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Effects of stress on magnetostrictive properties of low silicon non-oriented electrical steel
Journal of Magnetism North-Holland
and Magnetic
Materials
83 (1990) 177-178
177
EFFECTS OF STRESS ON MAGNETOSTRICTIVE NON-ORIENTED ELECTRICAL STEEL G.H. SHIRKOOHI Wolfson Centre
PROPERTIES
OF LOW SILICON
and A.J. MOSES
for Magnetics Technology, Uniuersity of Wales College of Cardiff, UK
Magnetostrictive properties of low silicon, non-oriented steels are investigated under uniaxial compressive levels of up to 40 MPa. Materials exhibited high magnetostriction when compressed. The magnetostrictive under compressive and tensile uniaxial stress are discussed by extension of a simple theoretical model.
1. Introduction Silicon-iron alloys suffer localised strains due to the presence of domains which naturally occur to minimise the free energy in the material. When external mechanical stresses are applied, the energy distribution of the steel is altered and the magnetoelastic energy is reduced for the presence of tension along the direction of the applied field, and increased when compression is applied in the same direction. The resulting change in domain direction can cause a large magnetostriction [l] which causes noise or vibration in electrical machine cores. This paper presents magnetostriction measurements made on non-oriented electrical steels and shows the basis of a simple way of estimating the effect of stress.
2. Experimental procedure Epstein size (30 x 310 mm2) samples of low silicon non-oriented steels were sinusoidally magnetised at a constant flux density of 1.5 T, 50 Hz and simultaneously uniaxial tensile or compressive stresses up to 40 MPa were applied. A piezoelectric transducer was used to measure the fundamental and second harmonic components of magnetostriction. The transducer output was fed via a condenser amplifier to a high resolution spectrum analyser, and the magnetostriction was measured with an accuracy better than + 3%.
and tensile stress curves obtained
tion of materials E and F was very similar to that of material D. Magnetostriction measurements were widely variable and showed poor repeatability in all the samples. All samples showed a large and steady increase in the fundamental component of magnetostriction as the compressive stress was increased to around 10 MPa, and a more erratic variation was observed above these stress levels. Under applied tensile stress, the magnetostriction of most samples exhibited less fluctuation. In material A however, large sample to sample variation was noticed. Although material D has a higher silicon content, its magnetostriction is not lower than that of material A which contains less silicon. The trend of the stress sensitivity results is similar to that previously reported for grain-oriented steels [2-41. The highest magnitudes of peak fundamental magnetostriction for these non-oriented materials which occurred under compressive stress, did not exceed 5 X 10m6 which was low compared with the results previously
3. Results and discussions Six batches of fully processed non-oriented steels were investigated, three of these (A, B and C) contained 0.2% silicon and the rest (D, E and F) contained 1.3% silicon. Fig. 1 shows the stress sensitivity of the magnetostriction of five samples of material A (0.2% silicon) magnetised at 1.5 T. Materials B and C responded in a similar manner. Fig. 2 shows the magnetostriction of five samples of material D (1.3% silicon) under the same magnetisation and stress conditions. The magnetostric0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
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Fig. 1. Variation of peak fundamental and second harmonic components of magnetostriction of five specimens of material A, magnetised at 1.5 T, 50 Hz with linear stress applied along their rolling directions.
G. H. Shrrkoohi, A.J. Moses / Effects of stress on non-oriented steel
178
COW,‘K~ISION
,
8°F..
ILNSION
Fig. 2. Variation of peak fundamental and second harmonic components of magnetostriction of five specimens of material B, magnetised at 1.5 T, 50 Hz with linear stress applied along their rolling directions.
