State-of-the art eddy current NDT control allows for inspecting a widest variety of conductive materials:
Eddy current NDT inspections detect discontinuities, measure the exact dimensions, reveal vibrations and determine physical and mechanical properties and condition of facilities.
Currently, the eddy current NDT method facilitates technical diagnostics of the following:
• conductive rods;
• coatings (including multi-layer coatings);
• nuclear reactor vessels;
• bearing balls and rolls;
• fastening parts and other industrial products.
Its advantages are normally said to include, first of all, high sensitivity to microscopic defects located on surfaces or in the immediate vicinity of the tested area of a metal object. The eddy current NDT method is efficient even if there is a minor gap between the inspected item and the transducer (ranging from parts of a millimeter to several millimeters).
This method differs for the ultrasound method in that an eddy current sensor does not require a coupling liquid. Many see the eddy current NDT method attractive due to its comparatively fast process. Such inspection is quite feasible and efficient even with objects of complex geometry or those located in difficult spots.
Among the disadvantages of the eddy current method are, first of all, the possibility to test only conductive materials: metals, alloys, graphites and semi-conductors. Secondly, the method offers a low penetration depth, which does not exceed several millimeters, since it is determined by the depth of penetration of electro-magnetic field into the controlled medium. The eddy current method is used to detect superficial or shallow discontinuities, as well as various cracks, stratifications, rolling laps, cavities, non-metal inclusions, etc. A superimposed transducer allows for detecting cracks 0.1–0.2 mm deep and 1–2 mm long under favorable test conditions and with little impact of negative factors. A passing transducer detects cracks about 1 mm long and 1–5% of the diameter of the controlled wire or rod in depth. A fault on an item's surface or at a small depth changes the path of eddy currents. This results in change of EMF in the measuring coil. The structural condition of metals and alloys influences their electrical and magnetic properties. This enables inspection not only of chemical composition homogeneity, but also of metals and alloys structure, as well as determination of mechanical stress. Eddy current devices are used to control the quality of thermal and thermochemical processing of parts, and the condition of superficial layers after mechanical processing (grinding, cold working), to detect residual mechanical stress and fatigue cracks in metals at early stages of their development, etc.
Eddy current NDT applications:
• Aircraft engineering. The method takes a special place in this industry, since here it is used both during airplane, helicopter and other aircrafts manufacturing, and during active operation. The eddy current NDT method is used to diagnose wings, body, wheel disks, components of engines, rotors, shafts, mounting holes, etc.
• Construction. First of all, eddy current NDT should be mentioned in connection with control of weld seams and metal structures. It detects cracks, burns, etc.;
• Automotive industry, car maintenance.
• Ship building (here, in particular, eddy current NDT is used on vessel shells).
• Oil and gas industry. The main objects here are oil lines, pipelines, gas lines, tanks, etc.
Overall, the eddy current NDT method is applicable wherever one needs to inspect painted, plated, protective, insulation coatings and other coatings on a metal base.
In some cases, eddy current NDT inspection is more efficient than capillary weld control.