GB2385133A

GB2385133A - Magnetic field imaging device

Info

Publication number: GB2385133A
Application number: GB0212089A
Authority: GB
Inventors: Jonathan Roger Max Johnson
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-12-08
Filing date: 2002-05-27
Publication date: 2003-08-13
Also published as: GB0212089D0; GB0129464D0

Abstract

A device (101) is disclosed that is configurable to image discontinuities in a magnetic field. The magnetographic device comprises a lower polymer membrane (102) sealed to a transparent upper polymer membrane (103), wherein the two membranes form a pocket. A dispersion of magnetized particles (104) in a fluid (105) is encapsulated in the pocket. When placed close to a discontinuity in a magnetic field, the magnetized particles are operable to align and form an image in response to the discontinuity in the magnetic field. The image formed by the magnetized particles shows up features such as surface cracks or regions of different hardness in ferromagnetic materials. Further, the image shows the direction of magnetic flux of a magnetic field.

Description

MAGNETIC FIELD IMAGING DEVICE Field of the Invention The present invention relates to the field of imaging discontinuities in a magnetic field, in particular although not exclusively for the purposes of nondestructive testing, education and indelible marking of goods.

Background to the Invention Ferromagnetic materials such as iron and steel are used in many critical applications, for example in aerospace applications such as the orbital gear of a helicopter rotor. Surface flaws in the component, such as surface cracks are usually too small to be observed visually by a human operator. However, it is known that surface flaws in a ferromagnetic material give rise to a discontinuity in the magnetic field at the surface of the material.

To examine each component that is made, non-destructive testing (NDT) methods have been developed to test each component for surface flaws without damaging the component itself. To test a ferromagnetic component for surface defects that cannot be observed visually, it is known to magnetize the component and flood it with magnetic ink. The component is usually magnetized by passing a high current through the component. The magnetic ink comprises a mixture of a fluid and magnetic particles. The magnetic particles in the fluid are drawn to discontinuities in the component, and so the operator can observe if any discontinuities arise from surface flaws. This technique is known as magnetic particle inspection (MPI).

The effects of gravity and surface tension ensure that the magnetic particles in the fluid are attracted to the regions with the strongest discontinuities in the magnetic field. This usually comprises the largest surface cracks. The known technique of MPI will not show up finer cracks in the surface of ferromagnetic component, or smaller discontinuities arising from other surface defects such as P0842 spec

inclusions or localized regions of hardness. Further, the technique of MPI is expensive and damaging to the environment, as the magnetic ink can only be used a certain number of times before it is discarded. Some types of magnetic ink are not water-based, but use a volatile organic compound as the fluid. It is an object of the present invention to overcome these technical problems.

Furthermore, it is known that ferromagnetic materials can be marked magnetically with data, such as a batch number or serial number. This identification can be easily removed by demagnetizing the component. It is a further object of the present invention to overcome this technical problem by local hardening of the component and imaging the magnetic discontinuities arising from the localized areas of hardness.

Further, it is known to use iron filings, magnetic ink or the like to image magnetic field around magnets such as bar magnets for educational purposes.

The iron filings form a pattern clearly showing the field around the magnet.

However, this sort of demonstration can be messy as the iron filings are difficult to remove cleanly from the bar magnet. It is a further object of the present invention to provide a means for imaging the magnetic field around a magnet without the use of iron filings.

Summary of the Invention According to a first aspect of the present invention, there is provided a device configurable to image discontinuities in a magnetic field comprising; a polymer membrane defining a lower first surface; a substantially transparent polymer membrane defining an upper second surface; a continuous seal between the first surface and the second surface, the first surface and the second surface forming a sealed pocket;

a fluid encapsulated in the sealed pocket; a dispersion of magnetized particles being position in the fluid ; wherein the magnetized particles are operable to align and move in said fluid to form the image in response to the discontinuities in the magnetic field.

Preferably, the lower polymer membrane is a dissimilar colour to the magnetized particles to provide contrast with the formed image.

According to a further aspect of the present invention, there is provided a method for detecting discontinuities in a magnetic field using the device, comprising; placing the device close to discontinuities in the magnetic field ; viewing an image formed by the magnetized particles ; interpreting the image.

