J. Chicois

Barkhausen noise from plain carbon steels: analysis of the influence of microstructure

The characteristics of the Barkhausen noise phenomenon are investigated for various crystalline microstructures of plain steels. The measurements of the magnetic Barkhausen noise (MBN) and of the acoustic Barkhausen noise (ABN) are performed using the same magnetisation rate for different materials, specially developed to provide fully reproducible experimental conditions. The MBN fingerprints of single constituent steels (ferrite, pearlite and martensite) are first studied. Then, examples of MBN characteristics for more complex microstructures are presented. The results concerning single constituent steels are discussed in terms of shape, amplitude and position of the MBN and ABN fingerprints, taking into account two main aspects: the elementary Barkhausen events (sources), linked to the interaction of the magnetic domain microstructure with the crystalline microstructure, and the propagation of the electromagnetic waves in the material as well as its detection at the pick-up coil, both strongly influencing the frequency spectrum of the detected signal. This approach enables us to get a better understanding of the dependency of Barkhausen noise on the crystalline microstructure, which is then used to explain the characteristics of MBN observed for more complex microstructures.

USE OF BARKHAUSEN NOISE FOR DETERMINATION OF SURFACE HARDENED DEPTH

Abstract The Barkhausen noise technique is sensitive to the microstructure of the ferromagnetic materials. The parameters amplitude and position of the peak-shaped Barkhausen signal can be applied for non-destructive characterisation of steel (heat treatment, composition). For a surface treated material, the measurement of the Barkhausen signal shows two microstructural states: the bulk one and the surface one. The use of the control of the applied field yields the same condition of magnetisation for all the sample and allows us to compare the signals for different treatment depths. We establish the ability of the technique for the determination of hardened depth in the case of induction hardening, ion nitrogen hardening and case hardening.

MONITORING FATIGUE DAMAGE IN INDUSTRIAL STEEL BY BARKHAUSEN NOISE

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 sign...

A Characterization of the Fibre — Matrix Bond in Single SiC filament — Aluminium Matrix Composites by Theoritical and Experimental Approach of the Multifragmentation

The multiple fibre cracks phenomenon of a single filament Metal Matrix Composite was investigated all along a tensile test by using acoustic emission measurements. It appears that the same mean fibre critical length, from Kelly’s model /4/, can be reached for very different composite strains, so this parameter cannot characterize entirely the fibre-matrix bond behaviour. After a theoritical analysis of the multiple fibre crack phenomenon, a new method is proposed for characterizing the fibre-matrix bond from a load transfer point of view.