1. The principles of Eddy Current Testing (ECT):

Eddy Current Testing (ECT)  is based on the principles of electromagnetic induction and is used to identify or differentiate among a wide variety of physical, structural, and metallurgical conditions in electrically conductive ferromagnetic and nonferromagnetic metals and metal parts. Eddy current inspection can be used to:

- Measure or identify such conditions and properties as electrical conductivity, magnetic permeability, grain size, heat treatment condition, hardness, and physical dimensions.

- Detect seams, laps, cracks, voids, and inclusions.

- Sort dissimilar metals and detect differences in their composition, microstructure, and other properties.

- Measure the thickness of a nonconductive coating on a conductive metal, or the thickness of a nonmagnetic metal coating on a magnetic metal.

Picture 1: Equipment of Eddy Current Testing (ET).

2. The process of Eddy Current Testing (ECT):

- The alternating current flowing through the coil at a chosen frequency generates a magnetic field around the coil.

- When the coil is placed close to an electrically conductive material, eddy current is induced in the material.

- If a flaw in the conductive material disturbs the eddy current circulation, the magnetic coupling with the probe is changed and a defect signal can be read by measuring the coil impedance variation.

Picture 2: The process of ET

3. Advantages and disadvantages of Eddy Current Testing (ECT)

- Advantages:

• Sensitive to surface defects
• Can detect through several layers
• Can detect through surface coatings
• Accurate conductivity measurements
• Can be automated
• Little pre-cleaning required
• Portability

- Disadvantages: 

• Very susceptible to permeability changes
• Only on conductive materials
• Will not detect defects parallel to surface
• Not suitable for large areas and/or complex geometry's
• Signal interpretation required
• No permanent record (unless automated)
• Expensive equipment