Detection of Surface and Back-Surface Defects on Metal Plate via Rectangular Wave Eddy Current Testing Using Magnetoresistive Sensor

Eddy current testing (ECT) at different frequencies can identify surface and back-surface defects caused by the skin effect. Traditional ECT uses a pickup coil to measure the magnetic-field signal caused by eddy current. However, according to Faraday’s law of electromagnetic induction, the voltage signal detected at low frequencies is weak; thus, the signal is susceptible to interference from noise. By contrast, magnetoresistive (MR) sensors have high sensitivity at low frequencies. Therefore, we developed a rectangular wave ECT (RECT) system using an MR sensor. The system uses rectangular wave excitation, which can generate both low- and high-frequency signals, and the MR sensor detects the magnetic-field components. A 12 mm-thick aluminum plate with an artificial circular defect 10 mm in diameter and 4 mm in height was used as a specimen. The measurement results indicated that the developed system can accurately and quickly distinguish between surface and back-surface defects.