According to a further aspect of the present invention, there is provided a method for detecting discontinuities in a magnetic field using the device, comprising; placing the device close to the discontinuities in the magnetic field ; moving the device over a surface at a speed of no more than 1 ms-1 ; viewing a dynamic image formed by the magnetized particles ; interpreting the image.

Preferably, the device is moved over a surface at a speed of no more than 0.1 ms. 1.

According to a further aspect of the present invention there is provided a method of detecting discontinuities in a magnetic field comprising: forming a sealed pocket using at least one polymer membrane, the pocket having a lower first surface and an upper second surface; encapsulating within the pocket a fluid, said fluid containing a dispersion of magnetized particles; placing the device in close proximity to said magnetic field ; allowing the particles to align in the magnetic field and form an image of discontinuities.

Brief Description of the Drawings For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Fig. 1 is illustrates schematically a cross-sectional side view of the device according to a specific implementation of the present invention; Fig. 2 illustrates schematically a cross-sectional side view of the device when in close proximity to a discontinuity in a magnetic field according to a specific implementation of the present invention;

Fig. 3 illustrates schematically a cross-sectional side view of surface features that can give rise to discontinuities in the magnetic field of a ferromagnetic material; Fig. 4 illustrates schematically one specific embodiment of the device for imaging discontinuities in a magnetic field according to a specific implementation of the present invention; Fig. 5 illustrates the process of obtaining an image of discontinuities in a magnetic field according to a specific implementation of the present invention; Fig. 6 illustrates schematically a further specific embodiment of the device in close proximity to an end of a cylindrical ferromagnetic component showing an image generated by discontinuities in a magnetic field at a surface according to a specific implementation of the present invention; Fig. 7 illustrates schematically an embodiment of the device in close proximity to an end of a further cylindrical ferromagnetic component showing an image generated by discontinuities in a magnetic field at a surface according to a specific implementation of the present invention; Fig. 8 illustrates schematically a bar magnetic and the magnetic field associated with the magnet; Fig. 9 illustrates schematically a further specific embodiment of the device in close proximity to the bar magnet depicted in figure 8, showing an image generated by the device according to a specific implementation of the present invention; Fig. 10 illustrates schematically a further specific embodiment of the device in close proximity to a bar magnet with different magnetic poles to that depicted in

figure 6, showing an image generated by the device according to a specific implementation of the present invention; Fig. 11 illustrates schematically a further specific embodiment of the device in close proximity to a ferromagnetic surface showing an image generated by discontinuities in a magnetic field of a surface arising from localized regions of different hardness device according to a specific implementation of the present invention; Fig. 12 illustrates schematically a further specific embodiment of the device in close proximity to a ferromagnetic surface that has previously been marked magnetically with identifying information device according to a specific implementation of the present invention; Fig. 13 illustrates schematically a further specific embodiment of the device in close proximity to a magnetized ferromagnetic surface that has previously been marked with identifying information by locally altering the hardness of the surface according to a specific implementation of the present invention; Fig. 14 illustrates schematically a further specific embodiment of the device in close proximity to a printed circuit board containing two components that generate a magnetic field and a third component that cannot operate in a magnetic field according to a specific implementation of the present invention; Detailed Description of the Best Mode for Carrying Out the Invention There will now be described by way of example the best mode contemplated by the inventors for carrying out the invention. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well-known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

The following terms are used in the description: Ferromagnetic materials are materials that become strongly magnetized in a weak external field and may possess spontaneous magnetic moment even in zero field. Examples of materials include iron, cobalt and nickel.

Discontinuities in a magnetic field arise where the magnetic induction is higher or lower than the surrounding magnetic induction in a localized region.

This is also known as magnetic flux leakage.

Referring to Fig. 1 herein, there is illustrated schematically a cross-sectional side view of the device 101 according to the specific embodiment of the present invention. The device comprises a lower polymer membrane 102, a transparent upper polymer membrane 103, a seal 104 between the upper polymer membrane 102 and the lower polymer membrane 103, wherein the upper and the lower polymer membrane form a pocket. A mixture of magnetic particles 104 and a fluid 105 is encapsulated in the pocket formed by the polymer membranes.

