On the Combined Effect of Elastic and Plastic Strain on Magnetic Barkhausen Noise Signals

Detection of tensile residual stress in steel, introduced by non-uniform plastic deformation, is critical for avoidance of failure of structures such as aircraft landing gear. Magnetic Barkhausen noise (MBN) is the result of abrupt changes in local magnetization, due primarily to 180° domain walls (DWs) as they jump from one pinning site to the next. MBN is sensitive to the presence of elastic tensile stress, due to an increase in 180° DW population, and therefore, can be used to detect tensile residual stress in steel. However, MBN exhibits a reduction under plastic strain deformation conditions. This confounds the detection of tensile stress by MBN in the presence of plastic deformation. In this study mild steel samples were plastically deformed to 2.9, 5.4, 11.7 and 20.5%, respectively, under uniaxial tensile strain. Measured MBN signals obtained by sweeping sample flux density exhibited a successive reduction in amplitude with increasing plastic deformation, as previously reported in the literature. However, subsequent application of elastic tensile stress showed a recovery and slight increase of the observed MBN signal. Recovery was attributed to the presence of compressive residual stress in the samples, which could be overcome by the application of tensile stress. Greater MBN signal amplitude at the highest stresses in deformed samples was attributed to increased dislocation density, introduced under plastic strain deformation conditions, which increase pinning of DWs and therefore, MBN. These results have implications for the detection of residual stress if plastic deformation is present.