IE47295B1

IE47295B1 - Ultrasonic testing

Info

Publication number: IE47295B1
Application number: IE2052/78A
Authority: IE
Original assignee: Ti Group Services Ltd
Priority date: 1977-10-20
Filing date: 1978-10-16
Publication date: 1984-02-08
Also published as: IT1100970B; DK465778A; FR2406824A1; DE2845579A1; FR2406824B1; IT7828944A0; NL7810530A; LU80398A1; IE782052L; BE871341A

Abstract

Ultrasonic waves are used for non-destructive testing of an electricity conducting sample and are induced in the sample by an RF coil. Transmitting and receiving coils (14, 17) are mounted coaxially w.r.t. a pole piece (11) and the central pole (1). The coils are spaced apart on the magnetic axis and surrounded by a tubular external pole piece (3). A winding (2) between central (1) and external (3) poles creates electromagnetic pole pieces. The pole faces are close to the surface of the body (10) under test which concentrates the magnetic flux.

Description

This invention relates to non-destructive ultrasonic testing of electrically conducting materials, primarily metals, and in particular to apparatus for generating ultrasonic waves in a specimen under test and for detecting reflected ultrasonic vzaves.

Ultrasonic waves can be generated in the surface of a conducting body by inducing a radio frequency field 'in the body by the use of a coil placed outside the body, if the body is itself in a stong magnetic field; the ultrasonic waves result from the interaction of the eddy currents, induced by the r.f. field, and the magnetic field. Reflected ultrasonic waves in the body can be detected by the same coil or a similar coil in the presence of a magnetic field, the ultrasonic waves interacting with the field to generate a radio frequency current signal in the body which induces a corresponding current in the coil.

Based on this electromagnetic-acoustic phenomenon, an ultrasonic non-destructive testing technique has been developed that requires no direct contact with the body. This is in contrast to normal ultrasonic testing which either requires a piezo-electric transducer to be tightly bonded to the surface of the body (which must be smooth and free from scale) or at least requires the interposition of a liquid to transmit the ultrasonic waves. Inspection without ζ*' contact has obvious advantages, such as the fact that the body under test can be hot, its surface can be rough, and the automatic inspection of bodies on a production line basis is simplified by not having to cause tight mechanical engagement between a transducer and the surface of the body.

« However, the echo signal is very small, several orders of magnitude smaller than in the normal piezoelectric direct-contact method. The detecting cir10 cuit for the echo signal therefore has to be extremely sensitive, and the r.f. coil has to be placed as close as possible to the surface of the body.

In our earlier Patent No. 44575 we propose apparatus for carrying out this electromagnetic15 acoustic testing technique comprising a magnet having a central pole and an outer concentric pole with their pole-faces arranged close to the surface of the conducting body so as to produce a constant magnetic field therein, and separate transmitting and receiv20 ing coils both wound spirally, i.e. in a substantially flat disc form, and spaced laterally apart on the .central pole-face in a plane substantially parallel to the surface of the body. When applied to the testing of round-section rod, bar or tube, the 25 outer pole-face and the two coils lie in curved planes close to and parallel with the outer curved surface of the rod. The two coils are also spaced apart in the direction of the circumference of the body so that the magnet and coils are symmetrical 30 about a central plane through the axis of the rod.

This apparatus is satisfactory for testing round-section bodies having a particular diameter or a diameter within a particular limited range corres4729 S ponding to the curvature of the outer pole-face and the plane of the coils, but it cannot be used satisfactorily to test bodies with widely differing diameters because the outer pole-face and coils do not lie close to the surface of the body in all cases. For example, if the body has a smaller diameter than that for which the apparatus is designed, those parts of the coils and outer pole-faces furthest from the central plane of symmetry will be spaced further from the body than those parts nearer the central plane of symmetry. The electromagnetic coupling between the apparatus and the body is therefore reduced.

An object of the present invention is to provide improved electromagnetic-acoustic testing apparatus that can be used to test cylindrical bodies having a relatively large range of diameters.

This object is achieved according to the invention by providing apparatus comprising a magnet with a central pole surrounded by an outer tubular pole and transmitting and receiving coils adjacent the pole-face of the central pole, the pole-face of the outer tubular pole lying in a curved cylindrical surface with the greater part of its area concentrated in regions lying at opposite ends of a diameter of the pole that is parallel to the axis of the cylindrical surface, so that when the pole-face is located adjacent the surface of a body being tested the magnetic flux is concentrated along that diameter. The magnetic flux is not significantly reduced, therefore, if the air gap between the pole-face and the body at the outer regions thereof along a diameter perpendicular to the said diameter is increased when testing bodies with a smaller diameter than the maximum diameter for which the apparatus is designed. 7 2 9 5 Furthermore, because the greater part of the area of the pole-face is concentrated along the said first-mentioned diameter, the width of the pole-face along a diameter perpendicular to that firstmentioned diameter can be reduced. This in turn further reduces the variation in the distance between the poles of the magnet and the surface of the body when the latter has a diameter different to the maximum diameter for which the apparatus is designed. Thus, larger variations in the body diameter can be tolerated.