According to the specific embodiment of the present invention, the lower polymer membrane 102 comprises a thin white polyester membrane with a thickness of no more than 300 um. The white lower polymer membrane ensures that the dark magnetic particles are visible against a white background. The upper polymer membrane 103 comprises a transparent acetate membrane with a thickness of up to 700 um, but preferably no more than 300 pm. The upper membrane is transparent to allow the operator to see the image formed by the dark magnetic particles against the white background. The seal 104 between the two layers such that a pocket is formed between the two layers may be effected using any suitable sealing technique, including cementing, but preferably heat sealing.

In the specific embodiment, the magnetic particles 104 comprise premagnetized iron oxide (fie203). The iron oxide particles are pre-magnetized in a DC magnetic field of greater than 200 Gauss. The iron oxide particles have an average particle size of between 0.1 and 30 urn.

The fluid comprises any suitable carrier fluid for the iron oxide particles. In the specific embodiment, the fluid comprises a mixture of water and at least one chemical dispersant. The chemical dispersant is present to prevent the iron oxide particles from agglomerating.

Referring to figure 2 herein, there is illustrated schematically a crosssectional side view of the device when in close proximity to a discontinuity in a magnetic field according to the specific embodiment of the present invention.

The device 101 is shown with a discontinuity in the magnetic field below it 201.

The magnetic particles 104 align in proximity to the magnetic field 201 in clearly defined patterns 202 to 204. The aligned particles 204 to 204 can be seen clearly by an operator through the transparent upper polymer membrane 103 against the white lower polymer membrane 102. The patterns formed by the aligned magnetized particles 202 to 204 form an image of the discontinuities in the magnetic field that are interpreted by an operator.

According to the specific embodiment of the invention, the suspension of magnetic particles in a fluid is captivated between two polymer membranes, and so the effects of surface tension and gravity are minimized. The particles are still attracted to the strongest discontinuities in a magnetic field, but are not free to move immediately to that region. Magnetic particles at a distance from the strongest discontinuities are therefore attracted to less strong discontinuities in a magnetic field. The device thereby images discontinuities in a magnetic field that could not be imaged by conventional MPI techniques.

According to the specific embodiment of the invention, the device is sensitive to very low levels of magnetic fields. When magnetized strongly, the

black iron oxide particles link to form rod shaped structures within the entrapped fluid. Being suspending in fluid, the effect of gravity and surface tension on the particles is minimal. The particles 104 are free to orientate themselves in line with the flux present at the surface of the device. A magnetic field of 25 Gauss is sufficient to pivot these particles 104, and thereby form an image. The image of the magnetic field will form almost instantly enabling the'scanning'of a surface.

There is contrast provided between the white lower polymer membrane 102 and the black magnetized particles 104. If a desired area is chosen for close inspection the film is rested above the area and left. Within 30 seconds the particles 104 creating the image of the settle onto the upper surface of the lower polymer membrane 102 owing to the effects of gravity. The device may then be carefully removed from the surface leaving the image temporarily held.

According to the specific embodiment of the present invention, the device detects magnetic fields as low as 25 Gauss, and yet is capable of imaging magnetic field in excess of 400 gauss. This broad range of detection limits allows the device to be used in a wide variety of applications, from testing flaws in permanent magnets to imaging the magnetic fields generated by electrical devices.

The minimum detectable field gradient for the specific embodiment of the invention has been shown to be 1. 75x104 Am-2, and the minimum detectable gradient in the induction is 22 Tam'. The absolute field and induction values that can be detected are 17.5 Am-'and 22/iT respectively. For comparison, the horizontal component of the Earth's magnetic induction is approximately 40 IT.

This high sensitivity to discontinuities in a magnetic field makes the device much more sensitive than prior art methods of MPI.

The specific embodiment of the invention device shows not only magnetic field discontinuities, but also the direction of the magnetic flux surrounding those discontinuities.

In addition to being very sensitive to small discontinuities in a magnetic field, the magnetic particles 104 in suspension in a fluid 105 are also very responsive to changes in a dynamic magnetic field according to the specific embodiment of the invention. The device can therefore create a dynamic moving picture of the magnetic fields present, and can be moved over a surface at a speed of more up to 1 ms. 1 to scan that surface for magnetic discontinuities. This allows an operator to scan a large surface, such as a long weld, quickly without the need for flooding it with magnetic ink.

