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Optical detection of magnetic stray fields
Volume 24, number 1
OPTICS COMMUNICATIONS
January 1978
OFTICALDETECTIONOFMAGNETICSTRAYFIELDS R. FALK and M. LAMBECK Optisches Institut der Technischen
Universithi, D 1000 Berlin 12, Germany
Received 21 September 1977
The magnetic stray field caused by defects in ferromagnetic parts is indicated by means of magneto-optics. Using thin ferromagnetic films as the magneto-optic medium, surface cracks in steel with a width of 0.015 mm and sub-surface defects (bores with a diameter of 1.75 mm) 21 mm below the surface are detected.
1. Introduction Faraday- glass
If a ferromagnetic part is exposed to a magnetic field, the part concentrates the magnetic field into its interior due to its high permeability. This concentration can only be perfect if the part itself is perfect. If, however, the part has defects like cracks, flaws or corroded spots the magnetic field is expelled to the exterior space again leading to a magnetic stray field. Therefore the detection of this stray field is an important technique of non-destructive testing [I]. To this purpose hitherto two groups of methods have been used: a) The stray field is detected by probes which are sensitive to a magnetic field like Hall generators or induction coils. b) Small ferromagnetic particles are put on the surface of the part as dry powder or as suspension in a liquid so that they gather at the places of the maximum stray field gradient. Both methods work rather slowly; additionally the second method requires the cleaning of the part after the inspection. To avoid these disadvantages, i.e. to produce a pictorial indication of the defects without inertia and without physical contact to the part under test, here the stray field is investigated by optical means, namely by the Faraday effect and the Kerr magneto-optic effect.
2. Faraday effect in glass A plate of Faraday glass (heavy lead glass, thickness 5 mm) is illuminated with linearly polarized light
0)
b)
Fig. 1. Experimental arrangement for the magneto-optic tection of stray fields.
de-
at a small angle of incidence (fig. 1a). The light penetrates the glass under the influence of the magnetic stray field caused by a test crack in a magnetized ferromagnetic specimen; consequently the plane of polarization of the light is rotated due to the Faraday effect proportional to the normal component of the stray field. At the surface facing the specimen the light is reflected by a mirror (evaporated on the glass plate) so that the light penetrates the glass in the reverse direction and the plane of polarization is rotated by the same amount due to the symmetry properties 129
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OPTICS COMMUNICATIONS
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of the Faraday effect. The light emerging from the glass passes through an analyzer into the lens of a camera. The resulting angle of rotation of the plane of polarization depends on the place on the specimen relative to the crack. At the left side of the crack the incident light propagates oppositely to the normal component of the magnetic field, producing a rotation of the plane of polarization e.g. counterclockwise; at the right side of the crack the rotation is clockwise. At the very place of the crack the light propagates perpendicularly to the magnetic field so that the Faraday effect is zero. There are two possibilities to convert these changes of the polarization to image contrasts [2,31: a) The analyzer is set perpendicular to the polarizer. In this crossed position the place of the crack appears dark since the incident light is extinguished by the analyzer (fig. 2). At both sides of the crack the field of
crOS5ed
Fig. 2. Surface
130
cracks
in steel (width
1978
view appears bright with the same intensity on either side of the crack. The contrast depends on the magnitude and on the extension of the stray field so that it decreases with decreasing width of the crack. b) The analyzer is rotated with respect to the crossed position by the angle of the maximum Faraday rotation produced by the given crack. So after the analyzer the light coming from different sides of the crack has different amplitudes. Thus in this amplitude position the crack is marked by the boundary between two areas of different intensities (fig. 3). Using this application of the Faraday effect. SW face cracks in steel with a minimum width ot‘0.05 mm could be detected. According to fig. 2 the crossed position is best suited for wide cracks, while the amplitude position has the highest sensitivity for small crack! The use of the glass as magneto-optic medium is advantageous consider-ing its simplicity and complete absence by hysteresis effects. However, the contrast is
position
2b) indicated
January
amplitude
by the Faraday
effect
position
in glass for different
positions
of the analyzer.
rather poor because of the weak Faraday effect in the glass. Cracks with a width below 0.05 mm and sub-surface effects can hardly be detected since their stray field is too weak to produce a sufficient magnetization of the glass. The lateral resolution is limited by the thickness of the glass plate so that the lateral resolution and the contrast which is proportional to the length of the path of the light within the glass vary inversely to each other.
