NDE_ElectromagneticTesting

Introduction

The scientific development of eddy current theory started with the discovery of the law of electromagnetic induction by Faraday. Faraday’s law states that when a magnetic field cuts a conductor or when a conductor cuts a magnetic field, an electrical current will flow through the conductor if a closed path is provided over which the current can circulate. Eddy currents are defined as oscillating electrical currents induced in a conductive material by an alternating magnetic field, due to electromagnetic induction.

Principle

In eddy current testing (ECT) an alternating current (frequency 1kHz - 2 kHz) is made to flow in a coil (also called probe), which in turn produces an alternating (primary) magnet field around it. This coil, when brought close to the electrically conducting surface of a metallic material that is to be inspected, induces an eddy current flow in the material due to electromagnetic induction. These eddy currents in turn generate an alternating (secondary) magnetic field in the opposite direction which may be detected either as a voltage across a second coil or by the perturbation of the impedance of the original coil. The opposing magnetic field coming from the material has a weakening effect on the primary magnetic field and the test coil can sense this change. In effect the impedance of the coil is reduced proportionally as eddy currents are increased in the test piece. A crack in the test material obstructs the eddy current flow, lengthens the eddy current path, reduces the secondary magnetic field and increases the coil impedance. The inductive reactance of the coil increases as the severity of the defect increases.

Techniques
  1. Constant current drive ECT techniques: Enhanced sensitivity and better discrimination are possible with the use of constant current excitation of test coils. Additionally use of this method can result in reducing non-linear responses.
  2. Scanning probe technique: Scanning probe is used for wire and tube testing for detecting very small defects, which are not detectable by encircling coils. Scanning probe can be used with normal hand rotation or automatic rotation around the test object. The resolution of the scanning probe is independent of the diameter of the test object because the probe can see only a limited area of the object. The scanning probe performs better as far as sensitivity of testing is concerned.
  3. Multi frequency ECT: Multi-frequency ECT is a suitable NDT technique for in service inspection of steam generator, heat exchanger and condenser tubing. Extraneous discontinuities such as tube support plate, tube sheet, internal corrosion deposits and dents interfere in conventional single frequency testing. Multi- frequency testing is the direct equivalent of operation of more than one single frequency unit with a common search coil. Independent information of material characteristics will be stored at different frequencies. Test results from individual frequencies can be combined in real time so as to obtain output, which is free of certain unwanted parameters.
  4. High frequency ECT: In specialized applications for detecting fine surface cracks like fatigue cracks, the use of higher frequencies is desired in order to reduce penetration depth thus achieving high current density on the surface. This means that eddy currents do not flow deep into the material. The main advantage of this feature is the maximum concentration of eddy currents at the flaw and gives maximum detectable sensitivity. To achieve this, electron spin precision resonance in ferromagnetic crystals such as Yttrium Iron Garnet (YIG) or Gallium doped YIG is employed.
  5. 3D or phased array ECT: Conventional eddy current probes cannot detect defects that are parallel to their windings, because of limited interaction of eddy currents with the defects. The 3D system uses a different bridge circuit where each of the three coils is driven with a unique phase of inspection frequency. (0, 120 and 240 degrees). The magnetic field generated by this three coil configuration results in a constant magnitude rotating magnetic field. The current flow in a 3D probe causes both circumferential and axial current flow to occur at every point within the tube at some point of time. This makes #D ECT sensitive to circumferential and longitudinal defects and pits.

Applications
  1. Determine the carbon content of various steels.
  2. Determine alloy composition of ferromagnetic materials based on permeability.
  3. Provide alloy identification and sorting.
  4. Determine the effects of corrosion thinning in pipes and vessels.
  5. Determining the hardness and depth of case hardening in bearing rings and other parts.
  6. Measure the length of unwelded seams in welded tubing using absolute coil technique.
  7. Test fine wire at maximum flaw sensitivity using absolute coils.
  8. Test ball pins with a single pointed eddy current test probe.
  9. Test motor valve seating rings using two moving probes while scanning a rotating test piece.
  10. Determine the installed position of piston rings in motors using the differential probe technique.

Limitations
  1. The major limitation of ECT is that only electrically conductive materials can be inspected.
  2. Since too many parameters affect the eddy current probe impedance, ECT is not effective when more than one variable is present.
  3. Another limitation of ECT is that it can inspect with reasonable sensitivity, metallic components of thickness up to only 6 mm.