It is a further advantage of the specific embodiment of the present invention that it can be reused many times, unlike conventional methods of MPI. Once an image of a magnetic discontinuity has been formed, the image can be erased by removing the device from close proximity to the discontinuity in the magnetic field and horizontally agitating it so as to redistribute the suspended particles 104 evenly in the fluid 105.

Referring to figure 3 herein, there is illustrated schematically a crosssectional side view 301 of surface features that can give rise to discontinuities the magnetic field of a ferromagnetic material. These features include cracks 302, inclusions 303, localized areas of different hardness 304 to the surrounding material and locally magnetized areas 305.

Surface flaws such as cracks 302 cause regions of high stress in the surrounding material. This alters the properties of the material. However, the extent of the discontinuity in the magnetic field is often much larger than the physical size of the crack, thereby allowing the imaging of the discontinuity in the magnetic field to indicate the presence of a crack.

Inclusions 303 comprise unwanted impurities in a component. An inclusion is likely to have different magnetic properties to the surrounding material, and so can be shown up by the discontinuity on the magnetic field around it.

Localized areas of different hardness 304 may be caused by heat damage.

For example, when magnetizing a component before MPI, a poor contact between the component and the electrical source may cause arcing. The heat generated locally can alter the physical properties of the component locally, usually by grain refinement. Altering the grain size in a localized area not only alters the hardness in a localized area, but also alters the magnetic properties in a localized area. Localized regions of differing hardness can therefore be shown up by imaging discontinuities in the magnetic field at the surface.

Ferromagnetic materials have a property known as remanence, whereby the material will hold a magnetic field after it has been magnetized once the original source of the magnetic field has been removed. If a portion of a ferromagnetic component has previously been in contact with a magnetic field, the ferromagnetic component can thereby have local magnetized areas that give rise to discontinuities in the magnetic field that can be imaged.

Fig. 4 illustrates schematically a further specific embodiment of the device

for imaging discontinuities in a magnetic field. This further specific embodiment (M) of the device, herein referred to as a Magnetograph fi) 401, has a substantially rigid supporting layer 404, which gives sufficient mechanical rigidity to allow easy handling of the Magnetograph The supporting layer has a cut-out region through which the viewing window 402 can be seen. The viewing window 402 comprises the first specific embodiment of the invention as illustrated in figure 1 herein, with the transparent upper polymer layer 103 positioned vertically above the lower polymer layer 102. The position of the viewing window 402 is offset in the plane of the supporting layer 401, to allow an operator to hold the Magnetograph in the region shown 403 without obscuring the image formed in the viewing window 402.

( M) The operator holds the Magnetograph t at one end 403 and places it over

a ferromagnetic surface to be imaged for discontinuities in the magnetic field such that the lower polymer membrane 102 is in contact with the ferromagnetic

surface. The image of any discontinuities in the magnetic field is allowed to form and the operator can view and interpret the image shown in the viewing window 402.

r Other embodiments of the Magnetograph @-are possible, wherein the (Itlrm) Magnetograph < & can be moved and positioned under automatic control in

response to instructions generated by a computer or computer numeric control device.

Referring to figure 5 herein, there is illustrated schematically the process of obtaining an image of discontinuities in a magnetic field.

The operator first distributes 501 the particles 104 evenly in the fluid 105. r, ,) This is achieved by rubbing the Magnetograph or by shaking it vigorously horizontally. This is necessary to break up any agglomerates of particles that have formed either through settling or from a previous image of a discontinuity in a magnetic field.

The operator selects the type of image required 502. A static image 503 of a small surface (typically smaller than the area of the viewing window 402)

may be obtained, or a dynamic image 504 may be required of a larger surface, for example a long weld.

Where a static image is required, the operator places 504 the fRTH) Magnetograph in contact with the magnetized ferromagnetic surface. An

image of the discontinuities is immediately formed, although settling of the particles 104 allows a detailed image to form 505 within around 30 seconds.

The operator may then interpret 506 the image to assess the discontinuities in the magnetic field of the component. These may arise from defects such as cracks that may require further investigation.

The operator then decides if another image is required of the same component or a different component 507. If another image is required the operator returns to step 501, otherwise the operation ends 508.

