GB2186372A

GB2186372A - Eddy current testing

Info

Publication number: GB2186372A
Application number: GB08602554A
Authority: GB
Inventors: Kenneth Duncan Boness
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1986-02-03
Filing date: 1986-02-03
Publication date: 1987-08-12
Also published as: GB8602554D0

Abstract

Surface or near surface discontinuities in a conductive body are detected by generating and detecting eddy currents in the body by a variety of exciting coil arrangements (31, 33, 35, 37) and receiving coil arrangements (32, 34, 36, 38). <IMAGE>

Description

SPECIFICATION Eddy current testing The present invention relates to eddy cu rrenttesting, and specificallyto the detection of cracks.

Eddy current testing is a method for testing the integrity of metal bodies in which eddy currents are induced in the body, and their effects are observed.

Changes in the observed effect indicate changes in the material of the body under test. Conventionally, the parameter measured isthemagnitudeofthe eddy currents. However, because the interaction is an electromagnetic one, if the technique is to be used for the detection of cracks and other discontinuities it is necessary to know the value of the permeability and resistivity of the body undertest, or at least to be sure that they do not vary over the body undertest.

Also, it is necessary to apply corrections forthe lift-off ofthe testing probe, that is the distance between it and the surface ofthe body undertest.

According to the present invention there is provided a method of detecting discontinuities within a conductive body, comprising the operations of generating eddy currents within the body, and detecting changes in the spatial distribution of the eddy currents within the body.

Also according to the invention there is provided an apparatus for detecting discontinuities within a body, comprising means for generating eddy currents within a conductive body undertest, and means for detecting the said eddy currents, wherein the means for detecting the said eddy currents is adapted to provide an indication of changes in the spatial distribution of the eddy currents in the body due to the said discontinuities within the body.

Preferably, the means for generating the eddy currents and the means for detecting the eddy currents are incorporated into a single probe, the means for generating the eddy currents comprising a first coil, and the means for detecting the eddy currents comprising at least one other coil so disposed in relation to the first coil that its response to eddy currents generated in the body by the first coil is primarily dependent upon the spatial distribution of the eddy currents.

For example, the first coil may be rectangular and the detecting means may be a singular rectangular coil surrounding the first coil and orthogonal to it with the shorter axes of symmetry of the coils coincident. Alternatively, the detecting means may be a pair of co-planar rectangular coils disposed orthogonally either side of the first coil with their common plane coincident with the shorter axis of symmetry of the first coil.

In anotherarrangement, the first coil may be circular, with two two-lobed detecting coi Is within the first coil, the two detecting coils being co-planar with their bisectors orthogonal to each other.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows schematically likely eddy current paths in a crack-free body and a body in which there is a surface, or near surface, crack Figure2 shows schematically, an apparatus embodying the invention, and Figure 3 shows a series of different coil formats which may be used in performing the invention, together with an indication of their most sensitive orientations with respectto a crack in a body under test.

Referring to the drawings, Figure 1 (a) shows the eddy current paths which normally would be induced in an homogeneous and crack-free metal specimen 1 byatestcoil (notshown)positioned above the specimen 1 and carrying an alternating current. Figure 1 (b) show how the eddy current pattern is altered in the presence of a surface, or near surface, crack 3 in the specimen 1. It can be seen that instead of a single system 3 of closed eddy current loops, there are two separate systems 4 and 4' of closed eddy current loops, one on each side of the crack 2.

The present invention operates by detecting the presence or absence of those parts of the eddy current systems 4 and 4' which are flowing parallel to the crack 3. It is therefore insensitive to changes in the electrical parameters of the specimen 1 or variations in the lift-off ofthetest coil 2.

Figure 2 shows schematically, an apparatus for carrying outthe invention, comprising a test probe 21 including a first, exciting, coil 22 and two detecting, or receiving, coils 23 and 23'. The arrangement of the coils 22,23 and 23' shown is schematic only, actual possible geometric arrangements are shown in Figure 3. The exciting coil 22 is fed by an a.c. generator 24, and the output from the receiving coils 23 and 23' is applied to a detection and display device 25 via a high impedance buffer amplifier 26. The detection and display device 25 is not described in detail because it is conventional in nature.

Figure 3(a) shows schematically a probe arrangement in which there is one exciting coil 31 and one receiving coil 32. Each of the coils 31 and 32 is rectangular in form, and they are arranged symmetrically and orthogonal with respect two each other. In use, they both are arranged to be perpendicularto the specimen 1 under test.

Figure 3(b) shows a variant of this arrangement in which the single receive coil 32 is replaced bytwo identical receive coils 32 and 32', one on each side of the exciting coil 31. It can be seen aiso from Figure 3 that the effect of this change is to rotate the alignments of most sensitivity by 45 degrees.

Figure 3(c) shows an arrangement in which the exciting coil is a fiat two-lobed coil 33 surrounding a two-lobed receiving coil 34 wound in a figure-of-eight configuration.

Figure 3(d) shows an arrangement in which a single flat circular exciting coil 33' surrounds two two-lobed receiving coils 34', 34", positioned with their bisecting axes of symmetry orthogonal.

Figure 3(e) shows an arrangement in which a rectangular exciting coil 35, as used in the arrangement of Figure 3(a) has a two-lobed receiving coil 36 similarto the coil 33, positioned co-planar with it, and with the central axis of itswindings coincident with the longitudinal axis of the exciting coil 35.

Figure 3(f) shows an arrangement which, is an extended version of the arrangement shown in Figure 3(a), having an elongated rectangular exciting coil 37 and a plurality of regularly spaced orthogonal receiving coils 38.

Figure 3(9) shows an arrangement which is an extended version ofthat of Figure 3(e) a plurality of two-lobed receiving coils 39 being regularly spaced within the elongated exciting coil 37 and co-planar with it.

Claims

1. A method for detecting discontinuities within a conductive body, comprising the operations of generating eddy currents within the body and detecting changes in the spatial distribution ofthe eddycurrentswithin the body.

2. An apparatus for detecting discontinuities within a body, comprising means for generating eddy currents within a conductive body undertest and means for detecting the said eddy current, wherein the means for detecting the eddy currents is adapted to provide an indication of changes in the spatial distribution of the eddy currents due to the discontinuities within the body.

3. An apparatus according to claim 2wherein there is included a test probe including a rectangular exciting coil at least one rectangular receiving coil positioned orthogonal to the exciting coil and with the shorter axis of symmetry ofthe exciting coil lying in the plane of the receiving coil.

4. An apparatus according to claim 2 wherein there is included a test probe including a rectangular exciting coil andatleastonepairofco-planar receiving coils disposedsymmetricallyaboutthe exciting coil and with their common plane orthogonal to that of the exciting coil.

5. An apparatus according to claim 2 wherein there is included a test probe including a rectangular exciting coil and at least one two-lobed receiving coil within the exciting coil and co-planar with it, the receiving coil having a central axis of symmetry which is parallel to the longitudinal axis ofthe exciting coil.

6. An apparatus according to claim 2 wherein there is included a test probe including a flat two-lobed exciting coil co-planarwith a two-lobed receiving coil with their bisecting axes orthogonal.

7. An apparatus according to claim 2wherein there is included a test probe including a flat circular exciting coil surrounding and co-planar with at least two two-lobed receiving coils positioned with their central axis of symmetry orthogonal.

8. A method for detecting discontinuities within a conductive body substantially as hereinbefore described with reference to the accompanying drawings.

9. An apparatusfordetecting discontinuities within a conductive body substantially as hereinbefore described with reference to the accompanying drawings.