Resonance systems for EC-NDT inspection

Eddy Current based Non Destructive Testing (EC-NDT) is an in-field inspection technique mainly adopted for detecting and characterizing flaws on metallic materials. The basic operating principle of this kind of test is very simple. An excitation coils induces eddy currents in the specimen under test while magnetic sensors, or sensing coils, measure the reaction magnetic fields. The presence of defects in the specimen under test modifies the magnetic reaction fields. This field difference, through suitable processing techniques based on inversion models or experimental analysis, can highlight the defect presence. A key element of an EC-NDT instrument is the EC probe. Typically it is composed by three elements: a) the exciting coil, b) the measurement sensor and, c), the mechanical facility where both the a) and b) elements are located. As for the exciting coil, the maximum allowed current and the factor of merit Q factor (meant as the ratio between the inductive and resistive impedance of the coil) are very important project parameters. The excitation coil must guarantee hundreds of milliamps and high values of Q to induce acceptable eddy current values in the specimen under test. The actual trend of EC-NDT is going to in-field inspection of very thin carbon fiber materials adopter both for aeronautic and industrial applications. The inspection of these materials is imposing the use of frequency much higher than the previous ones, up to some megahertz. In these cases, the higher frequency the higher the overall impedance of the excitation coil, the higher the voltage excitation required to achieve the desired current values. This leads to problems in developing and realizing the current amplifier that feed the excitation coil increasing the overall cost and reducing the overall bandwidth. To overcome this problem, in this paper the effect of some resonant circuits developed to reduce overall voltage and power required to feed the excitation coils is analyzed. In detail, after some theoretical remarks about the resonating circuits in EC-NDT testing, two set-up have been realized and compared for frequencies up to 1MHz.