Eddy Current Testing

Eddy Current NDT Inspection of a weld without the need to remove paint

Image: Eddy Current Testing of welds through paint

Eddy Current Testing (ET) is used for the detection of surface or near-surface defects in materials such as aluminium, stainless steel, copper, titanium, brass, Inconel alloys, and even carbon steel (surface defects only). ET does not require the removal of paint or coatings to be able to inspect for surface-breaking defects in carbon steel. This makes ET an economical NDT technique for inspecting cranes, lifting lugs, and various other assets that require routine inspection.

Eddy currents are fields of alternating magnetic current that are created when an alternating electric current is passed through one or more coils in a probe assembly. When the probe is placed close to the part under inspection, the alternating magnetic field induces eddy currents in the test part. Discontinuities or property variations in the test part change the flow of the eddy current and are detected by the inspection probe, enabling material thickness measurements or the detection of defects such as cracks and corrosion.

Eddy Current Testing

Image: Eddy Current testing through paint on heavy machinery components

Advantages of Eddy Current Testing

  1. Sensitive to Small Cracks and Other Defects: ET is highly sensitive to small cracks, especially those perpendicular to the coil movement.

  2. Versatile for Various Applications: It’s used for crack detection, material thickness measurements, conductivity measurements for material identification, and heat damage detection.

  3. Immediate Results: The results are immediate and can be displayed in real-time, aiding quick decision-making.

  4. Non-Contact Method: ECT does not require physical contact with the material, making it suitable for delicate surfaces and automated scanning systems.

  5. Can Inspect Through Coatings: Thin non-conductive coatings or paint do not need to be removed as ET can inspect through them.

  6. No Safety Hazards: Unlike radiographic testing, ET does not use hazardous radiation.

  7. Minimal Surface Preparation Required: It requires less surface preparation compared to methods like dye penetrant or ultrasonic testing.

Disadvantages of Eddy Current Testing

  1. Limited Penetration Depth: ET is mainly used for surface and near-surface defect detection and has limited depth penetration.

  2. Conductivity Requirements: It’s only applicable to conductive materials, limiting its use on non-metals.

  3. Surface Condition Effects: The method is sensitive to the surface condition of the material, and rough or irregular surfaces can interfere with readings.

  4. Coupling Consistency Needed: Inconsistent probe contact and liftoff can affect the reliability of results.

  5. Variability in Material Properties: Variations in material properties like conductivity and permeability can affect results.

  6. Risk of Electromagnetic Interference: ET can be susceptible to electromagnetic interference from external sources.

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