How Does Eddy Current Testing Work?

Eddy current testing, or ECT, is commonly used in manufacturing and service industries that use sheet metal, tubing, and other thin metal products for critical operations. In short, eddy current testing uses electromagnetic fields to identify cracks, metal thickness and hardness, heat damage, conductivity, and other characteristics of metal surfaces without damaging the test material.

How Does Eddy Current Testing Work?

If you think your facility might benefit from ECT, you might be wondering how this testing process works. Electromagnetic induction is at the heart of eddy current testing. An eddy current probe consists of a conductive wire coil (often made of copper) through which an alternating current flows, generating an oscillating magnetic field around the coil. The field oscillates at the same frequency as the current running through the coil.

When the probe is brought close to a conductive metallic surface, the circular flow of electrons, called an eddy current, moves across the metal like ripples on water. This causes the conductive test material to generate its own magnetic field, which interacts with the coil’s field through mutual inductance.

As the probe passes over cracks, pits, and other defects in the test material’s surface, the pattern of the eddy current (and the resulting magnetic field) is altered or interrupted. This changes the movement of the electrons and produces a signal that shows up on the attached test instrument screen. The technician performing the test can tell where the defect is located based on the changes in the phase angle and impedance amplitude displayed on the screen.

Factors that Impact Eddy Current Inspection Results

Interpreting the results of an eddy current test requires specialized training because many factors can impact test sensitivity, resolution, and penetration. These factors include:

  • Coil type and size: Larger coils can inspect larger materials because the magnetic field flows deeper, but smaller coils are more sensitive to minute defects.
  • The frequency of the alternating current running through the coil: This affects how far the eddy currents penetrate the test material. Higher frequencies result in better near-surface resolution, but they limit penetration.
  • The conductivity of the test material: The higher the conductivity, the more surface defects the probe can detect. However, penetration into the material is limited.
  • The test material’s magnetic permeability: Variations in this value generate background noise that can limit the resolution.

A knowledgeable technician selects the proper coil type, size, and test frequency to overcome limitations created by material conductivity and magnetic permeability. This provides the best possible test results despite conditions that are out of the technician’s control.

Eddy Current Testing in Chicago

Because ECT is so complex, it’s important to hire an experienced technician for the job. Murphy & Miller, Inc. has over 80 years of experience providing our customers in Chicago with eddy current testing and other mechanical and technical services. To learn more about how eddy current testing works, or to schedule services, please contact us today at 1-312-427-8900.