Change of the pulsed eddy Current signals by the variation of the thickness of an aluminum specimen

Eddy current testing has been used for detecting the defect like fatigue crack in the conductive materials such as aluminum. The conventional eddy current testing uses a sinusoidal signal with very narrow frequency bandwidth. Whereas, the pulsed eddy current method uses a pulse signal with a broad frequency bandwidth. This allows multi-frequency eddy current testing, and the penetration depth is greater than that of the conventional eddy current testing. In this work, the pulsed eddy current instrument was developed for evaluating the metal loss. The developed instrument was consist of the pulse generator generating the square pulse of maximum 40 V, the amplifier controlled to 52 dB, the A/D converter of 16 bit 20 MHz, and the industrial personal computer for operating with Windows program. And, the probe for the pulsed eddy current was designed as the pancake type in which the sensing coil was located in the driving coil. The peak voltage did not linearly increase with the voltage of the step pulse. For the driving coil with inductance of 670 H, the peak voltage linearly increased with the step pulse voltage to 30 V. But, for the other driving coils with the inductance of 1.7 mH, 2.7 mH, 3.6 mH, 22 mH, the peak voltage linearly increased with the step pulse voltage to 20 V. The output signals of the sensing coil rapidly increased when the step pulse driving voltage was off, and the latter part of the sensing coil output voltage exponentially decreased with a time. The decrement value of tile output signals of sensing coil increased with the thickness of the aluminum test piece.