Eddy Current inspection is one of several NDT methods using the principle of electromagnetism to conduct examinations. Other methods, such as remote field testing (RFT), flux leakage and Barkhausen noise, also use this principle.
Eddy Current Testing is completed through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to maximum and then collapses as the current is reduced to zero. When another electrical conductor is positioned in close proximity with this changing magnetic field, current will be induced into this second conductor. Eddy currents are induced electrical currents that flow in a circular path. Eddy Current testing takes its name from Eddies, which are formed when a liquid or gas flows in a circular path around obstacles under the right conditions.
ID probes, also referred to as Bobbin probes or feed-through probes, are inserted into pipelines or tubing to inspect from the inside out. ID probes have a housing that keep the probe centred in the product and maintain the orientation of the coil(s) constant relative to the test surface. The coils are most commonly wound around the circumference of the probe so the probe inspects an area around the entire circumference of the test object in one go.
Applus+ offers five NDT inspection methods for heat-exchanger tubing systems:
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.
Alternating current field measurement (ACFM) is an electromagnetic NDT technique capable of both detecting and sizing (length and depth) surface-breaking cracks in metals. The ACFM NDT technique alternates constant-current in a tangential solenoid, remote from the test surface, and 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 in this electromagnetic NDT process.
RTD INCOTEST (INsulated COmponent TESTing) is based on the pulsed eddy current principle to carry out PEC inspection in a wide range of industry sectors. Pulsed eddy current testing is a reliable way to survey ferrous pipes and vessels through their thermal insulation and protective coatings. Our jointly-developed PEC inspection technology is an excellent tool for prioritising corrosion under insulation 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 NDT probe. The average remaining wall thickness within the enclosed magnetic field is proportional to the decay time of the received signal.
Remote field testing (RFT) is one of electromagnetic NDT methods most commonly employed in the field of testing and inspection. Other electromagnetic inspection methods include magnetic flux leakage, conventional eddy current and alternating current field measurement testing.