GB2466849A

GB2466849A - Defect detection using magnetic field and particles

Info

Publication number: GB2466849A
Application number: GB0900490A
Authority: GB
Inventors: Mark Bernhard; Roland Richard Moser; Andres Peralta
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2009-01-13
Filing date: 2009-01-13
Publication date: 2010-07-14
Also published as: GB0900490D0

Abstract

A defect detection apparatus for detecting defects 5 in a Ferromagnetic or non Ferromagnetic test object 2 comprises magnetic flux generator 12 for inducing magnetic flux lines 14 in the object and a viewing device 20 that includes magnetic particle medium 21 in a casing 25. The casing has a transparent surface 26 which in use faces away from the object and through which encased magnetic medium can be observed and a second surface 27 which in use faces towards the test object. When the apparatus is placed near a test object, distortions of the induced magnetic flux lines by defects observably influence the alignment of the magnetic medium to enable detection of defects. A camera with light source may be used to observe the transparent surface. The apparatus may be used to inspect tubes and internal surfaces.

Description

I

Defect detection apparatus and method for detecting defects

FIELD OF THE INVENTION

The invention relates to the non-destructive testing of Ferro-magnetic and non Ferro-magnetic test objects using electromagnetic techniques. More particularly the invention relates to detecting of defects such as cracks or voids by observing the distortion of induced magnetic flux by apparatuses that may be miniaturised.

STATE OF THE ART

Magnetic particle testing is a non-destructive electro magnetic testing method used for detecting defects in Ferro-magnetic materials. The method includes the step of applying magnetic medium to the surface of a test object after preparing the surface by for example removal of surface coatings and cleaning it. Surface preparation is an important step as cleanliness and roughness significantly influences test reliability.

As test surfaces are not always easily accessible, to ensure adequate surface preparation, test objects may need to be disassembled. This can be a time consuming process.

Crack detection apparatuses have been developed to overcome some of these problems. US 4,443,350 for example, describes a magnetisable recording medium that can be applied to a test object to record defects. After removal from the test object a viewer, such as provided in US 3,013,206 can be used to view defects recorded on the recording medium. The viewer comprises a non-ferromagnetic hollow container, having a clear viewing window, filled with a medium that consist of a suspension of weakly magnetic crystals that are configured to align with magnetic flux lines in a viewable fashion, viewable to the human eye. In a related application US 3,978,398 discloses a magnetisable putt like substance that enables detection of defects in areas of the object that consists of holes, threads or other sharply curved areas.

US 5,754,044 discloses a magneto-optic device capable of detecting defects in articles of non magnetic and magnetic conducting material. The device comprises, a housing with electrical contacts for inducing magnetic flux in a test object and a magneto-optical transformer element, such as an uniaxial anisotropical single crystalline of Bi-substituted iron garnet film grown expitaxially on a substrate of gadolinium-gallium garnet, for recording induced magnetic flux distorts caused by defects in the test object. A polarised light source directed onto the surface of the magneto-optical transformer is reflected by the magneto-optical transformer and then passes through an analyser element before passing through an optical lens from which defects of the test object can be viewed.

A test method capable of testing conductive materials is the so-called eddy current method. In this method a magnetic field is induced into a test object by a changing electric field. Crack-like discontinuities in the test object create eddy currents in this magnetic field and are detected by so call Hall sensors or coils. As the disturbances are not directly visible the data from the sensors needs to be compared with standards and interpreted. This requires a skilled operator. A further limitation is that with conventional methods, Ferro-magnetic materials cannot be inspected due to magnetic effects resulting from magnetic permeability of the material. These effects overshadow the induced eddy currents and inspection is not possible. In very specific cases, tube inspections for example, Ferro-magnetic materials can be inspected with full I partial magnetic saturation or remote-field Eddy-current applications.

us 5,053,704 and us 5,446,378 overcomes some of these problems by disclosing a magneto-optic eddy current imaging apparatuses. The apparatuses comprise: sensors that rotate the plane of polarization passing through as a function of applied magnetic field; a means for applying a normal magnetic field to the sensor; and an optical means for applying polarised light to the sensor. Defects are viewed by a video camera after light reflected from the sensor is processed. While the apparatuses of these patents can be used for testing both Ferro-magnetic and non Ferro-magnetic materials they are limited by their size to the testing of surfaces with fixed shapes, for example flat planes, or cylindrical surfaces with large radii. A known method of testing of non-flat regions or regions comprising grooves, gaps or turbines comprising the use intermediate recording medium, such as that described in US 4,443,350.

