Theory Behind Eddy Current Inspection
A coil is used to induce an AC current into a conductive material. The current concentration is at the surface and in ferrous metal there is minimal penetration.
Impedance changes with a single coil are indicated with lift-off providing a significantly different signal to a defect.
Use of Eddy Current Probes for Weld Inspection
Reasons for using eddy current probes are:
- In MPI and UV inspection, the coating has to be removed before inspection, costing both money and time
- Eddy current probes enable efficient inspection allowing welds to be efficiently inspected for near-surface cracks
- Eddy current can inspect welds through metallic or paint coatings
Use of Special Probe for Weld Inspection
Using a differential coil configuration enables eddy current weld probes to perform inspection without the need of removing paint or coating. This helps the probe to be highly sensitive to lift-off and variations in material properties resulting from the heat affected zone through a simple absolute probe.
A probe was constructed with two interleaved coils. Coil-positioning is critical to producing signals as needed in the inspection procedure.
Scanning with an Eddy Current Weldscan Probe
The active part of the probe is 3 * 3 mm. For scanning the material, multiple scans need to be performed, which are zig zag scan of the toe in the heat affected area along the length of the toe, scan of the weld cap and sweep along the toe.
Specialty of the Weld Probe
The weld probe is very special due to minimal spurious signals caused by lift-off due to differential connection. The probe features a minimal lift-off sensitivity variation 8 dB per mm as compared with the pencil probe at 40 dB per mm.
Directionality implies that the defects in line with the scan offer a positive indication and defects at right angles offering a negative indication with a 45° null. The modelling of the probe has been done and a highly linear field and in the active area resulting in good sizing capability and superior lift- off characteristics.
Offshore applications include the following:
- It is used extensively for weld inspection where it replaces MPI since there is no need to remove the surface coating
- Aids in rope access inspection
- Widely used in the nuclear NDT industry
- Used for conventional manual inspection and also in automated scanners
- Used for manual inspection of wheels and rails
- Used for axial inspection where it is more reliable than UT
- Used for weld inspection on bogeys and chassis
- Used for testing bridges, steel framed buildings, prison bars for detection of saw cuts, amusement rides and traffic lights
Equipment Used to Perform a Weldscan Probe Inspection
The equipment include the following:
- Impedance play display eddy current instrument such as veritor or vantage
- Weld Probe
- Paint probe for assessing coating thickness and compensating calibration using shims
- Test block with 2, 1 and 0.5 mm notches and four 0.5 mm shims
The technique has been accepted widely by several bodies that include DNV, PCN and LLOYD register. IN 2000 BS EN 1711 was issued.
The weld probe offers a good alternative to MPI inspection as there is no need to remove the coating. It is highly suited for rope access inspectors. The probe is relatively convenient to use so it find applications in axle inspection. Training courses with operator certification, national standards and acceptance by certifying authorities imply that the weld probe becomes a recognised solution.
About Ether NDE
Ether NDE are a dedicated manufacturer of Eddy Current NDT equipment and accessories and the 'go to' company for all your Eddy Current NDT Solutions.
Ether NDE continually strives to provide innovative solutions to eddy current testing in all possible inspection conditions.
Offering a range of innovative eddy current testing instruments and probes, Ether NDE will endeavour to find the solution that best fits the clients specific needs.
ETher NDE pride themselves on the ability to remain client focussed, conducting their business with three simple customer promises:
- The ability to speak to someone who understands their products and your application
- Industry leading delivery on goods and the ability to respond to your unique challenges
- Provide products that are second to none in both performance and quality.
This information has been sourced, reviewed and adapted from materials provided by Ether NDE.
For more information on this source, please visit Ether NDE.