Evaluation of the stress gradient of the superficial layer in ferromagnetic components based on sub-band energy of magnetic Barkhausen noise

Since magnetic Barkhausen noise is very sensitive to the change of the stress or residual stress level in components, it is feasible to detect the stress or residual stress profile along the depth direction by the magnetic Barkhausen noise method. An engineering approach was used to validate a theoretical model describing the relationship between magnetic Barkhausen noise and stress magnitude. To generate stress gradients in the specimen of Q235 steel under different deflections, the four-point bending device was used. Then the detected signals obtained in the experiment were divided into different sub-bands to evaluate the stress at different depths. The testing depth was set to be around 100 μm due to the rapid exponential attenuation of the noise signals. In the experiments, the sub-band energy density showed sensitive as the loading displacement increasing. The experimental results showed that the signal energy was also sensitive to the plastic deformation of the material. Within the depth of 100 μm, the stress change was very small. The final experimental results showed that even under such small stress variations, it is sensitive and feasible to use the magnetic Barkhausen noise energy to characterise the change of stress gradients.