Magneto-Optical and Hall Effect Magnetic-Flux Leakage Sensing: A Validation Study

In this article, we investigate the feasibility of using magneto-optical (MO) films as sensors for the magnetic-flux leakage (MFL) inspection technique. When a ferromagnetic specimen is saturated with a strong magnetic field, a surface-breaking defect can cause the magnetic field to leak out, producing an MFL signal. The components of the MFL signal (axial, radial, and tangential) are postulated to contain important topological information of the surface-breaking defect and are usually measured point-by-point by a Hall effect sensor. The MO film sensor analyzed in this article is an iron garnet film grown on [111]-oriented substrates. It visually responds to the MFL field in the 2-D plane spanning the film. We derive an analytical model that addresses, explains, and justifies the fundamental physics of the MO technique. We applied the model to two distinct types of MFL data: 1) surface scan data measured with a triaxial Hall effect sensor in the laboratory and 2) surface scan data simulated with the magnetic dipole model (MDM). Both of these sources of data are widely used to measure and simulate MFL signals. The accuracy of the MO film sensor was tested through a comparison between MO film experimental visual responses and results computed through MO analytical model applied to measured and simulated MFL data for different defect shapes. Our analysis and experiments show a good agreement between MO-measured images and MO simulations using Hall effect sensor measured data and analytical MDM simulated data.