SUMMARY OF THE INVENTION

The invention provides an alternative apparatus for the non-destructive testing of metal test objects that is suitable for use in areas with restricted space such as inside grooves, small gaps, tubes and bores This problem is solved by means of the subject matters of the independent claims.

Advantageous embodiments are given in the dependant claims.

Provided in one aspect is a defect detection apparatus for detecting defects in a Ferro-magnetic or non-Ferro-magnetic test object. The apparatus comprises a magnetic flux generator for inducing magnetic flux lines in a portion of the test object and a defect viewing device. The defect viewing device includes: * a magnetic medium capable of observably aligning with induced magnetic flux lines; and * a casing, for encasing the magnetic medium wherein the casing further includes; o a transparent surface which in use is arranged to face away from the test object and through which encased magnetic medium can be observed; and o a second surface which in use is arranged to face towards the test object.

The apparatus is configured such that when placed on or near a test object, distortions of the induced magnetic flux lines by defects observably influence the alignment of the magnetic medium so by enabling detection of defects.

In a further aspect the apparatus the magnetic flux generator is an AC coil wherein the AC coil and the defect viewing device are arranged such that in use the defect viewing device is positioned between the AC coil and the test object.

In another further aspect the magnetic flux generator is a magnet with magnetic poles wherein the magnetic poles are juxtaposed around the defect viewing device.

In a yet further aspect the apparatus further includes a camera for recording images of the alignment of the magnetic medium.

In a yet further aspect the apparatus further includes a mirror for reflecting the image of aligned magnetic medium from the defect viewing device to the camera. In a further aspect the camera is located next to the defect viewing device and the mirror reflects the image through about a 90 degree angle to the camera. The mirror in these arrangements gives the designer the freedom to reduce the stack height of the apparatus. This flexibility can, for example, be particularly useful when the apparatus is required to test curved bores or alternatively the walls of small diameter bores or tubes.

In a further aspect the apparatus is miniaturised such that the diameter of the defect viewing device is between 5-20 mm.

In a yet further aspect apparatus includes a remote data processor for collating image data from the camera.

In a yet further aspect the apparatus includes a location device capable of providing location data of the apparatus to the remote data processor to enable location data stamping of the image data from the camera.

In a yet further aspect the apparatus is configured to provide testing capability in restricted areas by comprising an extension arm and a swivel joint with first and second ends wherein the defect viewing device and magnetic flux generator are combined in a unit which is attached to a first end of the swivel joint while the extension arm is connected to the second end of the swivel joint. In a further aspect the diameter of the unit is between 5 mm and 10 mm.

In a yet further aspect the apparatus includes a casing light source, formed in the second surface of the casing, configured and arranged to pass light through the magnetic medium.

In a yet further aspect the magnetic medium of the apparatus includes fluorescent particles and the apparatus further comprises a UV light source configured and arranged to illuminate the fluorescent particles.

Another aspect provides a defect detection apparatus for detecting defects in a Ferro-magnetic or non Ferro-magnetic tube. The apparatus comprises: * a cylindrical hollow shell, sized to fit inside the tube, with inner walls and a first end and a second end that form distal ends of the shell; a magnet inside the shell having o a first magnetic pole at the first end of the shell, o a magnet shaft around with wind ings and o a second magnetic pole wherein the magnet shaft extends between the first magnetic pole and the second magnetic pole and the magnet is configured and arranged to induce magnetic flux lines, when fitted in the tube, in a section of the tube section between the first magnetic pole and the second magnetic pole; * a casing, located between the first and the second magnetic pole, in contact with an inner wall of the shell wherein the casing comprises a first and a second opposing transparent surface each of which face towards the first and second ends of the shell respectively; * magnetic medium enclosed by the casing, configured to visibly align with the induced magnetic flux lines; * a light source located in the shell adjacent to the first magnetic pole located and configured to direct light through the first and second opposing transparent surfaces of the casing therethrough; and * a camera, located inside the shell, adjacent to the second magnetic pole, configured and arranged to record light passing through the casing, wherein the apparatus is arranged and configured such that distortions of the induced magnetic flux lines by defects change the alignment of the magnetic medium such that the light, passing through the casing and received by the camera, is altered in a way that enables the identification of a defect. In a further aspect, the magnetic medium includes fluorescent particles and the light source is UV light.

