Flaw Detection for Thin Sheet Using Acoustic Stimulated Electromagnetic Wave Technique

Acoustically stimulated electromagnetic (ASEM) waves in thin steel sheets have been investigated for flaw detection. In the ASEM wave technique, magnetization is temporally modulated at the radio frequency (rf) of the irradiated ultrasonic waves through magnetomechanical coupling. The induced rf magnetic fields are detected by a resonant coil antenna and spatial images are obtained by scanning the ultrasound focal spot. In this work, we detected artificial defects (through holes) in thin steel sheets. Specific patterns of magnetic flux density caused by the hole were observed. By improving the sensitivity with a small coil antenna, we visualized a through hole 0.1 mm in diameter with a lift-off of 10 mm.