Eddy Current Testing

Eddy current Testing (ET) is an electromagnetic testing method wherein a localized alternating current loop is induced in the test object and the inductive reactance of probe to magnetic field of induced current indicates subsurface flaws. It can be utilized on electrically conductive materials only.

All nonmagnetic metals such as copper, aluminum and some stainless steels have measurable conductivities. The more conductive a material, the easier it is to induce an electric current in it. We know that every current carrying conductor has a magnetic field, and the field is perpendicular to the current flow.

As an example, let us take a plate of aluminum as our conductor. An eddy current test probe driven at a given AC frequency is brought very close to the surface of the aluminum plate. The test probe is wound in such a way that it produces a downward vertical magnetic field. These magnetic fields interact with the aluminum plate inducing eddy currents. The eddy currents induced in the aluminum plate move in circular patterns in the horizontal plane. These eddy currents themselves produce a magnetic field perpendicular to their flow, in essence an upward vertical field. The eddy currents magnetic field couples and interacts with the probes magnetic field. This is known as magnetic coupling and self inductance. The eddy currents magnetic field induces a secondary current in the test coil, opposing the original current. This opposition can be referred to as impedance. It is this impedance that is measured, compared and quantified in electrical terms in eddy current testing. A crack in the aluminum plate will change the plate’s conductivity and a cascade of changes will follow. The crack will change the induced eddy current’s magnetic field, which will change the inductance in the test probe and finally the impedance in the test probe. These changes are noticeable when compared to a non cracked area on the plate. Eddy current testing is a comparative test, comparing a test material to a standard or to itself. If improper test parameters are used, it is entirely possible to miss a longitudinal crack running the entire length because the crack is “constant” and no change is noticed.

The strength of magnetic coupling depends on how close the probe is to the surface of the aluminum plate. The closer the probe is, the greater the magnetic coupling. A good test will maintain a constant coupling or lift –off. Changes in lift –off can introduce errors in the test. Lift-off effects can be suppressed electrically so that they are not a significant variable. However, lift-off can be calibrated and exploited to measure coating or paint thicknesses.

Eddy current testing is basically measuring a change in conductivity in a material and a change of impedance in the test probe. Copper is used as a reference standard for conductivity and set at 100%. Conductivity is measured as a percentage of International  Annealed Copper Standard, with the units as %IACS. All conductive materials can be plotted on a convex downward sloping conductivity curve. The more conductive a material the lower it appears on the conductivity curve. Copper is close to the bottom of the conductivity curve, at 100 % IACS. Aluminum is in the 30% IACS region. Eddy currents can be used to measure conductivity and hence sort alloys, material identification, hardness, metallurgical differences or heat treat condition.

Eddy current tests have been used successfully to replace Penetrant (PT) and Magnetic particle testing (MT) in certain applications. PT is a surface tests, that is, the crack has to be open to the surface of a test object. PT and in most cases MT, require surface coatings to be removed, which can be time consuming and costly. Eddy currents can be carried out with the coating intact. In non-magnetic materials, depending on the materials location on the conductivity curve, with the correct frequency, eddy currents can inspect to a depth slightly over ½ inch deep. In ferromagnetic alloys ET is a shallow surface test with a depth slightly below 0.010 inches deep.Eddy Current Testing can be utilized to detect cracks up to 0.2 mm in length.

Eddy current tests have been more sensitive than PT tests in finding cracks in aircraft landing gear wheels. PT tests must be performed between 50-120 degree F, doing PT outside in winter is not always an option. Eddy current tests do not have this temperature restriction if calibrated accordingly, and are much faster.

ET tests are routinely done to inspect condenser tubing in power generation, petrochemical industries and HVAC units. Clad metal thicknesses, (conductive coating on conductive material) can be measured by eddy current testing.

Summary of applications:

• Flaw detection, bolt hole cracks, welding, corrosion mapping, PT replacement
• Material Sorting
• Nonconductive coating thickness, paint, epoxies, ceramics
• Conductive coating thickness, clad metals
• Material/dimensional thickness gauging
• Hardness measurements