Continuous wave eddy current testing is one of several non-destructive testing methods that use the electromagnetism principle. Conventional eddy current testing utilises electromagnetic induction to detect discontinuities in conductive materials.
A specially designed coil energised with alternating current is placed in proximity to the test surface generating changing magnetic-field which interacts with the test-part producing eddy The electrical conductivity variations or magnetic permeability of the test-part, or the presence of any discontinuities, will cause a change in eddy current and a corresponding change in phases and amplitude of the measured current. The changes are shown on a screen for easy interpretation.
Another method of using the eddy current principle is pulsed eddy current testing. Methods are being developed to investigate surfaces through protective coatings, weather sheetings, corrosion products and even the insulation materials.
Inspection of surfaces at high temperature are possible with the eddy current technique as it is a non-contact method.
Characteristics of the ECT technique :
- Suitable for non-ferrous tubing such as SS304/316, Brass, Titanium, Inconel, Cu, Cu-Ni, etc.
- Can detect pits, corrosion, erosion and axial cracking.
- Uses multiple frequencies for better analysis and flaw sizing.
- The probe needs a good fill factor to remain sensitive (about 0.85 to 0.9).
- Probe centering is important for uniform sensitivity and reduce lift-off signal.
- Very fast technique, up to 2m/s pulling speed.
- Checking for surface breaking cracks on metal
- Metal tube inspection for discontinuities
- Heat treat verification of metals
- Checking conductivity of metals, thickness of coatings and of thin metals
- Inspection of friction stir welds
- Testing gas turbine blades
- Inspection of a cast iron bridge
- Inspection of Hurricane propeller hubs
- Testing nozzle welds in nuclear reactor
- Inspection of girth welds of Pressure vessels