If a dynamic scan is required 504, the operator places the Magnetograph /T) CTM) . in contact with the surface 509 and moves the Magnetograph lover the

surface 510 at a speed of no more than 1 ms-1. The device is responsive enough to image discontinuities in the magnetic field as the device is moved over the surface. This is also the case where the magnetic field itself is

dynamic. If any feature is found that warrants further investigation 511, the (RTM) operator stops moving the Magnetograph, 512 and moves to step 505. If no

feature is found that warrants further investigation, the operator moves on to step 507.

Referring to figure 6 herein, there is illustrated schematically a (RTh) Magnetograp101 in close proximity to an end of a cylindrical ferromagnetic

component 601 showing the image generated 602 by discontinuities in a magnetic field at a surface according to a first specific implementation of the present invention.

(RE) The Magnetograpelo1 is placed over the end of a magnetized cylindrical

hollow bore component 601 and the operator allows an image to form in the viewing window 402. The particles 104 align to form an image of the magnetic field generated by the cylindrical component 602. In this instance, there are discontinuities in the magnetic field associated with the end of the cylindrical component 601, as shown by the breaks 603-605 in the image of the cylindrical component. These discontinuities may arise from cracks, hardness anomalies or magnetized regions and warrant further investigation by the operator.

Referring to figure 7 herein, there is illustrated schematically a ) Magnetograph 401 in close proximity to the end of a further cylindrical

ferromagnetic component 701 showing an image generated 702 by

discontinuities in a magnetic field at a surface according to a second specific implementation of the present invention.

(~) The Magnetograph IM is placed over the end of a magnetized solid

cylindrical component 701 and the operator allows an image to form in the viewing window 402. The particles 104 align to form an image of the magnetic field generated by the cylindrical component 702. In this instance, there are discontinuities in the magnetic field associated with the component as shown by the dark lines of the image 703-705. These lines are associated with cracks at the surface that are not visible by a visual inspection, and so the component warrants further investigation by the operator.

In a further specific embodiment of the invention, the thickness of the lower polymer membrane 102 is no more than 50 pm, to adhere to British Standards regarding the proximity of magnetic particles to the ferromagnetic surface during MPI and crack detection.

Referring to figure 8 herein, there is illustrated schematically a bar magnetic and a magnetic field associated with the magnet.

The bar magnet 801 has been magnetized such that the north pole is at one end of the magnet 802 and the south pole is at the other end of the magnet 803. The magnetic field around this bar magnet 801, and the direction of the magnetic flux, is also shown 804-813. When educating pupils in the nature of magnetism, it is a very vivid and clear demonstration to be able to show the pupils in a practical setting that this is indeed the shape of a magnetic filed around a bar magnet 801.

Referring to figure 9 herein, there is illustrated schematically a (kT-) Magnetograph 401 in close proximity to the bar magnet 801 depicted in figure

8, showing an image generated by the device according to a third specific implementation of the present invention.

(zut) The Magnetograph @ 401 is placed over the bar magnet 801 such that the

viewing window 402 is over the bar magnet 801. The magnetized particles 104 align in the discontinuities in the magnetic field arising from the magnet to form an image of the field 901. Th particles 104 form an image of the bar magnet itself. The particles also form an image of both the bar magnet and the field around the bar magnet, thereby demonstrating the shape of the field around the bar magnet 801.

(Rr^) Referring to figure 10 herein, there is illustrated a Magnetograph 401 in close proximity to a bar magnet with different magnetic poles to that depicted in figure 6, showing an image generated by the device according to a fourth specific implementation of the present invention.

The bar magnet 1001 has been magnetized in a different direction to that shown in figure 8. In this instance, the upper long surface of the bar magnet as

depicted comprises the north pole 1002 and the lower long surface of the bar (RTM) magnet as depicted comprises the south pole 1003. The Magnetograph. 401 is

placed over the bar magnet 1001 such that the viewing window 402 is over the bar magnet 1001. The magnetized particles 104 align in the discontinuities in the magnetic field arising from the magnet to form an image of both the bar magnet 1001 and the field associated with the magnet 1004, thereby demonstrating the shape of the field associated with the bar magnet 1001.

fr) Referring to figure 11 herein, there is illustrated a Magnetograph @ 401 in

close proximity to a ferromagnetic surface 1101 showing an image generated by discontinuities in a magnetic field of a surface arising from localized regions of different hardness according to a fifth specific implementation of the present invention.