Another aspect provides a method for detecting defects within the core of a turbo generator. The method includes the steps of: a) inducing magnetic flux lines in a stator of the turbo generator by passing an AC flow through a rotor of the turbo generator; b) moving a defect detection device, capable of providing location data of the detection device and image data of induced magnetic flux lines with sufficient resolution to enable identification of magnetic flux lines distorted by defects, wherein the defect detection device is moved over surfaces of the stator; c) collecting the image data and the location data while completing step b); and d) collating data as it is collected in step c), to form an image map of induced magnetic flux lines of the surfaces of the stator over which the defect detection device was moved in step b).

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, an embodiment of the invention is described more fully hereinafter with reference to the accompanying drawings, in which: FIG. 1 is a block diagram showing the major components of a defect detection apparatus according to an embodiment of the invention; FIG. 2 is a perspective view showing an exemplary embodiment of a defect detection device of the defect detection apparatus of FIG. 1; FIG 3 is a perspective view showing another exemplary embodiment of a defect detection device of the defect detection apparatus of FIG. 1; FIG. 4 is a perspective view showing the defect detection device of FIG. 3 with a camera; FIG. 5 is a perspective view showing another exemplary embodiment of the defect detection device of FIG. 4; FIG. 6 is a perspective view showing another exemplary embodiment of the defect detection device of FIG. 4; FIG. 7 is a schematic view of an embodiment of the invention applied to the testing of a turbo generator; FIG. 8 is a sectional view of an embodiment of the invention applied to the testing of a tube or bore; and FIG. 9 is a perspective view of an embodiment of the invention applied to the testing of a bore of a test object.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It may be evident, however, that the invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the invention.

FIG. 1 shows the features of a defect detection apparatus for detecting defects 5 in a test object 2. Shown is a defect detection device 10 located on or proximal to a test object 2. The defect detection device 10 includes a magnetic flux generator 11 for inducing magnetic flux linesl4 in the test object 2 as shown in FIGs., 2,3 and 4, and a defect viewing device 20, used to enable the viewing of the induced magnetic flux lines 14 (not shown) by the human eye.

When placed over the region in which the induced magnetic flux lines 14 are present the medium 21 in the defect viewing device 20 aligns with the induced magnetic flux lines 14. When there is a defects 5 in the region of induced magnetic flux lines 14, the defects 5 distorts this alignment. This distortion is then reflected in the alignment of the magnetic medium 21, which in turn is viewable in the defect viewing device 20 as shown in FIGs 2,3 and 4. In this way defects 5 are made viewable by the defect viewing device 20.

In an exemplary embodiment, a visualisation device 30 visually records images from the defect viewing device 20 and then transmits a signal encoded with the image data via a communication means 40, to a remote data processor 50 where the data is analysed and processed. In a further embodiment the apparatus also includes a location device 45 that provides the transmitted signal with location data that can be used by the remote data processor 50 to create an image map comprising of a collage of image data taken from the regions over which the apparatus has been moved.

FIG. 2 is an exemplary embodiment of the defect detection device 10 of FIG. 1 suitable for testing a conductive test object 3 that has low conductivity and low permeability The magnetic flux generator 11 induces magnetic flux lines 14 by means of an AC coil 12 located above the area to the tested. The magnetic medium 21 within the casing 25 of the defect viewing device 20 aligns itself with the induced magnetic flux lines 14 the casing 25 is configured to enable viewing of magnetic medium 21 by having a transparent surface 26 which in use faces away from the conductive test object 3 as shown. When a second surface 25 of the casing 25 corresponding to the surface opposing the transparent surface 26 is proximal to the conductive test object 3, any distortions of the induced magnetic flux lines 14, caused by defects 5 can be recognised by simply viewing the alignment pattern of the magnetic medium 21 through the transparent surface 26.

In this exemplary embodiment the defect viewing device 20 diameter defines the defect detection apparatus diameter AD.

A suitable magnetic medium 21 preferably comprises free flowing magnetic particles or else particles suspended in gel or liquid. Preferably the particles are fine particles as resolution improves with decreasing particle size. An example of suitable magnetic medium 21 is disclosed in US 3,013,206.

FIG. 3 is an exemplary embodiment of the defect detection device 10 of FIG. 1 suitable for testing a Ferro-magnetic test object 4. A magnet 15 provides the function of a magnetic flux generator for generating induced magnetic flux lines 14, and has magnetic poles 16 juxtaposed around a defect viewing device 20. In operation these magnetic poles 16 are located proximal to or in contact with the Ferro-magnetic test object 4. The exemplary magnet 15 shown is an electro magnet with windings 18 however other magnets such as permanent magnets could also be used. The magnetic medium 21. encased by the casing 25 of the defect viewing device 20 and viewable through a transparent surface 26 of the casing 25 that in use faces away from the Ferro-magnetic test object 4, aligns itself with the induced magnetic flux lines 14 due to is proximity to the Ferro-magnetic test object 4. Any distortions of the induced magnetic flux lines 14 caused by defects 5 can be recognised by simply viewing the alignment pattern of the magnetic medium 21. A suitable magnetic medium 21 for this application is disclosed in us 3,013,206.

