Use of the Barkhausen effect in the measurement of residual stresses in steel

For on-line monitoring of the state of materials, various non-destructive methods exist, among them the micro-magnetic method with which a ferromagnetic material is magnetised by an alternating current while the Barkhausen noise (BN) generated is monitored. A suitable experimental set-up was composed. The paper deals with determination of residual stresses in samples of heat-treatment steel C45E which were hardened and tempered at three different temperatures and eventually flat ground. By means of a measuring coil of Hull probe, the changes in the induced voltage in magnetisation, which are due to various influences of the analysed material, were detected. Further processing of the captured voltage signals was carried out on samples being in different states, and calibration curves were determined. A final decision on the state of the material was obtained from the calibration curves. On the basis of the micro-magnetic parameter obtained, the parameter of surface integrity searched for can be determined. The assessment of the efficiency of determination of residual stresses by the micr-magnetic method based on the BN was carried out by means of measurement of residual stresses with the relaxation method. Comparative through-thickness measurements showed that the micro-magnetic method was suitable for determination of residual stresses since it gives a real-time variation of residual stresses in a thin surface layer of the material concerned in a very short time.