On the use of differential permeability and magnetic Barkhausen Noise Measurements for Magnetic NDT Applications

Mapping microstructural to macroscopic magnetic properties is challenging yet necessary for the development of novel non-destructive testing techniques based on magnetic measurements. The macroscopic properties of a material reflect the underlying magnetization processes and the microstructural configuration as a result of the history of the material. Magnetic nondestructive evaluation techniques are needed for the health monitoring in critical structures and the quality monitoring of materials used in components, such as transformers and motors. In this work, we assess the usability of metrics related to AC magnetometry and magnetic Barkhausen Noise through measurements on several series of samples of various grain sizes and levels of plastic deformation. AC magnetometry is preferred to magnetic Barkhausen Noise as it provides more repeatable and reliable metrics than mBN which suffers from higher uncertainty. Also, differential permeability curves are preferred to hysteresis loops because they can be obtained directly through ac magnetometry and provide several types of information, such as the peak value, the coercivity, the overall shape, the emergence and location of multiple peaks which are related to stresses. In the case of stress-dependent magnetization processes, both differential permeability and mBN results are consistent, regardless of the way strain is applied, and corroborate results previously reported. However, permeability measurements are inconclusive on the effect of grain size on magnetization processes while the mBN voltage increases monotonically with decreasing grain size, in both types of samples. The mBN energy associated with each applied field step is proposed as a complementary measure for the assessment of the contribution of the irreversible processes in a material where the permeability measurements are inconclusive. Finally, the experimental observations have been used to set up micromagnetic calculations by modeling a plastically deformed material as a soft matrix with grains enclosed by boundaries with high transverse anisotropy which act as pinning centres and prevent further rotation of the magnetization and its alignment with the applied field thus reproducing the typical phenomenological features of plastically strained materials.