reported for grain-oriented materials. The second harmonic component of magnetostriction was also measured and its peak values were noted to be well below 1 X 10e6 in all samples which were much less than those previously reported for grain-oriented steels (41. The saturation magnetostriction in polycrystalline materials is given by [5]:
x,
= 3,,,
+ :x,,,
,
tion will be changed to around 14 x 10m6 when the flux density is set at 1.5 T hence the measured peak to peak value of magnetostriction will be the difference between 14 X lop6 and -OS&,,,, i.e. around 12 X 10e6 (peak). This is around three times the measured values. This is mainly because of the assumption that full domain rotation occurs under compressive stress. This however will not be the case because of other energy constraints which are not taken into account here. It is also partly because of the approximate calculation of the magnetostriction at 1.5 T. A more thorough statistical calculation of strain due to the predicted domain patterns would be necessary to obtain a closer value. Far more accurate knowledge of material properties and domain structures in non-oriented steels are necessary before the experimental results can be quantitatively explained. 4. Conclusions The magnetostrictive properties of the non-oriented steels investigated under applied uniaxial stress were similar to those observed for grain-oriented steels although the stress sensitivity was not so great. The higher harmonics of magnetostriction are small and not stress sensitive so they should not produce any problems in electrical machines. It is difficult to accurately predict the stress sensitivity of non-oriented steels without better knowledge of their domain structures.
(1)
constants. where &, and A,,, are magnetostriction Their values for iron are approximately 20 x 10m6 and -20 x 10m6, respectively. From (1) at zero stress and high applied fields, the value of magnetostriction becomes -4 x 10m6. Under high tensile stress and in the absence of an external magnetic field all domains are rotated into the stress direction and the strain becomes X,,. Ideally, application of a field will not change the magnetostriction so the measured value will be zero as is found to be the case in material D. The higher magnetostriction under tensile stress in material A is probably caused by the stress not producing so much rotation in this material in the demagnetised state. Under an applied high compressive stress and in the absence of an external field, the magnetostriction strain for full domain rotation is -0.5X,,. The magnetostric-
The authors wish to thank Stahlwerke Bochum for financial support and for supplying the materials for investigation. Thanks are also due to Dr. K.H. Schmidt. Mr. H. Huneus and Dr. K. Peters for useful comments and discussions throughout the work. References [I] P. Allia, A. Ferro, G.P. Soardo and F. Vinai. J. Appl. Phys. 50 (1979) 7716. [2] A.J. Moses and P.S. Phillips. IEEE Trans. Magn. MAO-14 (1978) 353. [3] A.J. Moses, IEEE Trans. Magn. MAG-15 (1979) 1575. [4] A.J. Moses and D. Davies. IEEE Trans. Magn. MAC-16 (1980) 454. [5] F. Brailsford. Physical Principles of Magnetism (Van Nostrand, London. 1966) p. 150.
and Magnetic
Materials
83 (1990) 177-178
177
EFFECTS OF STRESS ON MAGNETOSTRICTIVE NON-ORIENTED ELECTRICAL STEEL G.H. SHIRKOOHI Wolfson Centre
PROPERTIES
OF LOW SILICON
and A.J. MOSES
for Magnetics Technology, Uniuersity of Wales College of Cardiff, UK
Magnetostrictive properties of low silicon, non-oriented steels are investigated under uniaxial compressive levels of up to 40 MPa. Materials exhibited high magnetostriction when compressed. The magnetostrictive under compressive and tensile uniaxial stress are discussed by extension of a simple theoretical model.
1. Introduction Silicon-iron alloys suffer localised strains due to the presence of domains which naturally occur to minimise the free energy in the material. When external mechanical stresses are applied, the energy distribution of the steel is altered and the magnetoelastic energy is reduced for the presence of tension along the direction of the applied field, and increased when compression is applied in the same direction. The resulting change in domain direction can cause a large magnetostriction [l] which causes noise or vibration in electrical machine cores. This paper presents magnetostriction measurements made on non-oriented electrical steels and shows the basis of a simple way of estimating the effect of stress.
2. Experimental procedure Epstein size (30 x 310 mm2) samples of low silicon non-oriented steels were sinusoidally magnetised at a constant flux density of 1.5 T, 50 Hz and simultaneously uniaxial tensile or compressive stresses up to 40 MPa were applied. A piezoelectric transducer was used to measure the fundamental and second harmonic components of magnetostriction. The transducer output was fed via a condenser amplifier to a high resolution spectrum analyser, and the magnetostriction was measured with an accuracy better than + 3%.
