Challenges for Detection of Small Defects of Submillimeter Size in Steel using Magnetic Flux Leakage Method with Higher Sensitive Magnetic Field Sensors

Recently, several types of miniaturized magnetic field sensors with high sensitivity have been developed, thereby allowing us to detect small defects in steel using the magnetic nondestructive testing technique. Trials to detect a small-sized defect in the submillimeter scale in steel using the magnetic flux leakage method are described herein. Magnetic field sensors with high sensitivity such as giant magneto-resistance (GMR) and giant magneto-impedance (GMI) were used in this study to scan leakage field on the surface of a specimen. When we used a GMI sensor, a defect of size 100 mm was detected by applying a low field; its limitation was restricted by the size of the sensor and the dynamic range. Meanwhile, we detected a small defect of diameter 30 mm using a GMR sensor acting as a gradiometer.