In the frame of the Fusion Underlying Technology Work programme, novel electromagnetic non destructive techniques were developed in the Research Institute for Technical Physics and Materials Science in Budapest (Hungary). The physical background of this technique is based on the fact, that structural changes in materials are accompanied by modification of magnetic properties. One of the performed tasks was the characterisation of structural deformation of ferromagnetic materials by the measurement of the stray magnetic field around the specimens. For this purpose a new, fluxgate type magnetic field sensor (Fluxset sensor) was applied for the measurement of magnetic leakage flux in plastically deformed, ferromagnetic, low alloy steel samples.

The method can be illustrated by the measurements, performed on a series of A533B type steel specimen. The investigated specimens were shaped in a standard loading test sample as shown in the Figure (top). Different tensile stress up to P=530 MPa were applied on these samples. After loading, prior to the measurement, samples were polarised in 1 kOe magnetic field. The measurement of the flux leakage was performed in zero external field by applying different sensor orientations, parallel (X and Y direction) and normal to the sample surface (Z direction). As an illustration, the leakage flux density, measured by the X direction oriented sensor (along the axis of the sample), scanning over the surface is shown in the bottom part of the Figure, as a function of loading. (Only the highlighted area of the sample was measured.)

The results prove the applicability of this technique: the obtained pictures show the fine details of the flux distribution, which is due to the degradation of the material. It has been found, that high resolution and high sensitivity can be achieved, strong in homogeneities can be revealed in the investigated sample, and the fine structure of the magnetic flux distribution can be detected. Another advantage, is that the measurement can be performed within a short time. The method is especially suitable for the measuring of magnetic fields, parallel to the sample surface.

Samples, having small remanence, can also be measured by this technique, due to the high sensitivity of the applied magnetic field sensor. This means that in many cases the previous magnetisation of the samples is not necessary, which makes the application easier. Last, but not least, the described technique is suitable for the simultaneous measurement of the DC magnetic field and even the AC leakage field as well. An applicable local AC exciting coil makes it possible to apply localised magnetic excitation instead of global magnetic polarisation of the huge samples to be tested, and in addition, it introduces the ability of the eddy current type measurements.