Multi-frequency imaging with non-linear calibration of magnetoresistance sensors for surface and buried defects inspection

Abstract

Timely inspecting defects has both safety and economic significance in industry. Eddy current testing (ECT) probes with array sensors are widely used for metal structures inspection. However, the sensitivities of array sensors are typically not consistent, resulting in different responses when measuring the same defect signal. Therefore, the array sensors of an array probe are desired to be calibrated. This paper proposes an ECT probe with an array of tunnel magnetoresistance (TMR) sensors and two excitation coils for surface and buried defects inspection. Alternating currents with different frequencies and amplitudes are driven through the two coils simultaneously. The TMR sensors have non-linear resistance vs. magnetic field curves. A non-linear calibration method based on the non-linear characteristic is proposed to calibrate the high frequency signals of the sensors. This calibration method does not need any extra equipment and takes account of the effect of the non-uniformity of the excitation magnetic field to some extent. Experimental result shows that the signal becomes much flatter after the calibration. The relative flat index is reduced from 38.6% to 11% by the calibration process. To validate the detectability of the proposed probe for different kinds of defects, surface and buried defects in aluminum samples are tested by the probe. It is seen that a small surface defect with sizes 1 mm (length) x 0.5 mm (width) x 0.5 mm (height) can be clearly identified from the high-frequency image. Two defects buried 3 mm below the top surface are also inspected. The peak signal values of the two defects are 0.69 V and 0.97 V, respectively, demonstrating that the probe can distinguish the 0.1 mm height difference of the two defects. In addition, a defect buried 10 mm under the surface is imaged by the probe. Thanks to the special probe design and excellent sensitivity of the TMR sensors, the probe has good sensitivity for both surface and buried defects.