Giant Magneto-Resistive (GMR) Sensors for Non-Destructive Detection of Magnetic Flux Leakage from Sub-Surface Defects in Steel

There is an increasing demand for using high sensitive sensors which can detect very low levels of magnetic flux leakage from hidden defects or far-side wall loss due to pitting or local corrosion in ferromagnetic steels. This paper discusses the use of highly sensitive tiny GMR sensors for detection of sub-surface defects in carbon steel plates. Notches located at different depths below surface are machined in 12 mm thick carbon steel plates and the tangential component of the leakage magnetic field is measured. The sensor output is amplified selectively for reliable detection of feeble leakage magnetic fields from sub-surface notches. MFL images of notches are obtained by raster scan imaging of surfaces using GMR sensor. Application of GMR sensor to detect natural and artificial defects in steel wire ropes is also discussed.