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Normally carried out with pencil probes or 'pancake' type probes on ferrous or non-ferrous metals. Frequencies from 100 kHz to a few MHz are commonly used. Depending on surface condition it is usually possible to find cracks as small as 0.1 mm or so deep.
Differential probes are sometimes used, particularly in automated applications, care must be taken to ensure that the orientation of flaws is correct for detection.
This is essentially conductivity testing and for dedicated applications a conductivity meter may be a better choice. From the impedance plane diagram it will be seen that the indication from a conductivity change is essentially the same as from a crack, and both meter and impedance plane type crack detectors can be successfully used to sort similar metals using a suitable absolute probe. It should be remembered that widely different metals may have similar conductivity and that the allowable values for similar alloys my overlap, so conductivity measurement should only be used as an indication that a metal is of correct composition or heat-treatment.
Primarily used in Airframe inspection. By using a low frequency and a suitable probe eddy currents can penetrate aluminium or similar structures to a depth of 10mm or so, allowing the detection of second and third layer cracking, which is invisible from the surface, or thinning of any of the different layers making up the structure.
Heat exchangers used for petrochemical or power generation applications may have many thousands of tubes, each up to 20m long. Using a differential ID 'bobbin' probe these tubes can be tested at high speed (up to 1 m/s or so with computerized data analysis.) and by using phase analysis defects such as pitting can be assessed to an accuracy of about 5% of tube wall thickness. This allows accurate estimation of the remaining life of the tube allowing operators to decide on appropriate action such as tube plugging, tube replacement or replacement of the complete heat exchanger.
The operating frequency is determined by the tube material and wall thickness, ranging from a few kHz for thick-walled copper tube up around 600 kHz for thin-walled titanium. Tubes up to around 50mm diameter are commonly inspected with this technique. Inspection of ferrous or magnetic stainless steel tubes is not possible using standard eddy current inspection equipment.
Dual or multiple frequency inspections are commonly used for tubing inspection. In particular for suppression of unwanted responses due to tube support plates.. By subtracting the result of a lower frequency test (which gives a proportionately greater response from the support) a mixed signal is produced showing little or no support plate indication, thus allowing the assessment of small defects in this area.
Almost all high-quality steel tubing is eddy current inspected using encircling coils . When the tube is made of a magnetic material there are two main problems:
Because of the high permeability there is little or no penetration of the eddy current field into the tube at practical test frequencies.
Variations in permeability (from many causes) cause eddy current responses which are orders of magnitude greater than those from defects.
These problems may be overcome by magnetically saturating the tube using a strong DC field. This reduces the effective permeability to a low value, allowing effective testing.
Tubes up to around 170mm diameter are commonly tested using magnetic saturation and encircling coils.
When tubes are welded this is usually where the problems occur, and so welded tubes are commonly tested in-line using sector coils which only test the weld zone.
The geometry and heat-induced material variations around welds in steel would normally prevent inspection with a conventional eddy current probe, however a special purpose "WeldScan" probe has been developed which allows inspection of welded steel structures for fatigue-induced cracking, the technique is particularly useful as it may be used in adverse conditions, or even underwater, and will operate through paint and other corrosion-prevention coatings.
Cracks around 1mm deep and 6mm long can be found in typical welds.
There are a number of excellent books available on eddy current testing. Many are available from ASNT or other national institutes.
The following are particularly recommended to 'fill in' if a more complete understanding is required.
Fundamentals of eddy current Testing,
Donald J. Hagemaier, ASNT, 1990
Advanced Manual For: Eddy Current Test Method
Canadian General Standards Board
NDT Handbook 2nd Edition Volume 4 b,
Ed. Paul McIntyre/Mike Mester, ASNT, 1986
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