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 current in the vicinity.
Variations in the changing phases and magnitude of these eddy currents is then monitored through the use of receiver-coil(s), or by measuring changes to the alternate current flowing in the primary excitation-coil.
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.
TWI Resources
Equipment available at TWI:
- R/D Tech® Olympus MultiScan MS5800 (computerised eddy current systems)
- MultiView™ software
- GE Hocking Phasec 2d portable (dual frequency systems)
- Nortec 500 dual frequency portable with Eddymaster software
- EtherNDE veritor single frequency laptop based system with automated controls
Techniques available with this type of testing are:
- Single probe testing at a wide range of frequencies
- Array scanning technique
- Multi Frequency technique
- Pulsed Eddy Current technique
- Remote Field Technique
Benefits
- Detection of very small cracks in or near the surface of the test part
- Physically complex geometries can be investigated
- Electrical conductivity measurement
- Coating thickness measurement
- Provides immediate feedback
- Portable
- No couplant is required
Applications
- 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
For more information about TWI's NDT services, please email contactus@twi.co.uk.