Eddy Current NDT (ET)
Like magnetic particle testing, eddy current testing is another electromagnetic testing technique. It works on the principle of electromagnetic induction.
When current passes through any current-carrying conductor (primary conductor), it generates a magnetic field (primary field).
When we place a second conductor (test specimen) in this magnetic field, the primary magnetic field induces an opposing electric current in this conductor.
This current is known as eddy current and it is proportional to the change in the magnetic field as the alternating current in the primary coil rises and falls during every cycle.
The fluctuating eddy current creates its own magnetic field (secondary field) that opposes the primary field and affects the voltage and current flowing through the primary conductor.
As the test specimen’s magnetic permeability and electrical conductivity changes because of the defects, the magnitude of the eddy current changes. These changes can be recorded by using the primary or the secondary coil and analysing them to get more information about the defect.
Advantages of eddy current testing:
- Immediate results
- Minimal part preparation
- Can be a non-contact process
- Works with complex part geometries
- Capable of automation for uniform parts
- Can spot surface and subsurface defects up to 0.5 mm in size
- Multipurpose. It can also measure coating and material thickness, identify materials and their heat treatment conditions
Disadvantages of eddy current testing:
- The penetration depth depends on many factors
- Cannot detect flaws parallel to the part surface
- Only works with electrically conductive materials
- Requires high-skill personnel for accurate interpretation of results