Effect of cyclic deformation on Barkhausen noise in a mild steel

The rms value of the Barkhausen noise and residual stresses were followed as a function of loading cycles in alternating bending fatigue of mild steel specimens. The measurements were carried out in several directions so that the changes in the angular anisotropy of the Barkhausen noise and reorientation of the macroscopic easy axis of magnetisation could be detected. In the loading direction the Barkhausen noise amplitude decreased due to build-up of compressive residual stresses. This was followed by either an increase or a decrease in the Barkhausen noise amplitude depending on the direction of the initial magnetic easy axis with respect to the loading direction. After the commencement of macroscopic plastic deformation the angular anisotropy of Barkhausen noise augmented substantially. In the transverse direction to the loading axis Barkhausen noise decreased in the beginning of loading, reached thereafter a minimum followed by an increase in Barkhausen noise. When cycling under larger strain amplitudes the loading direction and the direction transverse to the loading axis exhibited least changes as a function of loading cycles compared to other directions. The results indicated that following Barkhausen noise only in one direction, usually in the loading direction, as a function of loading cycles, may lead to misevaluation of the fatigue damage development. Monitoring of several directions gives much more reliable results and therefore it is highly recommended.