IE43294B1 - Arrangement for magnetographic defect inspection - Google Patents

Abstract

1503050 Detecting flaws magnetically M FORSTER 13 Oct 1976 [13 Dec 1975] 42593/76 Heading G1N An arrangement for magnetographic defect inspection of non-flat surfaces of ferromagnetic bodies comprises a frame 9 movable over a surface of the body 1 under test, two spaced magnet units 79 for magnetising the surface of the body and a feed unit 5 carried by frame 9 and including a pressure wheel 39 arranged to apply a magnetic recording medium (tape) 53 to the magnetised surface between the two magnet units 79. At least one of the magnet units 79 is carried on an arm 7 pivoted to the frame 9 so that the radius of curvature of an arc through the two magnet units 79 and the region therebetween at which the pressure wheel 39 applies tape 53 to the magnetised surface is variable. The magnet units 79 may be mounted on a supporting structure 65 which is pivoted for two degrees of movement on arm 7.

Description

The present invention relates to an arrangement for the magnetographic defect inspection of surfaces of ferromagnetic bodies, comprising a frame carrying a magnetization unit for the magnetization of a surface zone of a body under test and a feed unit including a pressure Wheel arranged, in operation, to apply a magnetic recording medium to the magnetized surface so as to record stray fluxes emanating from defects.

German Offenlegungsschrift 2 065 130 describes an arrangement of this type with two reels for a magnetic storage tape and one pressure roll which are installed in a casing together with a magnet yoke. The casing (serving as a frame) can be passed over the test body surface by means of a handle. During this operation the storage tape is rolled off over the surface while it is being rewound from one of the two reels to the other.

L5 In a place between two poles of the magnet yoke the storage tape is forced by the pressure roll against the surface and records the stray fluxes emanating from defects. The recording of the stray defect fluxes thus obtained can be evaluated in any desired manner, e.g. by scanning with the aid of probes which are sensitive to magnetic fields.

Arrangements of this type already enable a whole series of inspection problems to be solved and are particularly suitable for the inspection of welding seams, since the magnetic storage tape can be brought into very close contact with the irregular surface of the welding seam by means of a spring-loaded pressure roll. Such a close contact with the test specimen surface is of decisive importance for obtaining a good signal-to-background ratio of the defect signals and cannot be achieved with any other magnetic testing method.

The use of the described arrangement - whether for the inspection of welding seams or for quite general purposes - is limited by the geometry of the surface. Admittedly, the arrangement is not only suited for flat surfaces. On the contrary, uniformly curved surfaces can also easily be inspected in the direction of the profile, such as longitudinal welding seams of welded pipes, transverse welding seams of butt-welded pipes or the surfaces of special section tubes in the longitudinal direction of the tubes. In these cases the pole shoes of the magnetization yoke need only be adapted to the curvature or profile of the surface in order to achieve a uniform and sufficient magnetization of the test body surface along the testing path. However, if the surface has an irregular angular or curved geometry, adequate magnetization and consequently reliable inspection cannot be achieved any more by means of the known arrangement. Examples for a geometry of this type are the joint welds of pipes crossing at arbitary angles. In this case the known arrangements fail even in the simplest applications.

Furthermore, the known arrangements will require special pole shoes for many applications which they can handle. The object of the present invention therefore is to provide an arrangement which can yield reliable test results even on surfaces of irregular angular or curved geometry.

According to the present invention, there is provided an arrangement for magnetographic defect inspection of non-flat surfaces of ferromagnetic bodies, comprising a frame moveable over a surface of a body under test, two spaced magnet units carried by the frame for magnetizing the surface of the body under test, and a feed unit carried by the frame and including a pressure wheel arranged, in operation, to apply a magnetic recording medium to the magnetized surface between the two magnet units, at least one of the magnet units being carried on an arm so pivoted to the frame that the radius of curvature of an arc through the two magnet units and the region therebetween at which the pressure wheel applies the recording medium to the magnetized surface is variable.

The frame with the feed unit can form a very narrow main unit whose width is essentially determined by the necessary recording medium width only, i.e. by the width of the surface zones to be inspected, such as a welding seam. The main unit can be supported on the surface to be tested by means of the pressure roll and a light tail wheel or even only by one pressure roll alone and is therefore easily movable. Both magnet units can be separate units carried on outrigger arms pivoted to the frame so as to swing in a plane parallel to the pressure wheel axis. If necessary, and as explained below, one or more degrees of freedom of pivotal movement may be provided between each magnet unit and its corresponding arm.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a front view, and Figure 2 a side view of a testing arrangement embodying the invention.

