Eddy-current testing uses electromagnetic induction to detect flaws in conductive materials. A circular coil carrying current is placed in proximity to the component to be inspected. The alternating current in the coil generates changing magnetic field which interacts with component and generates eddy currents. Variations in the phase and magnitude of these currents are monitored either using a second 'search' coil, or by measuring changes to the current flowing in the primary 'excitation' coil. The presence of any flaw will cause a change in eddy current and a corresponding change in the phase and amplitude of the measured current.
Advantages
- Provides a faster scanning speed than conventional ultrasonic testing (UT)
- Unlike UT, requires no fluid couplant
- Again, in contrast to UT, the surfaces need minimal preparation and eddy current testing can be used through several millimetres of coating whereas ultrasonics can only be used through relatively thin layers (a few hundred microns).
- Can detect very small cracks in or near the surface of the material
- Physically complex geometries can be investigated
- Can be used to make electrical conductivity and coating thickness measurements
- The testing devices are portable
- They provide immediate feedback
- There is no need for the device to contact the component
TWI's Eddy Current Resources
Computerised systems. TWI owns two R/D Tech® Olympus MultiScan MS5800 computerised eddy current systems. These are used for the detection and sizing of surface breaking flaws in any conducting material. The MS5800 is a multi-channel unit for use with array probes with up to 32 coils. It is operated under the control of MultiView™ software. Output can be in the form of a colour-coded map, where the severity of any flaw is indicated by colour. The data can also be presented in the form of isometric views of the component. The probe can be manipulated in a robotic scanner, whose position is fed back to the computerised data acquisition system. However, complex shapes can also be inspected in a single probe pass without the need of scanners or robotic systems.
Conventional systems. TWI also owns several GE Hocking Phasec 2d portable dual frequency systems. One is shown above being used with a pencil probe to inspect for cracking at the edges of bolt-holes in a Ti alloy turbine disc.
Selected clients and applications
TWI is using its eddy current systems for many applications, typical ones being:
- Inspection of friction stir welds
- Testing gas turbine blades for Rolls- Royce
- Inspection of Hurricane propeller hubs for the Battle of Britain Flight
- Inspection of a cast iron bridge built by Thomas Telford
- Testing nozzle welds in nuclear reactor pressure vessels for power generation utilities. In this work, TWI is using the ability of the system to cope with complex geometries
For more information, please contact us.
