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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 configurationsIn 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 peakThe 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 microcracksFinally characteristics of Barkhausen peaks are proposed as indicators of the fatigue damage (i.e. microcracks initiation).

Barkhausen noise is generated by the irreversible and discontinuous motion of Bloch walls induced by an external magnetic field or a mechanical stress. These wall motions depend on the magnetic structure of the material (magnetocrystalline anisotropy for instance) and on the microstructural flaw density and distribution within the crystal. Barkhausen noise allows to characterize the microstructural state of ferromagnetic materialsThe aim of this work is to study the influence of various metallurgical and crystallographic parameters on the Barkhausen noise. High purity iron is studied as a reference material and is used to prepare carbon iron binary alloys (130 and 400 ppm)Characterization tests are carried out with an alternating magnetic field to reach one state of saturation to the other in the sample. The transducer used is a coil surrounding the sample. This transducer converts the variations of magnetic flow during magnetization in an electrical voltage. The signal is then amplified and filtered to record only the high frequency components. A quantitative analysis of the signal spectrum can be performed. RMS voltage is measured and provide characteristic parameters such as maximum amplitude, peak areaThe influence of different microstructural parameters is studied:— A pure iron sample is strained by monotonic tensile testing from 0 to 5% during loading or after unloading. Barkhausen noise is very sensitive to plastic strain an dinternal stress state, probably within the grains—Barkhausen noise depends on the carbon content as intersticial atoms and intra or intergranular iron carbides. The noise level increases slightly with the intersticial carbon content. Various aging treatments are performed to achieve £ carbide and Fe3C intragranular precipitates densities which are characterized by transmission electron microscopy observations and thermoelectric power measures. Barkhausen noise is also studied with samples in which all the carbon is precipitated as Fe3C at the grain boundaries. The Barkhausen noise level increases with the precipitation density for e carbides as well as Fe3C. Barkhausen noise also depends on precipitates coalescence. The level decreases with the degree of coalescence in the case of Fe3C precipitatesThe results are analyzed in terms of interaction between Bloch walls and different kinds of obstacles like crystal defects (grain boundaries, dislocation ) and precipitates (role of interfaces, mismatch stresses between precipitates and matrix, and magnetic nature of precipitates).

For pure rolling fatigue conditions, the effect of microstructural changes and internal stresses due to grinding and fatigue processes itself was studied. Using Sachs’ method, the field of internal stresses was determined and it has been shown that its evolution is correlated with microstructural changes. These microstructural changes lead to changes in mechanical properties, especially to the reversible strain limit stress (R.S.L.S.) which has been determined in each point of the subsurface contact. Assuming a given relation between R.S.L.S. and fatigue limit, and using fatigue criterion which includes R.S.L.S., the effect of internal stresses, microstructural evolution, and Hertz stresses was analyzed. It has been found that the grinding internal stresses have no significant effect. On the contrary, the R.S.L.S. value was strongly reduced by grinding thermal effect in subsurface region, and was also strongly changed by the fatigue process. Finally, this analysis method was used to determine the best field of internal stresses necessary to minimize the fatigue damage.