Figures 1 and 2, in a simplified representation, show a manually guided magnetographic testing arrangement which is suited for the production of magnetographic recording even on surface of a complicated geometry, although, in order to simplify the representation, the arrangement is actually shown as placed on flat metal sheet 1 with a welded seam 3.

The frame construction 9 carrying a tape roll-off unit 5 and two cross-arms 7 essentially consists of two frame legs 11 and 13 depending from a connecting piece 15 which connects the frame legs together. Frame leg 11 has a cranked part above the connecting piece 15 designed as a guide handle 17. The frame leg 11 expands into front and rear wings 19 and 21 while, at its lower end, it terminates in a trailing arm 23 which carried a tail wheel 27 running on shaft 25. Two brackets 29 are laterally disposed on the two frame legs 11 and 13.

The roll-off unit 5 comprises removable take-off and take-up reels 31 and 33 which are carried by shafts 35 and 37 on the wings 19 and 21 respectively, a pressure wheel 39 which has an elastic layer 41, e.g. of foam rubber, on its periphery and which is journalled between the two frame legs 11 and 13 on a shaft 43 as well as an auxiliary drive 45 which consists of two pulleys 47, 49 and a driving belt 51. The pulleys 47 and 49 are rotationally fast with the pressure wheel 39 and tape-up reel 33 respectively. Reel 31 carries a stock of magnetic storage tape which is led-around the lower periphery of the pressure wheel 39 and taken up by reel 33 during the roll-off procedure. The rotation of the pressure wheel 39 is transmitted to the reel 33 via shaft 43, wheel 47, driving belt 51, wheel 49, shaft 37 and a friction clutch which is not represented in the drawing for reasons of simplicity. The transmission ratio is such that, even in the most unfavourable case, reel 33 attains a peripheral speed via the friction clutch that is sufficient to keep the storage tape 53 tight. A brake at reel 31 which is also not Lo represented in the drawing ensures that an adequate tension of the storage tape 53 is also maintained between reel 31 and pressure wheel 39.

The two cross-arms are identical in all details. The inner ends of the two cross-arms 7 are arranged in the brackets .5 29 to swivel in Fig. 1 in the plane of the paper about pins 54.

From the outer end of each arm 7, a stirrup 57 is suspended in which an articulated stirrup 59 is arranged to swivel about a pin 61. By means of an additional pin 63 and a link 67 a support carriage 65 is arranged below the articulated stirrup 59 to swivel about the pin 63. Each support carriage comprises a frame which carries the link 67; the frame is provided with a set of four support wheels 70. Magnetic contact wheels prove very advantageous as support wheels as they retain the support carriage on the surface by their own force. i On each frame 69 a magnet unit 79 is suspended through a link 74 and a bracket 75 to swivel about a pin 77. The magnet unit has an electromagnet 80 with a ferromagnetic core 81 and a coil -83 which are potted with cast resin to form a block 85. The core 81 has two free poles and is inclined upwardly away from the pressure wheel 39 and its upper portion is bent out laterally in the same direction. Thus, it is ensured that there is practically no interference with the storage tape 53 by the magnet unit 79. A laterally arranged wheel 87 supports the magnet unit 79 on the surface of the metal sheet 1. A compression spring 89 which acts between the magnet unit 79 and an angle piece 91 attached to the frame 69 tends to force the magnet unit 79 against the surface of the metal sheet 1.

When using the arrangement described a takeoff reel 31 with storage tape is inserted, the storage tape 53 is passed under the periphery of the pressure wheel 39 to the take-up reel 33 and the arrangement together with pressure wheel 39 and the tail wheel 27 is placed on the zone to be inspected, e.g. a welded seam. During this phase the support carriage 65, due to the two possible degrees of freedom closely contacts the surface of the test specimen with the support wheels 70 and maintains this favourable position while it is passed over the test specimen surface in the direction of arrow 93 together with the roll-off unit. During this operation the magnet units 79 constantly remain closely above the test specimen surface and ensure adequate and constant magnetization of the zone of the test specimen lying between the magnet unit.

The arrangement described is suited for surfaces of very irregular geometry. In many cases, however, simpler alternative solutions are possible. In the simplest case two supporting devices with only one supporting wheel each may be provided at the magnet units, each supporting device being rigidly connected to an arm 7. If the supporting device is to have two wheels which are spaced from one another in the running direction, this device is arranged on the cross-arm 7 to swivel about the pin 61 only. However, if the supporting device is to have two wheels which are spaced from one another in the direction of the sixes, the supporting devide is arranged in the cross-arm 7 to swivel about the pin 63 only.