3. Kerr effect on ferromagnetic
January 1978
OPTICS COMMUNICATIONS
Volume 24. number 1
films
This difficulty of the weak contrast is overcome by using materials the magnetization of which is not produced by an external field but by the mechanism of intrinsic spontaneous magnetization, i.e. ferromagnetic materials like iron or nickel-iron alloys. Since these materials are always magnetized spontaneously to a high value of the magnetization the stray field is not necessary to produce the magnetization, it must only be high enough to change the direction of the already existing magnetization, i.e. to exceed the coercivity of the ferromagnetic material. The magneto-optic rotation angle and the corresponding image contrast are independent of the magnitude of the stray field, so that very weak stray fields can be detected. This advantage of high sensitivity is achieved at the expense of the fact that the magnetization cannot be infinitely finely divided since the spontaneous magnetization exists only in uniformly magnetized magnetic domains (Weiss domains) which are separated from each other by zig-zag shaped boundaries (magnetic walls) [4,5]. The magnetization of thin films of iron and nickeliron with a thickness of about 100 nm is always in the plane of the film. In order to produce the Kerr magneto-optic effect, the illumination has to be inclined with respect to the normal of the film (fig. lb). The ferromagnetic film acts as the magneto-optic medium and as a mirror simultaneously. The observation is performed in the amplitude position of the polarizing elements. The magnetization of the film is reversed at the places where the tangential component of the stray field is higher than the coercivity of the film. Using iron films, surface cracks with a minimum width of 0.015 mm could be detected (fig. 3). The lateral resolution is determined by the zigzag shape of the mag-
netic domain boundaries. Fig. 4 shows that two parallel surface cracks with a width of 1 mm and a separation of 2.5 mm could be resolved. A further increase of sensitivity was obtained by using films of Permalloy (80% Ni, 20% Fe) due to the low coercivity of this material. The ability of this material to detect sub-surface defects was tested using a hollow cylinder made of steel (height of the cylinder 2 1.9 mm, inner diameter 3 1.3 mm, outer diameter 125 mm) with several bores (diameter 1.75 mm) parallel to the axis of the cylinder at a depth t below the outer surface. This specimen has the same dimensions as the test object used by Foerster [6,7] in accordance with standards issued by the American Society for
2 b = 0.015 mm
2b
= 0.05 mm
2 b = 0.10 mm
ml lmm
2 b : 0.20
mm
2 b: 0.50 mm
Fig. 3. Surface cracks in steel (width 26) indicated Kerr effect
by the
on an iron film.
5mm-
Fig. 4. Two parallel surface cracks in steel (width 1 mm, distance 2.5 mm) indicated by the Kerr effect on an iron film.
131
Volume
24. number
1
OPTICS COMMUNICATIONS
.lanuary
1978
Testing and Materials (ASTM). Fig. 5 shows that all bores up to a depth of 31 mm below the surface of the steel ring could be detected.
ti
References 3.5 mm
t=70mm
t :lL.Omm
t =17.5mm
-
t=10.5mm
tz21
Omm
lOmm-
Izig. 5. Sub-surface defects (bores with a diameter of 1.75mm) in steel at a depth t below the surface indicated by the Kerr effect on a Permollay film.
Metals Handbook (American Society for Metals. Metals Park, Ohio, 1976)Vol. 11, 8th edition, p. 44. M. Lambeck, ILEE Trans. on Magnetics, Vol. MAC;-4 (1968) 51. M. Lambeck, Optica Acta 24 (1977) 643. 1:. Kneller, I~erromapnetismus (Springer-Verlap. IlerlinGottingen-Heidelberg, 1962) p. 294. M. Lambeck, Barkhausen-Effekt und Nachwirkunp in lerromagnetika (Walter de Gruyter, Berlin 197 1) p. 7. Forster report Nr. 8 (1967). H. Heptner and 11. Stroppe, Magnetische und magnetinduktive Werkstoffpriifung (Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1972) 1’. 105.