In this embodiment the diameter of the magnetic poles 16 defines the defect detection apparatus diameter AD.

FIG. 4 is an exemplary embodiment of the defect detection device 10 of FIG. 1 further comprising a visualisation device 30 of FIG. 1 in the form of a camera 31, located in view of the transparent surface 26 of the defect viewing device 20. The camera 31 records images of the magnetic medium 21. To improve the visibility of the magnetic medium 21, a casing light source 33 is formed in the second surface 27 of the casing 25 that in use faces the test object 2. In this arrangement light can illuminate the magnetic medium 21 by passing up through it.

In order to view a defect 5 the diameter of the transparent surface 26 of the defect viewing device 20 must be several times greater than the size of the defects 5, otherwise it is difficult to discern defects 5 by eye as they will extend beyond the viewing window of the defect viewing device 20. Defects 5 of importance are typically up to 3 mm long, have a depth of 0.2 mm and are up to 0,1 mm wide. The transparent surface 26 of the defect viewing device 20 should therefore have a defect viewing device 20 diameter preferably at least greater than the largest crack length Therefore in an exemplary embodiment a suitable minimum diameter is at least 5 mm.

FIG.5 and FIG. 6 show exemplary embodiments corresponding to the defect detection device 10 of FIG. 4 in an arrangement suitable for miniaturisation.

To overcome the limitations of a small viewing window, in exemplary embodiments shown in FIGs. 5 and 6 a miniaturised camera 31 is positioned over the defect viewing device 20 surrounded by magnets 15. The camera 31 is connected to a remote data processor 50, via communication means 40 as shown in FIG. 1 using either wired or wireless communication methods. The remote data processor 50, of FIG. 1, pieces together image data, using known programming techniques, to create an image map larger than the field of view of the defect viewing device 20.

The remote data processor 50 of FIG. 1, when applied to the miniaturised embodiments of FIG. 5 and FIG. 6, is preferably a PC or other computing device. As the defect detection device 20 is moved along the surface of the test object 2 an image map is developed by software in the remote data processor 50, shown in FIG. 1, from images taken by the camera 31.To enable improved image map quality in an exemplary embodiment a location device 45, as shown in FIG. 1, provides location data for association with the imaging data of the camera 31. This association can be used by the remote data processor 50, shown in FIG. 1, to piece together an accurate image map.

In some instances it is required to test the walls of cracks and slots. The exemplary embodiment shown in FIG. 5 having a mirror 32 fitted between the camera 31 and the defect viewing device 20 enables the camera 31 to lie flat against the test object 2 and fit into cracks and slots while providing a right angle view of the crack, slot, bore or tube wall that is the test object 2.

FIG 7 shows an exemplary use of the defect detection apparatus. The method comprises first forming an AC current loop 66 within the core of a turbo generator 61 utilising the rotor 62 wherein the AC current loop 66 forms an induction loop that induces magnetic flux lines 14, not shown, in the stator 63. A robotic sled 67, which preferably includes a location device 45 (not shown), positions and moves the defect detection device 10 proximally along the stator 63. During this movement images generated from the defect detection device 10, preferably location stamped by the location device 45 (not shown) are sent via communication means 40 (not shown) to a remote data processor 50 (not shown). The remote data processor 50 (not shown) then pieces together images from the defect detection device 10 to form an image map in which even a low skill person can easily identify defects.

In this way it is possible to test magnetic yokes of large electrical machines like motors, generators or transformers. These yokes are typically manufactured by stacking insulated ferromagnetic sheets in order to reduce eddy currents during operation. If these sheets are shorted, local currents will flow which will generate local overheating. These local areas can be check by for example the exemplary arrangement shown in FIG. 7. Especially during the manufacturing and repair of large yokes a detailed understanding about the locality and shape of failures is required. The exemplary arrangement has the advantage that the failure can be visualized easily and repair work minimized.

