Design, construction and characterisation of ECT sensors with rectangular planar coils

The design, construction and characterisation of eddy current sensors with rectangular planar coils is reported in the present paper. The rectangular planar coils have a maximum testing area of 2.25 cm(2) and an output impedance compatible with the input impedance of normal commercial equipment: (5-200) Omega. The design was based on a simple semi-empirical model, with which coil impedances can be estimated, but not the fields associated with them. Two different windings have been studied and their performance for crack detection and material sorting through resistivity assessment is discussed here. A frequency range between (0.4 and 2.0) MHz was selected for crack inspection. Signals showing good amplitude discrimination and rather poor phase discrimination were observed. For material sorting, a resistivity standard set from Zetec and a series of Zircaloy-4 samples with increasing hydrogen content were used. The best results were those for high resistivity materials in the frequency range 0.3-1.0 MHz. The advantages of these probes are: elimination of end effect in the inspection of edge cracks, good performance in the resistivity measurement at high frequencies, high sensitivity to crazing and other shallow imperfections, and the fact that any number of identical planar coils can be made from a single photographic negative. The probes' weak points are: low inductance values and coil assembling difficulties because of the unwanted signals that unstable probe stands can produce.