As referred to above, localized regions of hardness can be caused by arcing when passing high current through a magnetic component to magnetize it prior to

MPI. In some cases, this is visible in the form of small'pits'on the surface of the

material. However, this type of damage is not always easily visible. The Magnetograp'f, ir,-) Magnetograph e 01 is placed on to the surface of a magnetized ferromagnetic

component 1101 such that region required for analysis is under the viewing window 402. In this instance, image formed has not revealed any surface cracks. However, the particles have aligned in the magnetic field around areas of localized hardness to form an image, as shown by the rings 1102-1104 in the image, which may warrant further investigation. This type of image cannot be obtained using conventional MPI techniques.

(RI Referring to figure 12 herein, there is illustrated a Magnetograph e401 in

close proximity to a ferromagnetic surface 1201 that has previously been marked magnetically with identifying information according to a sixth specific implementation of the present invention.

It may be desirable to mark the surface of a component with information, such as a serial number, part number, batch number, manufacturer, quality control information and the like. In this instance a ferromagnetic material, owing

to its inherent property of remanence, is marked magnetically with such

information 1202-1203. The information is subsequently read by placing the () Magnetograph 8 401 over the surface of the component 1201 such that the

viewing window 401 is above the information 1202-1203 on the ferromagnetic surface. The particles 104 quickly align to form an image in the viewing window 402 of the magnetized markings 1202-1203. In this way, a component may be invisibly marked with information that can subsequently be read by imaging the discontinuities above the surface of the ferromagnetic component. In this instance, the markings comprise a name 1202 and a barcode 1203.

A disadvantage of this marking technique is that the markings can be erased by demagnetizing the component. Demagnetizing the component is easily achieved by applying a magnetic field to the component that is stronger

than the remanence. This is a disadvantage if the markings are applied as a measure to discourage theft.

Referring to figure 13 herein, there is illustrated schematically a (b Magnetograph. ko1 in close proximity to a magnetized ferromagnetic surface

1301 that has previously been marked with identifying information 1302-1303 by locally altering the hardness of a surface according to a seventh specific implementation of the present invention.

As discussed above, the surface hardness of a material may be altered by applying heat in a localized area. This is usually in the form of unwanted heat generated by arcing of an electrical current, and is not a desirable feature.

However, this effect is used to mark a component with identifying information 1302-1303, such as a serial number, part number, batch number, manufacturer, quality control information and the like.

A laser or any suitable heat source is used to mark the surface of a ferromagnetic component. The localized heating causes grain refinement, and in some cases may form a small melt pool. The grain refinement may not be visible to the eye, but effects the magnetic properties and hardness of the component in

a localized area. To read this information, the operator must first magnetize the

component 1301. The component 1301 is then placed under the Magnetograph (T 401 such that the area containing the information is underneath the viewing window 402. The discontinuities in the magnetic field associated with the regions heat-treated regions of different hardness are imaged by the magnetic particles 104, and the operator reads the information 1302-1303. In the figure shown herein, the information marked on the component using a laser comprises a name 1302 and an identifying serial number 1303. This type of image cannot be obtained using conventional MPI techniques.

Referring to figure 14 herein, there is illustrated schematically a {RT ) Magnetograph-@ 401 in close proximity to an electrical printed circuit board (PCB)

1401 according to an eighth specific implementation of the present invention. The PCB contains components that generate a magnetic field 1402-1403, and a

component 1404 that must be positioned in a region where there is zero field.

(erA) The Magnetograph. 401 is placed over the surface of the PCB 1401 and a

designer of a PCB allows the magnetized particles to align to form an image of discontinuities in a magnetic field in the viewing window 402. The particles 104 form an image of discontinuities in the magnetic fields 1405 - 1406 around the two components 1402-1403 that generate magnetic fields. This allows the designer of a PCB to see immediately the regions where there is zero magnetic field, and so position the magnetic-sensitive component 1404 in the region of zero magnetic field on the printed circuit board.

Claims

Claims :

1. A device configurable to image discontinuities in a magnetic field comprising: a polymer membrane defining a lower first surface; a substantially transparent polymer membrane defining an upper second surface; a continuous seal between said first surface and said second surface, said first surface and said second surface forming a sealed pocket; a fluid encapsulated in said sealed pocket; and a dispersion of magnetized particles being positioned in said fluid ; wherein said magnetized particles are operable to align and move in said fluid to form said image in response to said discontinuities in said magnetic field.