In each of the aforementioned alternatives, the suspension of the magnet units 79 laterally inside of the supporting devices 65 is particularly advantageous for the magnetization of the surface zones to be inspected. This is so for several reasons.

In the case of the magnetographic inspection of angular 0 or curved surfaces, an alternating current supply of the electromagnets 80 is recommended, in particular alternating current with a considerably higher frequency than the mains frequency. Thus, due to eddy current generation, magnetization of a thin skin only on the test specimen surface is achieved and edges and fillets are also uniformly magnetized. However the surface zone under each contact wheel 70 is magnetically saturated by the magnetism of these contact wheels. The magnetic permeability of ·' this zone is, therefore, practically reduced to the value 1. In case an alternating field electromagnet were arranged laterally outside a contact wheel of this type the magnetization of the zones to be tested could be badly disturbed. This is safely prevented by the arrangement of the magnet unit 79 between the support carriage 65 and the pressure wheel 39.

Another advantage is that the magnet unit 79 is i positioned close to the zone to be magnetized. In addition, the radius of curvature of the path described by the magnetization unit becomes smaller if, due to the curvature of the test specimen surface, the cross-arm 7 performs a swivel motion about the pin 54.

In any case it is recommended to press the magnet unit which is arranged to swivel about pin 77 against the surface by spring force instead of designing the running wheel 87 as a magnetic contact wheel. Thus, magnetic saturation in the zone to be magnetized by the magnet unit 79 is avoided.

The use of magnetic contact wheels can be completely dispensed with, a spring-loaded pressure of the supporting device against the test specimen surface being provided. This can be achieved by arranging springs between the arms 7 and the frame legs 11, 13.

Claims (13)

1. An arrangement for magnetographic defect inspection of non-flat surfaces of ferromagnetic bodies, comprising a frame movable over a surface of a body under test, two spaced magnet units carried by the frame magnetizing the surface of the body under test, and a feed unit carried by the frame and including a pressure wheel arranged, in operation, to apply a magnetic recording medium to the magnetized surface between the two magnet units, at least one of the magnet units being carried 0 on an arm so pivoted to the frame that the radius of curvature of an arc through the two magnet units and the region at which the pressure wheel applies the recording medium to the magnetized surface is variable.

2. An arrangement according to claim 1, wherein the two 5 magnet units are carried on two arms which extend in opposite directions from the frame and are pivotally mounted for swinging in a plane parallel to the axis of the pressure wheel.

3. An arrangement according to claim 1 or 2, wherein the or each magnet unit is carried by a supporting unit on a ) corresponding pivoted arm, the supporting unit having at least one supporting wheel which, in use runs over the surface of the test body.

4. An arrangement according to claim 3, wherein the or each supporting unit has only one supporting wheel and is rigidly i attached to the corresponding arm.

5. An arrangement according to claim 3, wherein the or each supporting unit has supporting wheels spaced apart in their running direction and is pivotally attached to the corresponding arm for pivotal movement about an axis parallel to the supporting wheel axes. - 10 43294

6. An arrangement according to claim 3 or 5, wherein the or each supporting unit has supporing wheels axially spaced apart and is pivotally attached to the corresponding arm for pivotal movement about an axis normal to the supporting wheel axes.

7. An arrangement according to any of claims 3 to 6, wherein at least one supporting wheel is a magnetized wheel tending to adhere to the surface of the test body.

8. An arrangement according to any of claims 1 to 6, wherein the or each arm is spring-biased to urge its magnet unit towards the surface of the test body.

9. An arrangement according to any of claims 3 to 7, wherein the or each magnet unit is mounted between the corresponding supporting unit and the pressure wheel.

10. An arrangement according to claim 9, wherein the or each magnet unit has a running wheel for running on the surface of the test body and is pivotally attached to the corresponding supporting unit.

11. An arrangement according to claim 10, wherein the or each magnet unit is spring biased to the corresponding supporting units so as to be urged towards the surface of the test body.

12. An arrangement according to any preceding claim, wherein each magnet unit has a coil on a core with a first free pole which, in use, is in juxtapation to the surface of the test body, the core inclining away from the first pole away from the pressure wheel to a portion which terminates in a second free pole and is bent laterally outwardly so as to face away from the pressure wheel.

13. An arrangement for magnetographic defect inspection 11 43294 of non-flat surfaces of ferromagnetic bodies substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.