OPTICS COMMUNICATIONS
January 1978
OFTICALDETECTIONOFMAGNETICSTRAYFIELDS R. FALK and M. LAMBECK Optisches Institut der Technischen
Universithi, D 1000 Berlin 12, Germany
Received 21 September 1977
The magnetic stray field caused by defects in ferromagnetic parts is indicated by means of magneto-optics. Using thin ferromagnetic films as the magneto-optic medium, surface cracks in steel with a width of 0.015 mm and sub-surface defects (bores with a diameter of 1.75 mm) 21 mm below the surface are detected.
1. Introduction Faraday- glass
If a ferromagnetic part is exposed to a magnetic field, the part concentrates the magnetic field into its interior due to its high permeability. This concentration can only be perfect if the part itself is perfect. If, however, the part has defects like cracks, flaws or corroded spots the magnetic field is expelled to the exterior space again leading to a magnetic stray field. Therefore the detection of this stray field is an important technique of non-destructive testing [I]. To this purpose hitherto two groups of methods have been used: a) The stray field is detected by probes which are sensitive to a magnetic field like Hall generators or induction coils. b) Small ferromagnetic particles are put on the surface of the part as dry powder or as suspension in a liquid so that they gather at the places of the maximum stray field gradient. Both methods work rather slowly; additionally the second method requires the cleaning of the part after the inspection. To avoid these disadvantages, i.e. to produce a pictorial indication of the defects without inertia and without physical contact to the part under test, here the stray field is investigated by optical means, namely by the Faraday effect and the Kerr magneto-optic effect.
2. Faraday effect in glass A plate of Faraday glass (heavy lead glass, thickness 5 mm) is illuminated with linearly polarized light
0)
b)
Fig. 1. Experimental arrangement for the magneto-optic tection of stray fields.
de-
at a small angle of incidence (fig. 1a). The light penetrates the glass under the influence of the magnetic stray field caused by a test crack in a magnetized ferromagnetic specimen; consequently the plane of polarization of the light is rotated due to the Faraday effect proportional to the normal component of the stray field. At the surface facing the specimen the light is reflected by a mirror (evaporated on the glass plate) so that the light penetrates the glass in the reverse direction and the plane of polarization is rotated by the same amount due to the symmetry properties 129
Volume
24, number
OPTICS COMMUNICATIONS
1
of the Faraday effect. The light emerging from the glass passes through an analyzer into the lens of a camera. The resulting angle of rotation of the plane of polarization depends on the place on the specimen relative to the crack. At the left side of the crack the incident light propagates oppositely to the normal component of the magnetic field, producing a rotation of the plane of polarization e.g. counterclockwise; at the right side of the crack the rotation is clockwise. At the very place of the crack the light propagates perpendicularly to the magnetic field so that the Faraday effect is zero. There are two possibilities to convert these changes of the polarization to image contrasts [2,31: a) The analyzer is set perpendicular to the polarizer. In this crossed position the place of the crack appears dark since the incident light is extinguished by the analyzer (fig. 2). At both sides of the crack the field of
crOS5ed
Fig. 2. Surface
130
cracks
in steel (width
1978
view appears bright with the same intensity on either side of the crack. The contrast depends on the magnitude and on the extension of the stray field so that it decreases with decreasing width of the crack. b) The analyzer is rotated with respect to the crossed position by the angle of the maximum Faraday rotation produced by the given crack. So after the analyzer the light coming from different sides of the crack has different amplitudes. Thus in this amplitude position the crack is marked by the boundary between two areas of different intensities (fig. 3). Using this application of the Faraday effect. SW face cracks in steel with a minimum width ot‘0.05 mm could be detected. According to fig. 2 the crossed position is best suited for wide cracks, while the amplitude position has the highest sensitivity for small crack! The use of the glass as magneto-optic medium is advantageous consider-ing its simplicity and complete absence by hysteresis effects. However, the contrast is
position
2b) indicated
January
amplitude
by the Faraday
effect
position
in glass for different
positions
of the analyzer.
rather poor because of the weak Faraday effect in the glass. Cracks with a width below 0.05 mm and sub-surface effects can hardly be detected since their stray field is too weak to produce a sufficient magnetization of the glass. The lateral resolution is limited by the thickness of the glass plate so that the lateral resolution and the contrast which is proportional to the length of the path of the light within the glass vary inversely to each other.
