Abstract
Conventional eddy current testing (ECT) using a pickup coil probe is widely employed for the detection of structural cracks. However, the inspection of conventional ECT for steel structures is difficult because of the magnetic noise caused by the nonuniform permeability of steel. To combat this challenge, we have developed a small magnetic sensor probe with a dual-channel tunneling magnetoresistance sensor that is capable of reducing magnetic noise. Applying this probe to a complicated component of steel structures—such as the welds joining a U-shaped rib and deck plate together—requires the reduction of signal fluctuation caused by the distance (liftoff) variations between the sensor probe and the subject. In this study, the fundamental crack signal and the liftoff signal were investigated with the dual-channel sensor. The results showed that the liftoff signals could be reduced and differentiated from the crack signals by the differential parameters of the dual-channel sensor. In addition, we proposed an extraction technique for the crack signal using the Lissajous curve of the differential parameters. The extraction technique could be applied to the inspections not only for flat plates but also for welded angles to detect cracks without the influence of the liftoff signal.
Highlights
Eddy current testing (ECT) using a pickup coil probe is a widely used technique employed to detect cracks in various conductive structures [1,2,3]
The change of baseline both sensors shifted due to the liftoff variation. These results suggest that the crack signal is buried caused by the magnetic noise and liftoff obtained by each sensor were almost the same
An extraction method for the detection of cracks in complicated structures was developed. It was shown by simulation and measurement that crack signals could be obtained with developed. It was shown by simulation and measurement that crack signals could be obtained with a small sensor probe using a dual-channel tunneling magnetoresistance (TMR) sensor and that the differential parameters reduced the a small sensor probe using a dual-channel TMR sensor and that the differential parameters reduced influence of the liftoff signal
Summary
Eddy current testing (ECT) using a pickup coil probe is a widely used technique employed to detect cracks in various conductive structures [1,2,3]. The welds joining U-shaped ribs and deck plates together are difficult to inspect where fatigue cracks often appear by the vibration of vehicles To inspect these structures, a small sensor probe of a gradiometer using dual channel magnetic sensors and an analysis method based on differential parameters are required. The distributions of eddy current on the flat plate and magnetic field inside the induction coil were simulated to confirm that the crack signal can be detected with liftoff which will occur in the probing of the complicated structure. An extraction technique of the crack signal using the Lissajous curve of the differential parameters obtained from a dual-channel magnetic sensor probe was proposed. Using the developed extraction method of the Lissajous curve, the complicated structure was measured
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