To maintain the safety of infrastructure, it is important to detect not only cracks but also corrosion due to aging. Ultrasonic testing is thus widely used to detect reduced material thicknesses from corrosion; eddy current testing, however, is not presently utilized for such investigations. A rapid testing method is nonetheless needed because of pretreatment protocols (such as the peeling of coatings) associated with ultrasonic inspection. In this study, we attempted to measure magnetic signals of the back sides of thick steel plates by applying low-frequency magnetic field measurements via large skin depths and a magnetoresistive (MR) sensor with a wide-range uniform frequency response, as opposed to simply utilizing a pickup coil. The developed measurement system consisted of an excitation coil, a compensation coil, an MR sensor, a lock-in amplifier, a function generator, an amplifier, and an A/D converter. The compensation coil was mounted in a sensor position to cancel any direct excitation magnetic field and thereby improve the signal to noise ratio. The lock-in amplifier detects a frequency component signal to the same extent as an applied magnetic field. The magnetic field vector was obtained by the in-phase and quadrature components of the detected signal. The sensor was arranged in the normal (z-) axis direction to obtain magnetic field from samples. The magnetic response characteristics using the magnetic field vector (with a range of low frequencies from 1 Hz to 1 kHz of different thickness samples) were measured. Consequently, the magnetic response characteristics showed a general dependency with thickness. Next, 12-mm-thick steel plate test samples that exhibited partial corrosion of different depths on their back sides were measured and subsequently imaged by two-dimensional scanning. As a result, the back side corrosion levels were visually characterized, with associated thickness-dependence images ultimately obtained. The results showed that the developed system is useful and convenient as a nondestructive testing technique for detecting the corrosion of ferromagnetic material objects.
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