Eddy current testing (ET) is a Non-Destructive Testing technique in which the examiner utilizes a principle based on electromagnetic induction. With this technique, both surface and deep-lying defects can be detected, even in multi-layer constructions. Whereby sub-surface indications can only be detected in non-ferrous materials.
During the examination a probe is used which is suited with a small copper coil that runs an alternating current. This coil thus produces an alternating magnetic field. If this alternating field is brought near a conductive material it will induce an eddy current field in the material under test. This eddy current field will in turn induce an alternating magnetic field which is in opposite direction to the one generated by the coil.
In the material under test the two magnetic fields are brought in balance. Once a defect is present which interrupts the path of the eddy current field it will cause an imbalance between the two magnetic fields. This imbalance will be translated by the handheld equipment to display a unique signature, which will be identified by the examiner as a defect. This is a method that requires a lot of field experience due to the indirect form of signal interpretation.
A major advantage in the use of eddy current examination is that this technique allows the inspection to be done whilst a layer of coating is still present. Provided that the coating layer is of a good quality and not to thick.
Side note; When the inspector detects an indication, its relevance has to be verified with a second NDT technique, like Magnetic Testing or Dye-penetrant Testing. It could be that the applied coating or conservation has to be (locally) removed to allow for this.
Typical applications are:
- (Weld)examination of coated or conserved construction parts,
- (Weld)examination of submerged construction parts,
- (Weld)examination of parts that can’t be contaminated with testing consumables like MPI ink, Dye-Penetrant oil or Ultrasonic coupling gel.
- (Weld)examination at higher (surface) temperatures
- (Semi) automated examination