MONITORING FATIGUE DAMAGE IN INDUSTRIAL STEEL BY BARKHAUSEN NOISE

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Abstract Barkhausen noise is generated by the rapid and irreversible motion of Bloch walls within ferromagnetic materials, when submitted to mechanical stresses or external magnetic field. Bloch walls motion is influenced by obstacles like dislocations, precipitates Therefore Barkhausen noise depends on the microstructure and the stress state of material, that can change during fatigue tests. Indeed the Barkhausen noise measurements can be used as a fatigue damage indicator when the microstructure of the fatigue specimens reaches critical configurations In the present study Barkhausen noise measurements were performed in order to detect the microstructure changes and the initiation of the damage in the case of a cyclic strained industrial steel. The sample is placed in an AC magnetic field to reach saturation. The variations of the magnetic flux within the sample are converted into electric voltage by a surrounding coil transducer. The Barkhausen noise is the high frequency component of this electric signal. The RMS voltage, is used as a quantification of the Barkhausen noise versus the applied field. A maximum in the RMS voltage of the Barkhausen noise is observed. So the RMS voltage appears as a peak shaped curve that we called the Barkhausen peak The influence of the number of cycles and the unloading mode, on the Barkhausen noise, is studied. Fatigue tests are performed according to a push-pull mode under total strain controlled conditions. After the completion of the test, the sample is magnetically characterized. Several cycle numbers (1,10,1000.) were studied to increase the damage. Four unloading modes are used: 2 in traction and 2 in compression (with no unloading plastic strain and from peak stress). The main results of this study are the following: —for given numbers of cycle, the Barkhausen peaks are very different for varying unloading modes. Moreover it is not symmetrical and it is probably depending on the sign of internal stress introduced by cyclic deformation. —for a given unloading mode a progressive boardening of the Barkhausen peak is observed as a function of the cycle number specially for an unloading mode from the tensile peak stress. Moreover a double Barkhausen peak appears from a fatigue level corresponding to approximately one thousand cycles. All these results are analyzed from the microstructural changes by cyclic deformation of the steel specially taking into account the sign and the level of the internal stress field as well as changes in the density of crystal defects, the precipitation state, and the initiation and the development of microcracks Finally characteristics of Barkhausen peaks are proposed as indicators of the fatigue damage (i.e. microcracks initiation).