FIG. 8 shows another exemplary embodiment used to detect defects 5 in the sidewalls of tubes or bores 60. The exemplary embodiment comprises a cylindrical or cuboidal like shaped shell 70 whose shape depends on the shape of the tube or bore 60 to be tested. First and second magnetic poles 16 corresponding to North and South poles are located at each end of the shell 70 respectfully. The polarity of the magnetic poles 16 maybe fixed or may be cycled depending on the chosen test methodology. In one form, the magnet 15 of the exemplary embodiment is an electra magnet comprising a magnet shaft 17 with windings 18, wherein the magnet shaft 17 forms a link between the two magnetic poles 16. The polarity of the magnetic poles 16 is induced by current flow through the windings 18 and changed by reversing the direction of the current flow. A light source 35, adjacent to one of the magnetic poles 16, directs light through a defect viewing device 20 located between the magnetic poles 16. The defect viewing device 20 comprises a casing 25 that encloses magnetic medium 21 and has opposing transparent surfaces 26 that enable the directed light to pass through the casing 25. A visualisation device 30, for example a camera 31, adjacent to the other magnetic poles 16 receives light passing through the casing 25.

Distortion, by defects 5, of the induced magnetic flux lines 14 (not shown) induced in the tube 60 by the magnetic poles 16 causes alignment and concentration of magnetic medium 21 in the casing 25. The distortion of light passing through the casing 25 by this alignment and concentration of magnetic medium 21 is detectable by the visualisation device 30. By this means the presence and location of defects 5 in the tube 60 can be determined.

FIG. 9 shows another exemplary embodiment used to defect defects 5 in bore ends and walls. The defect detection device 10 can be inserted into the bore by attachment to an extension arm 68. Once inserted into the bore a swivel joint 69 joining the defect detection device 10 to the extension arm 68 enables the defect detection device 10 to scan the surfaces of the bore for defects 5.This arrangement enables the checking of bores where the bore diameter BD is at least twice the diameter of the defect detection apparatus diameter AD. For example, a defect detection device 10 sized to have a diameter of between 5 to 10 mm is suitable for inspecting bores with bore diameters BD as small as 20mm.

In any of the exemplary embodiments the magnetic medium 21 can be enhanced with fluorescent particles and viewed with the assistance of UV light or with other light emitting systems such as phosphorescent particles. Another possibility is to use emitting light components, such as fluorescent, phosphorescent, or organic light emitting diodes, on the second surface 27 of the defect viewing device 20 as shown in FIG 4. The light components are not however limited to being formed in the second surface 27 of the casing 25 but may, for example, take the form of a ring around the lens of the camera 31.

Miniaturisation of the defect detection device 10 is further limited by the resolution of the magnetic medium 21 and by the contrast between areas of high and low magnetic particle concentration in the magnetic medium 21. By the mentioned light assistance means it is possible to realise an improve contrast so by overcoming this limitation.

Although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiment, it is recognised that departures can be made within the scope of the invention, which is not to be limited to the details described herein but is to be accorded the full scope of the appended claims so as to embrace any and all equivalent devices and apparatus.

REFERENCE NUMBERS

2 Test object 3 Conductive test object 4 Ferro-magnetic test object 5 Defects Defect detection device 11 Magnetic flux generator 12 AC coil 14 Induced magnetic flux lines 15 Magnet 16 Magnetic poles 17 Magnet shaft 18 Windings Defect viewing device 21 Magnetic medium Casing 26 Transparent surface 27 Second surface Visualisation device 31 Camera 32 Mirror 33 Casing light source Light source Communication means 45 Location device Remote data processor Tube or bore 61 Turbo generator 62 Rotor 63 Stator 66 AC current loop 67 Robotic sled 68 Extension arm 69 Swivel joint Shell AD Defect detection apparatus diameter BD Bore diameter

