Physics and Modeling of Magnetic Non Destructive Testing Techniques

Abstract

Magnetic Non Destructive Testing (MNDT) methods are a tool not limited in the detection of cracks and defects, like traditional NDT methods for ferrous structures, but they have shown a potential for the monitoring of the structure and crack prevention. MNDT techniques include surface Magnetic Barkhausen Noise measurements (MBN) yielding localized information about the surface stresses and magnetization processes in the vicinity of the measurement; the use of Magnetostrictive Delay Lines (MDL) for the measurement of surface stresses; the Magneto Acoustic Emission (MAE), revealing information about the magnetic domain wall propagation and indirectly about the underlying structure’s role in the magnetization process of the material; magnetic major and minor loop (B-H) bulk measurements which yield information on the macroscopic magnetic properties of the material such as, the coercivity, Hc, the remanence, Br, or the permeability, µ. Results show that changes in these properties are definite signs of non-uniformly distributed stresses along the material and reveal a definitive dependence of the various magnetization reversal mechanisms such as domain wall propagation and domain rotation on the microstructure of the material,eg, the domain wall structure, the effect of dislocations, the grain size, built-in stresses. However, the quantitative mapping of the MNDT results to the microstructure and from there to the possibility of crack generation and propagation is still a very attractive but open question. Modeling at the atomic level involving Ising Models, at the microscopic level using micromagnetic calculations and at the macroscopic level employing the Preisach formalism, has so far provided useful insight. The use of modeling in order to not only explain experimental results but in forecasting is expected to greatly enhance the position of the MNDT techniques in industrial NDT.