What is the principle of eddy current inspection?

• A. It relies on penetrating a surface with dye.
• B. It uses magnetization and particle visualization.
• C. A current is induced into the metal by a test probe. The amount of induced current is determined by the conductivity, mass, and permeability of the material, and the presence of and faults or voids.
• D. It measures acoustic waves.

Answer: (C)

Eddy current inspection relies on electromagnetic induction to generate currents in the surface region of a conductive metal. An alternating current in the probe creates a changing magnetic field that induces circular eddy currents in the material. The amount and pattern of these currents depend on the metal’s conductivity, permeability, and thickness. When flaws such as cracks or voids are present, they disrupt the flow of eddy currents, causing changes in the probe’s impedance or in the signal it senses. By monitoring these impedance changes, you detect and evaluate near-surface defects. This approach is different from dye penetration (which uses dye ingress to reveal surface-breaking flaws), magnetic particle inspection (which uses magnetization and visualization of particles), and ultrasonic testing (which relies on measuring sound waves).