Magnetic Anomaly Detection in Ferromagnetic Material

摘要

This paper discusses a non-destructive testing (NDT) technique based on the magnetic flux leakage (MFL) method, which is able to detect magnetic anomalies in ferromagnetic material. The goal is the detection of very small inhomogeneities not only on the surface but also anywhere in the cross section of magneto-conductive objects by scanning their surfaces using low-cost GMR (giant magneto resistance) sensors at rather high scanning speeds (up to 1 m/s) in order to enable an automatic inspection and to minimize test time. The measured magnetic flux density variations due to the expected inhomogeneities are down to only some nT (depending on the inhomogeneity size and position) on a pedestal of some mT, therefore the system must realize a very high resolution and all parameters affecting the system's sensitivity (geometrical arrangement, sensors and electronics, signal acquisition, signal processing, etc.) have to be optimized in order to get a robust and highly sensitive measuring system. As a pilot survey, some test specimen were prepared (by inserting artificial inhomogeneities into otherwise homogeneous, isotropic magneto conductive objects) and inspected with a prototype measuring system.