Not just for large surfaces:
Delivering precise results
As well as identifying cracks, this test also measures coating thicknesses and material quality. For especially critical areas of the engine, such as the blades and welding seams, other non-destructive tests (NDT) for materials are prescribed in addition to the mandatory fluorescent penetrant inspection (FPI) and magnetic particle inspection (MPI).
Unlike FPI, MPI and x-raying, under which the results depend on the experience and watchful eye of the technician concerned, eddy-current and ultrasound testing produce precisely measured results.
Like FPI and MPI, eddy-current measurement is also a surface technology, as it permits conclusions to be drawn only for abnormalities on or immediately below the surface. On the other hand, the two volume methods of x-raying and ultrasound cast a light on the inside of the component as well. As these three non-destructive material testing procedures require highly specialized measuring equipment, often the manufacturer of the components also supplies the equipment for testing it.
It is primarily due to the large-scale use of aluminum in aircraft construction that a procedure for determining material weaknesses and defects is necessary. Testing using this approach requires that the material is electrically conducting, which, in the case of engine parts, means it can be applied to most steel, aluminum, titanium, nickel-base, cobalt and magnesium alloys. As in the case of other surface techniques, the depth to which eddy-current testing is effective is limited.
In the engine area, eddy-current measurement is primarily conducted manually using a probe with a small copper coil on the rotating head, which creates the magnetic field. This magnetic field, which changes over time, induces an eddy-current in the interior of the component which, in turn, creates an opposite magnetic field. As soon as the eddy-current in the component encounters any damage to the material, it changes its flow and consequently the size of its own magnetic field. The difference is indicated by a deflection in the measuring device. As the test is conducted with a constant rate of advance, the measured signal allows the position of the fault to be accurately determined. The engine shop is also able to conduct eddy-current measurements automatically. A turbine disk is, as it were, "sampled– in the test device by the probe as if it were a record.
The measuring instruments are calibrated with reference to known defects. Ideally natural cracks on tested components which have been taken out of service would be used, but frequently a collection of assorted parts with artificially produced cracks is employed instead. For this purpose the department uses parts from different engine types and many rotating parts from the compressor and turbine areas, which are heavily stressed.
Drilled holes and rows of rivets can also be assessed using this technique. The aim here is primarily to ensure that the drilled hole is cleanly set and that the surrounding material has not sustained any damage. Cracks concealed below a coat of varnish can also be identified using this technology.
As well as detecting cracks, eddy-current testing is additionally used to determine coating thicknesses. A conductivity comparison permits conclusions to be drawn as to loss of strength after exposure to heat and even as to the material used. Incorrectly declared parts can be unveiled by this procedure.
The limitations of eddy-current measurement are felt when it comes to damage further down in the material or where it is important to find out about the form and characteristics of the defect. These aspects are covered by ultrasound testing and x-raying of components.