Subtle changes in magnetic Barkhausen noise before the macroscopic elastic limit

A mild steel specimen was loaded in tension up to ≈0.4% strain, with Magnetic Barkhausen Noise (MBN) measurements made at increasing strain levels. In addition, an MBN investigation was performed on a mild steel specimen bent progressively to ≈0.2% longitudinal surface strain. Overall, the parameter termed ‘MBNenergy’ increased significantly in the elastic, yet remained basically unchanged in the plastic range of deformation. While still in the elastic range, particular strain levels displayed abrupt changes in the MBNenergy. Variations in scatter about the average MBNenergy value, as well as distinct modifications in pulse height distributions, occurred simultaneously with the abrupt changes in MBNenergy. The non-uniform stress distribution among grains in the polycrystalline samples initiated dislocation formation in some grains before extending to other grains. Dislocation strain fields contributed to redistribution of strain within a grain leading to non-uniform changes in magnetic texture. Results indicate that the magnetic Barkhausen noise technique can detect microyielding.