Non-destructive testing and surface evaluation of electrical steels
Konadu, Sam Nyamekye.
The ultimate objective of this project has been to develop a non destructive and surface evaluation on-line system for the determination of localised power loss in electrical steels Three different approaches were made by employing three different methods or measuring systems on steel samples to study sequentially: (1) the position of a grain boundary from both the front and the exact opposite side at the back of the grain-oriented electrical steel sample, (2) the position of grain boundaries at the surface of grain-oriented electrical steels through the measurement of the field distribution at the surface of the magnetised steel samples, and (3) the localised power loss in 3.25% grain-oriented electrical steels. In the first case, a computer based optical microscope and a 3-Chip colour video camera system was used to establish positions of grain boundaries in order to explain differences in field profiles outside the systematic accuracy of measurements In the second case a Magnetic (Atomic) Force Microscopy (MFM) system was used to raster scan the top surface of samples such as high permeability Grain oriented 3.25% Silicon Steel, Cobalt based nanocrystalline material and Nickel Iron alloy W597 Vacoperm 100, producing images for the establishment of grain boundaries. Grain boundaries of width between 50nm and 150nm were established. Thirdly, a three dimensional integrated Hall effect sensor has been used together with two pairs of needle probes to measure localised power loss in two dimensions P(x) and P(y) in a 3.25% silicon iron material, with the total localised power loss, P(t) being equal to the sum of the power loss in x-direction, P(x) and the power loss in y-direction P(y). The 3-D i.e. Hall effect sensor and the needles together in a single package were moved on an arm of Perspex material attached to an X-Y-Z precision position control system. A Lab View software package was used for the purpose of controlling the movement of the precision position control system as well as the data acquisition and processing. Colour images and contours, and also other graph representations showing variations in the intensity of power loss, strayed vertical magnetic field and flux density have been obtained. The overall mapped results showed a good correlation with predictions made from observed static domains. The correlation not only showed that loss varies from grain to grain, but it is non-uniform in the individual grains.