Principles of Eddy Current NDT

Eddy Current NDT Technology Overview

Eddy Current NDT is a non-destructive testing method commonly used to detect surface and near-surface defects in conductive materials. Eddy current NDT can be used for various conductive materials and does not have strict requirements on the material's conductivity. It can be used to detect metal pipes, surface cracks, welding defects, coating corrosion, etc.

Eddy Current NDT has the advantages of being fast, non-contact, and highly sensitive. It is widely used in aerospace, automotive, power, petrochemical, and other fields. Depending on the application scenarios, different types have been derived: multi-frequency eddy current NDT, remote field eddy current NDT, and pulsed eddy current NDT.

Principle of Eddy Current NDT

To understandeddy current testing, we must first know what eddy currents are.

When a conductor made of metal is placed in a changing magnetic field, according to Faraday's law of electromagnetic induction, an electric current resembling the vortex of water flow is induced inside the metal conductor. This induced current, due to its vortex-like flow pattern, is called eddy current, and the described physical phenomenon is known as the eddy current effect.

Eddy Current NDT technology utilizes the physical phenomenon of the eddy current effect. When there is a defect in the tested metal specimen, the distribution of induced eddy currents and the magnetic field in the specimen will be correspondingly altered due to the defect. By analyzing these changes, one can understand relevant details about the defects in the specimen.

When using an alternating excitation source to apply an excitation signal to the coil, a corresponding excitation magnetic field is induced in the coil. When the exciting coil approaches the surface of the tested metal specimen, the excitation magnetic field generated by the coil induces an eddy current inside the metal specimen. This eddy current propagates through the specimen and simultaneously induces a magnetic field known as the induced magnetic field of the eddy current.

When there are no defects in the metal specimen, the eddy currents form a loop. When there are defects in the metal specimen, the defect is air, and the impedance of air is significantly higher than that of the metal. Therefore, the eddy currents change their original path when they reach the defect area, by passing the defect.

According to Lenz's law, the direction of the magnetic field induced by eddy currents is opposite to the direction of the coil's magnetic field. The magnitude, phase, and flow pattern of the eddy currents are influenced by factors such as the material's electrical conductivity, magnetic permeability, size, and structural properties. The magnetic field induced by the eddy currents, in turn, affects the original magnetic field, causing changes in the impedance of the detection coil. When a defect on the specimen interrupts the flow of the original eddy currents, it alters the distribution of the eddy currents. The effect of this interaction with the magnetic field is reflected in changes in the coil's impedance. By measuring these changes in the detection coil's impedance, it is possible to determine whether the material being tested contains defects.