Olivier Hubert

Effect of punching on electrical steels: Experimental and numerical coupled analysis

Punching electrical steels drastically alters their magnetic properties near the cutting edges, which should be accounted for when designing machines. A simple magneto-mechanical coupled finite element analysis, suitable for fully processed grades, is proposed in order to model the consequences of this local degradation of the material on the global behavior of a machine. The model is used to study the effective section of the teeth of a punched stator; calculated results are compared with experimental data and a good agreement is found.

Experimental analysis of the magnetoelastic anisotropy of a non-oriented silicon iron alloy

This paper deals with experimental measurements of the mechanical, magnetic and magnetostrictive behaviours of a non-oriented 3%SiFe alloy. The results show that the low crystallographic texture of the material brings important anisotropic effects and that the coupled magnetomechanical properties are much more sensitive than uncoupled ones.

An equivalent stress for the influence of multiaxial stress on the magnetic behavior

A main limitation of most models describing the effect of stress on the magnetic behavior is that they are restricted to uniaxial—tensile or compressive—stress. An idea to overcome this strong limitation is to define a fictive uniaxial stress, the equivalent stress that would change the magnetic behavior in a similar manner than a multiaxial one. A new definition of such an equivalent stress is proposed based on an equivalence in terms of magnetoelastic energy. This equivalent stress is compared with former proposals and validated using experimental results carried out under biaxial mechanical loading.

Effect of internal stresses on the magnetic properties of non-oriented Fe–3wt.% Si and (Fe,Co)–2wt.% V alloys

Abstract The magnetic properties of soft magnetic materials are very sensitive to mechanical strengthening and especially to plastic straining. In a first approximation, the degradation of the magnetisation with the plastic strain level is supposed to be roughly isotropic. However, even after uniaxial strengthening, the multiaxial residual stresses induce directional fluctuations of these degradations. An experimental device has been created in order to study the effect of such internal stresses on magnetic behaviour. Experimental results are given for non-oriented Fe–3wt.% Si and (Fe,Co)–2wt.% V thin sheets. An attenuated effect of the strengthening is observed in the direction perpendicular to the applied stress whatever the direction of the latter inside the sheet plane. This phenomenon is discussed in terms of metallurgical defects (dislocation structures, grain boundaries and antiphase boundaries), internal stresses and magnetocrystalline anisotropy.

Equivalent Stress Criteria for the Effect of Stress on Magnetic Behavior

A main limitation of most models describing the effect of stress on the magnetic behavior is that they are restricted to uniaxial, tensile or compressive, stress. An idea to overcome this strong limitation is to define a fictive uniaxial stress, the equivalent stress that would affect the magnetic behavior in a similar manner than a multiaxial one. Several authors have tried to define such a criterion. We propose in this paper to compare several equivalent stress definitions, and to apply them in the case of uniaxial and biaxial mechanical loadings for which experimental results are available.

Some aspects of the magnetomechanical coupling in the strengthening of nonoriented and grain-oriented 3% SiFe alloys

An investigation has been carried out on the effect of plastic strains on the magnetic properties of grain-oriented and nonoriented 3% by weight SiFe alloys. A drastic degradation of these properties with increasing deformations is observed for magnetic field amplitudes ranging between 0 and 2000 A/m. Empirical relationships between plastic strain and magnetic characteristics are obtained. Materials exhibit a Luders strain state under tensile loading in a low plastic deformation range. Meanwhile, the classical Ramberg-Osgood law is verified. The observation of the dislocation features at various plastic strain levels shows three typical configurations: hexagonal cells in the Luders strain state, small tangles and isolated screw dislocations at medium values of strain, and finally high-density tangles at higher deformations. In the same way, the densities of main and secondary magnetic domains follow an evolution in three stages with increasing strains. It is shown that the transverse domain patterns take place to counterbalance the increase of magnetoelastic energy due to the strengthening. The evolution of the coercivity and the initial relative permeability with the strains can be explained using potential model theories for the grain-oriented alloy in the range [2-8]%. Domain wall bowing theories could successfully be applied to both alloys at the ultimate stage of the strengthening. The relationship between the coercivity and the strengthening displays two linear stages for both 3% SiFe alloys instead of the three stages ordinarily reported in the case of polycrystalline high-purity iron.

Influence of a multiaxial stress on the reversible and irreversible magnetic behaviour of a 3%Si-Fe alloy

The research presented in this paper is motivated by the design of electrical devices submitted to mechanical stress. It aims at studying the magnetic behaviour of ferromagnetic materials submitted to biaxial stress. It deals with the evolution of the magnetic susceptibility, coercive field and magnetic losses with respect to stress, magnetic field level and frequency. These quantities are of primary importance in the design of rotors for high speed rotating machines. The work is focused on the magnetic behaviour of a standard grade of Iron-Silicon alloy under the form of thin sheet. Non conventional experiments are performed on cross-shaped samples in order to apply biaxial stress representative of the loadings experienced by rotors of rotating machines. These experiments are performed on a multiaxial testing machine, ASTREE. The magnetic loading is applied using a single U-yoke. The measurement of magnetic induction, magnetic field and strain is conducted by the means of needle-B sensor, H-coil sensors and Digital Image Correlation (DIC) respectively. Both anhysteretic and dissipative responses to magneto-mechanical loadings are considered. The results allow to identify the more critical stress configurations for this material.

Influence of the plastic anisotropy on the magnetic properties of a nonoriented 3% silicon iron

Modifications of the magnetic properties of a nonoriented 3% SiFe alloy with plastic strains are reported. The samples have been tested along rolling and transverse directions lying in the sheet plane by means of a suitable experimental system of measurements using a 50 Hz sinusoidal magnetic field. The magnetic properties markedly deteriorate when measurements are carried out in the direction of the applied stress. This phenomenon is less important perpendicular to the applied stress. Quasistatic experiments (f=0.1 Hz) have been accomplished to separate the total losses into hysteresis and dynamic components. Hysteresis losses strongly increase and dynamic losses sensitively decrease with the plastic deformation whatever the direction in sheet plane. Compression‐tension fatigue tests carried out have shown that the material displays a strong kinematic strengthening behavior representative of internal and long range stresses in the strained sample. The degradation of the magnetic properties of nonoriented 3% SiFe alloys with plastic strains is the result of two effects: one due to dislocations and another due to internal stresses. The latter display along the direction of magnetic measurements the same effects as an outside elastic stress.

Homogenisation of magneto-elastic behaviour: from the grain to the macro scale

The prediction of the reversible evolution of macroscopic magnetostriction strain and magnetisation in ferromagnetic materials is still an open issue. Progress has been recently made in the description of the magneto-elastic behaviour of single crystals. Herein, we propose to extend this procedure to the prediction of the behaviour of textured soft magnetic polycrystals. This extension implies a magneto-mechanical homogenisation. The model proposed is discussed and the results are compared to experimental data obtained on industrial iron-silicon alloys.

A Simplified 3-D Constitutive Law for Magnetomechanical Behavior

The magnetomechanical behavior of magnetic materials is the result of intricate mechanisms at different scales. These mechanisms have been described with satisfying accuracy from micro-mechanical approaches. But, the corresponding constitutive laws would lead to prohibitive computation time if they were implemented in structural analysis tools for the design of electromagnetic devices. In this paper, a simplified approach for the modeling of multi-axial magnetoelastic behavior is proposed. This approach includes hysteresis effects and their dependence to stress. The corresponding very low computational time makes it suitable for an implementation into numerical tools for structural analysis.