Institute of Fundamental Technological Research

The change in the dislocation density, induced by plastic deformation, influences strongly the magnetic domain structure inside the material. Being so, classic parameters, like the coercivity or magnetic permeability, can be a good measure of the deformation level, yet their reliable determination in a non-destructive way in industrial environment is problematic. The magnetoacoustic emission (MAE) which results from the non-180o domain walls (DW) movement in materials with non-zero magnetostriction can be used as an alternative. The intensity of the MAE signal changes strongly as a result of plastic deformation for both tensile and compressive deformation. It is however possible to discern those cases by analysing the changes in the shape of the MAE signal envelopes. The set of the martensitic steel samples (P91) deformed up to 10 % (for both tension and compression) was investigated. Due to geometrical limitations imposed by the special mounting system, enabling compression of the samples without buckling, had the shape resulting in low signal to noise (S/N) ratio. Being so the optimization of FFT filtering and wavelet analysis was performed in order to improve sensitivity of the proposed method of deformation level determination.

magnetic flux leakage, nondestructive testing, wavelet transform

The paper presents the results of investigation of the influence of plastic deformation on the magnetic properties of martensitic steel (P91 grade). The properties of the hysteresis loops as well as of the Barkhausen effect (BE) signal are analysed for both tensile and compressive loading up to ε=10% of plastic deformation. The choice of the steel and of the deformation range is unique, since for such combination one can expect high residual stresses (both compressive and tensile) in the material that does not exhibit saturation of the BE intensity as a function of elastic stress. The obtained relationships show that for the low level of deformation the dislocation density changes may play a dominant role, yet for higher deformation level the residual stress becomes a dominant factor. It leads to the strong decrease of the BE signal for tensile deformation and an increase for the case of compression. It agrees well with the assumption that the tensile plastic deformation results in the compressive stresses appearance in the soft (magnetically active) sub-regions of the material whereas for the compression one can expect a residual stress of a tensile nature in those areas. Both deformation modes result in the increase of coercivity of the samples, yet the increase observed for the tensile deformation is significantly higher since both the residual compressive stress and increase of dislocation density have a strong effect on the material coercivity. The change of the hysteresis loops steepness agrees well with the notion of the dominant role of residual stresses too.

Barkhausen effect, Plastic deformation, Residual stress, Magnetic hysteresis, Coercivity

Efekt Barkhausena (EB) kojarzony jest z impulsami napięcia indukowanymi w cewce detekcyjnej zbliżonej do magnesowanej płyty. Typowa sonda efektu Barkhausena składa się z elektromagnesu jarzmowego (typu C) i cewki detekcyjnej z rdzeniem ferrytowym [1]. Natężenie efektu Barkhausena zależy od mikrostruktury (wielkości ziarna, morfologii wydzieleń i dyslokacji) oraz od poziomu naprężeń [2]. Magnesowanie jest łatwiejsze, gdy kierunek pola magnetycznego sondy zgodny jest z kierunkiem działania naprężeń rozciągających i trudniejsze, gdy magnesowany jest materiał poddany działaniu naprężeń ściskających. Pomiar naprężeń z wykorzystaniem efektu Barkhausena ma wiele zalet: krótki czas pomiaru (kilkanaście sekund), względnie proste przygotowanie powierzchni (usunięcie warstwy tlenków szlifierką oscylacyjną z papierem ściernym o gradacji powyżej 100). W celu określenia składowych płaskiego stanu naprężenia konieczne jest wyznaczenie rozkładu kątowego natężenia EB [2, 3]. W artykule opisano wyniki badań stanu naprężenia za pomocą nowo opracowanego aparatu wyposażonego w sondę, która generuje w badanym elemencie namagnesowanie o skokowo zmiennym kierunku magnesowania. Aparat ten umożliwia automatyczny pomiar rozkładów kątowych natężenia EB. Badania te są realizowane w ramach projektu NCBiR nr PBS1/A9/14/2012 pt. Opracowanie magnetycznej metody oceny stanu naprężeń w materiałach konstrukcyjnych zwłaszcza anizotropowych. Zdaniem Autorów jest to rozwiązanie o lepszych walorach użytkowych niż proponowane w poprzednich opracowaniach [4, 5]. Anizotropowość właściwości magnetycznych (wynikająca np. z tekstury struktury ziaren) ujawnia się w eliptycznym, a nie kołowym rozkładzie kątowym natężenia EB. Oczywista jest konieczność uwzględnienia tej anizotropii przy badaniu stanu naprężenia tą technika. Poniżej podano przykłady badania stanu naprężenia nowym układem dla złącz spawanych wykonanych z praktycznie izotropowej magnetycznie stali oraz dla stali z wyraźną anizotropią magnetyczną.

The paper presents the results of multiparameter analysis of Barkhausen noise (BN) signal properties. In addition to the commonly used quantifiers of the BN signal, such as amplitude, integral of the BN envelope or results of pulse count analysis, we propose an additional analysis based on the change in magnetizing current amplitude. As it turns out the character of the change of the BN signal (as a function of the plastic deformation level) measured for various magnetizing currents differs significantly. Being so, a comparison of the results obtained for at least two magnetizing intensities gives a much better description of a plastic deformation level. In addition to that we observe two monotonic changes in the BN signal properties - a systematic shift of the BN signal peak position and the increase in the frequency for which the maximum in the BN FFT spectra occurs.

Barkhausen effect, plastic deformation, nondestructive evaluation

The correlation between magnetoacoustic emission intensity and mechanical Barkhausen effect intensity has been tested in the case of P91 steel having different microstructure states due to either plastic flow at room temperature or speeded-up creep, performed under stress 290 MPa at temperature 773 K. The plastic strain range for the first set of samples has been from 2.0% up to 10.5% and for the second one from 0.85% up to 10.0%. Both effects revealed analogous monotonous decrease of their intensities as a function of resulting plastic strain. The decrease was higher for plastic flow than for creep. Those results are well consistent with results of tensile tests of the as damaged samples.

Magnetic properties, Mechanical factors, Steel, Plastics, Creep, Testing, Temperature distribution, Microstructure, Performance evaluation, Stress