Stress dependent vector magnetic properties in electrical steel
Introduction
Stress and the resulting deformation cause the dislocation density to increase and shift, affecting the magnetic response of a material and its macroscopic magnetic properties, such as the coercivity, the differential permeability, the remanence, the shape and the area of minor and major hysteresis loops [1]. Magnetic NDE (mNDE) techniques currently under investigation include the use of magnetic Barkhausen Noise (mBN) and the hysteresis loops of materials [2]. These measurements must be carried out in the field without dissembling or interfering in any destructive way with the ferrous structure examined. Portable mBN devices as well as hysteresiographs are therefore needed. The use of an AC hysteresiograph [3], though by no means the standard way to measure hysteresis, presents the advantage that it is relatively easy and cheap to develop and customize to the under test magnetic materials and can be performed in the field in a non-destructive manner with a hand-held device. However, because it is an inductive AC measurement, it is important to test it against standard methods for measuring magnetic properties, such as using a Vibrating Sample Magnetometer (VSM) [4]. Because strain causes microstructural changes to the material it is of interest to also look into vector properties of deformed structures in order to determine whether a vector magnetometry method should also be developed and used [5].
Section snippets
Materials and methods
Tensile stress has been applied to commercial low carbon non-oriented electrical steel laminates 250×30×0.5 mm3 using an INSTRON machine [3]. The stress has been applied along the length of the samples at a constant strain rate of 0.5 mm/min. The samples were unloaded at strain levels varying from 3.5% till fracture, and had their magnetic properties measured with an AC hysteresiograph and a vector VSM.
The in-house AC hysteresiograph is a portable, uncalibrated device which can be used with the
Scalar measurements with AC hysteresiograph
In the results shown in Fig. 1, the magnetic field is collinear with the tensile stress and applied along the length of the lamination; the output is measured along the same direction. The output of the hysteresiograph is a voltage pulse proportional to the differential permeability, μdiff, of the sample, the integration of which yields the B(H) loop. The maximum inductance attained at field Hmax is denoted as Bmax to distinguish it from saturation which in the case of electrical steels is
Discussion
In the discussion hereinafter, the following differences between the two characterization methods must be kept in mind: i) the AC hysteresiograph is applied to electrical steel sheets in a non-destructive way by closing the magnetic circuit with the under test material, while for VSM measurements samples are cut into square pieces, less than 10 mm2, and the demagnetizing effect is more significant; ii) the VSM is an inherently DC magnetometer and measurements are affected only by the chosen
Conclusions
The well established dependence of magnetic properties on strain and residual stresses is measured using AC magnetometry along the direction of the applied stress as well as scalar and vector VSM measurements on samples of electrical steel strained at various levels from 0% till fracture. The agreement between the results obtained with the two methods shows that AC magnetometry, in spite of its frequency dependence and scalar nature, is a reliable technique for mNDE which, most importantly, can
Acknowledgments
This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) – Research Funding Program: ARCHIMEDES III. Investing in knowledge society through the European Social Fund.
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