2. A device as claimed in claim 1, wherein said lower polymer membrane comprises a dissimilar colour to said magnetized particles to provide contrast between said image and said lower first surface.

3. A device as claimed in claim 1 or claim 2, wherein said lower polymer membrane comprises polyester.

4. A device as claimed in any preceding claim, wherein said upper transparent polymer membrane comprises substantially transparent acetate.

5. A device as claimed in any preceding claim, wherein said continuous seal is effected by heat sealing said first surface to said second surface.

6. A device as claimed in any one of claims 1 to 4, wherein said continuous seal is effected by cementing between said first surface and said second surface.

7. A device as claimed in any preceding claim, wherein said

magnetized particles comprise iron oxide with an average particle diameter of less than 30 um.

8. A device as claimed in any preceding claim, wherein said magnetized particles comprise particles magnetized in a magnetic field, wherein said magnetic field comprises a field strength greater than 200 Gauss.

9. A device as claimed in any preceding claim, wherein said fluid comprises a mixture of water and at least one dispersant.

10. A device as claimed in any preceding claim that further comprises a substantially rigid border positioned at an outer region of said first and second surface, wherein said magnetized particles are viewed in said fluid through said substantially rigid border.

11. A device configurable to image discontinuities in a magnetic field as claimed in any preceding claim wherein said discontinuities have an induction gradient greater than 22 j. lTm-1.

12. A device as claimed in any one of claims 1 to 11, wherein said device is configured to image discontinuities in a magnetic field, said discontinuities arising from surface defects in a ferromagnetic surface.

13. A device as claimed in any one of claims 1 to 11, wherein said device is configured to image discontinuities in a magnetic field, said discontinuities arising from regions of different hardness in a ferromagnetic surface.

14. A device as claimed in any one of claims 1 to 11, wherein said device is configured to image discontinuities in a magnetic field, said discontinuities arising from impurities in a ferromagnetic surface.

15. A device as claimed in any one of claims 1 to 11, wherein said device is configured to image discontinuities in a magnetic field, said discontinuities arising from magnetized regions in a ferromagnetic surface.

16. A device as claimed in any one of claims 1 to 11, wherein said device is configured to image discontinuities in a magnetic field, said discontinuities arising from a magnet.

17. A device as claimed in any preceding claim, wherein said device is configured to detect flaws in ferromagnetic materials.

18. A method for detecting discontinuities in a magnetic field using the device of claim 1, said method comprising: placing said device close to said discontinuities in said magnetic field ; viewing an image formed by said magnetized particles ; interpreting said image;

19. A method for detecting discontinuities in a magnetic field using the device of claim 1, said method comprising: placing said device close to said discontinuities in said magnetic field ; moving said device over a surface at a speed of no more than 1 ms. 1 ; viewing a dynamic image formed by said magnetized particles ; interpreting said image;

20. A method as claimed in claim 19, wherein said device is moved over said surface at a speed of no more than 0.1 ms-1.

21. A method of detecting discontinuities in a magnetic field comprising: forming a sealed pocket using at least one polymer membrane, said pocket having a lower first surface and an upper second surface; encapsulating within said pocket a fluid, said fluid containing a dispersion of magnetized particles ; placing said device in close proximity to said magnetic field ; allowing said particles to align in said magnetic field and form an image of said discontinuities.

22. A method of detecting discontinuities in a magnetic field as claimed in claim 21, wherein said discontinuities have an induction gradient greater than 22 jJTm-1.

23. A method of detecting discontinuities in a magnetic field as claimed in claim 21 or claim 22, wherein said discontinuities arise from surface defects in a ferromagnetic surface.

24. A method of detecting discontinuities in a magnetic field as claimed in claim 21 or claim 22, wherein said discontinuities arise from regions of different hardness in a ferromagnetic surface.

25. A method of detecting discontinuities in a magnetic field as claimed in claim 21 or claim 22, wherein said discontinuities arise from impurities in a ferromagnetic surface.

26. A method of detecting discontinuities in a magnetic field as claimed in claim 21 or claim 22, wherein said discontinuities arise from magnetized regions in a ferromagnetic surface.

27. A method of detecting discontinuities in a magnetic field as claimed in claim 21 or claim 22, wherein said discontinuities arise from a magnet.