JOB KNOWLEDGE 29
on an oscilloscope screen as an X-Y display of resistance versus the inductive reactance. This gives a characteristic curve, the shape and size of which can be used to detect and size a defect to determine heat treatment condition or, as a quick sorting test, to establish the type of alloy.
In addition to selecting the optimum frequency the size of the probe can be varied – a large diameter coil will inspect a larger volume of metal and therefore reduce the inspection time – a small diameter probe, however, is more sensitive and better suited to detecting small flaws. The large diameter probes are often used for the detection of large sub-surface flaws in castings and forgings and for the detection of corrosion; the small diameter pencil type probes for detecting cracks. Weld examination requires special probes to reduce noise from the permeability change across a weld. Eddy current testing can be used for a variety of inspection tasks. Chief amongst these is the inspection of welded joints using pencil probes as a replacement for the more conventional magnetic particle or liquid penetrant inspection techniques. A major advantage is that the process may be used underwater and can be used to scan welds through paint and other coatings. With respect to detection of linear defects such as cracks and lacks of fusion the defect should break the lines of the eddy currents ideally at right angles – as with magnetic particle inspection defects parallel to the eddy currents are likely to remain undetected. It is important therefore that the weld is scanned in the correct direction. Cracks as small as 0.5mm deep and 5mm in length are capable of being detected.
Encircling coils are used for automated in-line tube inspection with welded tube lines using localised probes for weld examination. By measuring the conductivity of a metal it is possible to identify and sort both ferrous and non-ferrous metal and with certain alloys – in particular the aluminium alloys – it is also possible to establish the heat treatment condition. Low frequency probes are used to detect generalised corrosion, particularly in the aerospace industry for the examination of aircraft skins. Specially designed “bobbin” probes can be used to inspect the bore of tubes in service for signs of pitting or corrosion and there are also probes specially designed to examine the bores of bolt holes for cracks.
Measuring the proximity of a component to the probe can also be used to determine coating thickness provided the coating is non-conductive. The “lift-off”, the distance of the probe tip from the conductive surface, causes a change in eddy current flow which is measurable.
All of the systems must be calibrated using appropriate reference standards – as for any NDT method, this is an essential part of any eddy current examination procedure. The calibration blocks must be of the same material, heat treatment condition, shape and size of the item to be tested. For defect detection the calibration block contains artificial defects simulating defects; for corrosion detection a calibration block of different thicknesses is used. The eddy current method requires more skill on the part of the operator than, say, MPI and penetrant inspection – it goes without saying that operator training is essential.
TWI trains over 26,000 students each year around the world in welding, engineering, non-destructive testing and welding inspection. For more information on training courses please visit
www.twitraining.com, or call on +44 (0)1223 899500. l
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