Automatic Scanning System for Back-Side Defect of Steel Structure Using Magnetic Flux Leakage Method

An automatic nondestructive evaluation system using the magnetic-flux-leakage method was developed for detecting the back-side defects of large structures in order to realize fast detection in a wide measurement area. A magnetic field was applied to an object using a ferrite yoke with an induction coil, and the leaked magnetic flux from the object was detected by a magnetoresistive sensor. This measurement probe was fixed on an automatic scanning system. The developed automatic scanning system detected magnetic signal changes corresponding to the back-side defects of the steel plates. To demonstrate the effectiveness of the developed system, we measured the steel plate of a cargo crane, which was corroded on the back side. We found that the detected magnetic signal changed markedly in the area with corrosion, whereas only a slight magnetic signal change was detected in the area without corrosion. Therefore, the developed system is useful for detecting the back-side defects in a wide area, and can be used for screening tests.