between the coil and tube. RFT can be used to inspect any conducting tubular product, but it is generally considered to be less sensitive than conventional eddy current techniques when inspecting non-ferromagnetic materials. Near Field Testing (NFT) technology is a rapid and cost-effective solution intended specifically for fin-fan carbon-steel tubing inspection. This new technology relies on a simple driver-pickup eddy current probe design providing very simple signal analysis. NFT is specifically suited to the detection of internal corrosion, erosion or pitting in carbon steel tubing. The NFT probes measure lift-off or 'fill factor' and convert it to amplitude-based signals (no phase analysis). Because eddy-current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tube.