Nondestructive Detection of Inhomogeneity in the Magnetic Properties of Materials With a Moving Magnet Hysteresis Comparator

Microstructure, composition and residual stress determine the hysteresis properties of magnetic materials. Detection of inhomogeneity in any of these factors requires the assessment of local rather than global properties. This paper shows that the recently developed moving magnet hysteresis comparator can provide a simple, sensitive, and nondestructive means for detecting asymmetrical distributions of property variations. The method is based on measurement of the magnetic field pattern arising from magnetization gradients created in a test sample during motion of a small permanent magnet. The difference in this pattern during motion of the magnet in forward and reverse directions informs on the sample's hysteresis properties. Differences in this difference between normal and inverted sample orientations informs on the presence and extent of inhomogeneity. Results from experiments with sample groups having artificially created imhomogeneity in either microstructure, composition, or residual stress support the concept.