On the application of magnetoelastic properties measurements for plastic level determination in martensitic steels

Abstract

The change in the dislocation density, induced by plastic deformation, influences strongly the magnetic domain structure inside the material. Being so, classic parameters, like the coercivity or magnetic permeability, can be a good measure of the deformation level, yet their reliable determination in a non-destructive way in industrial environment is problematic. The magnetoacoustic emission (MAE) which results from the non-180° domain walls (DW) movement in materials with non-zero magnetostriction can be used as an alternative. The intensity of the MAE signal changes strongly as a result of plastic deformation for both tensile and compressive deformation. It is however possible to discern those cases by analysing the changes in the shape of the MAE signal envelopes. The set of the martensitic steel samples (P91) deformed up to 10% (for both tension and compression) was investigated. Due to geometrical limitations imposed by the special mounting system, enabling compression without buckling, the sample had the shape resulting in low signal to noise (S/N) ratio. Being so the optimization of FFT filtering and wavelet analysis was performed in order to improve sensitivity of the proposed method of deformation level determination.