3. Kerr effect on ferromagnetic
January 1978
OPTICS COMMUNICATIONS
Volume 24. number 1
films
This difficulty of the weak contrast is overcome by using materials the magnetization of which is not produced by an external field but by the mechanism of intrinsic spontaneous magnetization, i.e. ferromagnetic materials like iron or nickel-iron alloys. Since these materials are always magnetized spontaneously to a high value of the magnetization the stray field is not necessary to produce the magnetization, it must only be high enough to change the direction of the already existing magnetization, i.e. to exceed the coercivity of the ferromagnetic material. The magneto-optic rotation angle and the corresponding image contrast are independent of the magnitude of the stray field, so that very weak stray fields can be detected. This advantage of high sensitivity is achieved at the expense of the fact that the magnetization cannot be infinitely finely divided since the spontaneous magnetization exists only in uniformly magnetized magnetic domains (Weiss domains) which are separated from each other by zig-zag shaped boundaries (magnetic walls) [4,5]. The magnetization of thin films of iron and nickeliron with a thickness of about 100 nm is always in the plane of the film. In order to produce the Kerr magneto-optic effect, the illumination has to be inclined with respect to the normal of the film (fig. lb). The ferromagnetic film acts as the magneto-optic medium and as a mirror simultaneously. The observation is performed in the amplitude position of the polarizing elements. The magnetization of the film is reversed at the places where the tangential component of the stray field is higher than the coercivity of the film. Using iron films, surface cracks with a minimum width of 0.015 mm could be detected (fig. 3). The lateral resolution is determined by the zigzag shape of the mag-
netic domain boundaries. Fig. 4 shows that two parallel surface cracks with a width of 1 mm and a separation of 2.5 mm could be resolved. A further increase of sensitivity was obtained by using films of Permalloy (80% Ni, 20% Fe) due to the low coercivity of this material. The ability of this material to detect sub-surface defects was tested using a hollow cylinder made of steel (height of the cylinder 2 1.9 mm, inner diameter 3 1.3 mm, outer diameter 125 mm) with several bores (diameter 1.75 mm) parallel to the axis of the cylinder at a depth t below the outer surface. This specimen has the same dimensions as the test object used by Foerster [6,7] in accordance with standards issued by the American Society for
2 b = 0.015 mm
2b
= 0.05 mm
2 b = 0.10 mm
ml lmm
2 b : 0.20
mm
2 b: 0.50 mm
Fig. 3. Surface cracks in steel (width 26) indicated Kerr effect
by the
on an iron film.
5mm-
Fig. 4. Two parallel surface cracks in steel (width 1 mm, distance 2.5 mm) indicated by the Kerr effect on an iron film.
131
Volume
24. number
1
OPTICS COMMUNICATIONS
.lanuary
1978
Testing and Materials (ASTM). Fig. 5 shows that all bores up to a depth of 31 mm below the surface of the steel ring could be detected.
ti
References 3.5 mm
t=70mm
t :lL.Omm
t =17.5mm
-
t=10.5mm
tz21
Omm
lOmm-
Izig. 5. Sub-surface defects (bores with a diameter of 1.75mm) in steel at a depth t below the surface indicated by the Kerr effect on a Permollay film.
Metals Handbook (American Society for Metals. Metals Park, Ohio, 1976)Vol. 11, 8th edition, p. 44. M. Lambeck, ILEE Trans. on Magnetics, Vol. MAC;-4 (1968) 51. M. Lambeck, Optica Acta 24 (1977) 643. 1:. Kneller, I~erromapnetismus (Springer-Verlap. IlerlinGottingen-Heidelberg, 1962) p. 294. M. Lambeck, Barkhausen-Effekt und Nachwirkunp in lerromagnetika (Walter de Gruyter, Berlin 197 1) p. 7. Forster report Nr. 8 (1967). H. Heptner and 11. Stroppe, Magnetische und magnetinduktive Werkstoffpriifung (Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1972) 1’. 105.