Claims

Claims 1. A defect detection apparatus for detecting defects (5) in a Ferro-magnetic or non Ferro-magnetic test object (2), the apparatus comprising: a magnetic flux generator (11) for inducing magnetic flux lines (14) in a portion of the test object (2); a defect viewing device (20) that includes: a magnetic medium (21) capable of observably aligning with induced magnetic flux lines (14); a casing (25) ,for encasing the magnetic medium (21), having; a transparent surface (26) which in use is arranged to face away from the test object (2) and through which encased magnetic medium (21) can be observed; and a second surface (27) which in use is arranged to face towards the test object, wherein the apparatus is configured such that when placed on or near a test object, distortions of the induced magnetic flux lines by defects (5) observably influence the alignment of the magnetic medium (21) so by enabling detection of defects (5).
2. The apparatus of claim 1 wherein the magnetic flux generator (11) is an AC coil (12) wherein the AC coil (12) and the defect viewing device (20) is arranged such that in use the defect viewing device (20) is positioned between the AC coil (12) and the test object (2).
3. The apparatus of claim 1 wherein the magnetic flux generator is a magnet (15) with magnetic poles (16) wherein the magnetic poles (16) are juxtaposed around the defect viewing device (20).
4. The apparatus of any one of claims 1 to 3 further including a visualisation device (30) for recording images of the alignment of the magnetic medium (21)
5. The apparatus of claim 4 further including a mirror (32) for reflecting the image of aligned magnetic medium (21) from the defect viewing device (20) to the visualisation device (30).
6. The apparatus of claims 5 wherein the camera (31) is located next to the defect viewing device (20) and the mirror (32) reflects the image through about a 90 degree angle to the visualisation device (30).
7. The apparatus of any one of claims 1 to 6 wherein the diameter of the defect viewing device (20) is between 5-20 mm.
8. The apparatus of any one of claims 4 to 7 wherein the apparatus includes a remote data processor (50) for collating image data from the visualisation device (30).
9. The apparatus of claim 8 further including a location device (45) capable of providing location data of the apparatus to the remote data processor (50) to enable location data stamping of the image data from the visualisation device (30).
10. The apparatus of any one of claims 1 to 9 further comprising an extension arm (68) and a swivel joint (69) with first and second ends wherein the defect viewing device (20) and magnetic flux generator (11) are combined in a unit which is attached to a first end of the swivel joint (69) and the extension arm (68) is connected to the second end of the swivel joint (69).
11. The apparatus of claim 10 wherein the diameter of the unit is between 5mm and 10mm.
12. The apparatus of any one of claims 1 to 11 wherein the apparatus includes a casing light source (33), formed in the second surface (27) of the casing (25), configured and arranged to pass light through the magnetic medium (21).
13. The apparatus of any one of claims 1 to 11 wherein the magnetic medium (21) includes fluorescent particles and the apparatus further comprises a UV light source configured and arranged to illuminate the fluorescent particles.
14. A defect detection apparatus for detecting defects (5) in a Ferro-magnetic or non Ferro-magnetic tube (60), the apparatus comprising: a cylindrical hollow shell (70), sized to fit inside the tube, with inner walls and a first end and a second end that form distal ends of the shell (70); a magnet (15) inside the shell (70) having a first magnetic pole at the first end ofthe shell, a magnet shaft (17) with windings (18) and a second magnetic pole (16) at the second end of the shell wherein the magnet shaft (17) extends between the first magnetic pole and the second magnetic pole (16), wherein the magnet (15) is configured and arranged to induce magnetic flux lines (14), when fitted in the tube, in a section of the tube (60) section between the first magnetic pole (16) and the second magnetic pole (16); a casing (25) located between the first and the second magnetic poles (16) wherein the casing (25) is in contact with an inner wall of the shell (70) and comprises a first and a second opposing transparent surface (26) each of which face towards the first and second end of the shell (70) respectively; magnetic medium (21), enclosed by the casing, configured to visibly align with the induced magnetic flux lines (14); a light source (35) in the shell (70) adjacent to the first magnetic pole (16) located and configured to direct a light through the first and second opposing transparent surfaces (26) of the casing (25) thereth rough; and a camera (31) located inside the shell (70) ,adjacent to the second magnetic pole (16) and configured and arranged to record light passing through the casing, wherein the apparatus is arranged and configured such that distortions of the induced magnetic flux lines (14) by defects (5) change the alignment of the magnetic medium (21) such that the light, passing through the casing and received by the camera (31) is altered in a way that enables the identification of a defect (5).
15. The apparatus of claim 15 wherein the magnetic medium (21) includes fluorescent particles and the light source (35) is UV light.
16. A method for detecting defects (5) within the core of a turbo generator, including the steps of: a) inducing magnetic flux lines (14) in a stator (63) of the turbo generator by passing an AC flow through a rotor (62) of the turbo generator loop; b) moving a defect detection device (10), capable of providing location data of the detection device (10) and image data of induced magnetic flux lines (14) with sufficient resolution to enable identification of magnetic flux lines (14) distorted by defects (5), wherein the defect detection device (10) is moved over surfaces of the stator (63); c) collecting the image data and the location data while completing step b); and d) collating data as it is collected in step c), to form an image map of induced magnetic flux lines (14) of the surfaces of the stator (63) over which the defect detection device (10) was moved in step b).