and tensile stress curves obtained
tion of materials E and F was very similar to that of material D. Magnetostriction measurements were widely variable and showed poor repeatability in all the samples. All samples showed a large and steady increase in the fundamental component of magnetostriction as the compressive stress was increased to around 10 MPa, and a more erratic variation was observed above these stress levels. Under applied tensile stress, the magnetostriction of most samples exhibited less fluctuation. In material A however, large sample to sample variation was noticed. Although material D has a higher silicon content, its magnetostriction is not lower than that of material A which contains less silicon. The trend of the stress sensitivity results is similar to that previously reported for grain-oriented steels [2-41. The highest magnitudes of peak fundamental magnetostriction for these non-oriented materials which occurred under compressive stress, did not exceed 5 X 10m6 which was low compared with the results previously
3. Results and discussions Six batches of fully processed non-oriented steels were investigated, three of these (A, B and C) contained 0.2% silicon and the rest (D, E and F) contained 1.3% silicon. Fig. 1 shows the stress sensitivity of the magnetostriction of five samples of material A (0.2% silicon) magnetised at 1.5 T. Materials B and C responded in a similar manner. Fig. 2 shows the magnetostriction of five samples of material D (1.3% silicon) under the same magnetisation and stress conditions. The magnetostric0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
C”“I~nESSlON
;;iits(.
CLO
TFNbilN
Fig. 1. Variation of peak fundamental and second harmonic components of magnetostriction of five specimens of material A, magnetised at 1.5 T, 50 Hz with linear stress applied along their rolling directions.
G. H. Shrrkoohi, A.J. Moses / Effects of stress on non-oriented steel
178
COW,‘K~ISION
,
8°F..
ILNSION
Fig. 2. Variation of peak fundamental and second harmonic components of magnetostriction of five specimens of material B, magnetised at 1.5 T, 50 Hz with linear stress applied along their rolling directions.
reported for grain-oriented materials. The second harmonic component of magnetostriction was also measured and its peak values were noted to be well below 1 X 10e6 in all samples which were much less than those previously reported for grain-oriented steels (41. The saturation magnetostriction in polycrystalline materials is given by [5]:
x,
= 3,,,
+ :x,,,
,
tion will be changed to around 14 x 10m6 when the flux density is set at 1.5 T hence the measured peak to peak value of magnetostriction will be the difference between 14 X lop6 and -OS&,,,, i.e. around 12 X 10e6 (peak). This is around three times the measured values. This is mainly because of the assumption that full domain rotation occurs under compressive stress. This however will not be the case because of other energy constraints which are not taken into account here. It is also partly because of the approximate calculation of the magnetostriction at 1.5 T. A more thorough statistical calculation of strain due to the predicted domain patterns would be necessary to obtain a closer value. Far more accurate knowledge of material properties and domain structures in non-oriented steels are necessary before the experimental results can be quantitatively explained. 4. Conclusions The magnetostrictive properties of the non-oriented steels investigated under applied uniaxial stress were similar to those observed for grain-oriented steels although the stress sensitivity was not so great. The higher harmonics of magnetostriction are small and not stress sensitive so they should not produce any problems in electrical machines. It is difficult to accurately predict the stress sensitivity of non-oriented steels without better knowledge of their domain structures.
(1)
constants. where &, and A,,, are magnetostriction Their values for iron are approximately 20 x 10m6 and -20 x 10m6, respectively. From (1) at zero stress and high applied fields, the value of magnetostriction becomes -4 x 10m6. Under high tensile stress and in the absence of an external magnetic field all domains are rotated into the stress direction and the strain becomes X,,. Ideally, application of a field will not change the magnetostriction so the measured value will be zero as is found to be the case in material D. The higher magnetostriction under tensile stress in material A is probably caused by the stress not producing so much rotation in this material in the demagnetised state. Under an applied high compressive stress and in the absence of an external field, the magnetostriction strain for full domain rotation is -0.5X,,. The magnetostric-
The authors wish to thank Stahlwerke Bochum for financial support and for supplying the materials for investigation. Thanks are also due to Dr. K.H. Schmidt. Mr. H. Huneus and Dr. K. Peters for useful comments and discussions throughout the work. References [I] P. Allia, A. Ferro, G.P. Soardo and F. Vinai. J. Appl. Phys. 50 (1979) 7716. [2] A.J. Moses and P.S. Phillips. IEEE Trans. Magn. MAO-14 (1978) 353. [3] A.J. Moses, IEEE Trans. Magn. MAG-15 (1979) 1575. [4] A.J. Moses and D. Davies. IEEE Trans. Magn. MAC-16 (1980) 454. [5] F. Brailsford. Physical Principles of Magnetism (Van Nostrand, London. 1966) p. 150.