The area of the pole-face may be concentrated along the said diameter of the pole by forming the side wall of the tubular pole with portions at the end of a diameter perpendicular to the firstmentioned diameter that are of reduced thickness. This can be done by reducing the thickness of said portions along the length of the pole and/or by forming chamfers in said portions between the poleface and the outer surface of the side wall of the pole.

The transmitting and receiving coils are preferably arranged so that they occupy a restricted width of the pole-face of the central pole, whereby the electro magnetic coupling between the coils and the surface of the body does not vary significantly if the body has a smaller diameter than said maximum diameter. Thus, the coils may be arranged one within the other or may be arranged side-by-side along said first-mentioned diameter.

Preferably, both coils are of elongated form and extend longitudinally along said magnetic axis so as to produce a wide ultrasonic beam as measured along 7295 this axis, i.e. along the length of the body to be tested.

The invention will nov; be described by way of exanple with reference to the accompanying drawings in which:Figure 1 is a perspective view of an electromagnetic-acoustic test-head according to the invention, Figure 2 is an axial section through the testhead of Figure 1, Figure 3 is a plan view of the test-head of Figure 1, and Figure 4 is a plan view similar to Figure 3 but showing an alternative arrangement of the receiving and transmitting coils on the test-head.

The test-head of Figure 1 to 3 comprises an electro-magnet having a central cylindrical pole 1 with an energising winding 2 wound around it, and an outer co-axial tubular pole 3 joined to the inner pole by a yoke 4 at one end. The central pole 1 is received within a sleeve 6 connected to the yoke 4 so that the winding 2 is enclosed between the sleeve 6 and outer pole 3 and the yoke 4 and a brass ringshaped plate 5 opposite the yoke 4. O-ring seals 8 seal the space around the winding 2 and connections 7 allow the flow of cooling water of oil therethrough.

The outer pole 3 is of basically cylindrical form with flat faces 9 formed on diametrically opposite sides parallel to a central axial plane which, 47385 in use, is aligned with the axis of the cylindrical rod 10 to be tested, the free ends of the poles opposite the yoke being positioned closely adjacent the rod. The free ends of the poles 1, 3 are shaped to provide pole-faces 11, 12 respectively, that lie in a common, cylindrically-curved plane that is parallel to the outer surface of a rod 10 having the maximum diameter for which the apparatus is designed. The side portions of the outer pole 3 between the pole-face 12 and the flat faces 9 are also formed with chamfers 13.

The provision of the flat faces 9 and chamfers 13 at the sides of the outer pole 3 serves to restrict the pole-face 12 to regions therebetween along the length of the rod, thereby concentrating the magnetic flux along the length of the rod, i.e. along a magnetic axis parallel to the axis of the rod.

The central pole 1 comprises two co-axial polepieces 14 and 15, the inner cylindrical pole-piece 14 being axially adjustable within the outer tubular pole-piece 15, and both pole-pieces being axially adjustable within the yoke 4. The pole-pieces are shaped at their free ends so as to form said pole face 11, and radio frequency transmitting and receiving coils 16, 17 are mounted on the pole face 11 of the pole-pieces 14 and 15, respectively. Both coils 16 and 17 are elongated along the length of the rod 10. The transmitting coil 16 is made of a few turns of relatively large gauge wire and the receiving coil 17 is made of a large number of turns of relatively small gauge wire. Copper screens 18, 19 are provided around the coils to limit electromagnetic coupling between the coils, and each coil 472SS is sandwiched between an insulating backing layer 20 and a ceramic front facing 21.

Typically, apparatus such as illustrated can be used to test cylindrical bars in a range of diameters from 70 to 280 millimetres.

An alternative embodiment o£ the invention is illustrated in Figure 4 which is similar to that of Figures 1 to 3 except for the constructions of the central pole 1 and the arrangement of the transmitting and receiving coils 16, 17. Instead of the two coils 16, 17 being arranged coaxially on the end faces of tvzo coaxial pole-pieces 14, 15, each of the coils 16, 17 is mounted on the end face of a respective pole-piece 22, 23 that is axially adjustable within a respective bore in the one pole-piece 15, the two pole-pieces 22, 23 being spaced apart along said magnetic axis running parallel to the axis of the tested rod. Each coil is elongated along said magnetic axis so as to have a beam characteristic that is correspondingly elongated along the magnetic axis, whereby the transmitted and received beams overlap within the tested rod. A copper screen 24 surrounds each coil 16, 17 and each is sandwiched between an insulating backing layer and a ceramic front facing as in the first embodiment.

A particular advantage of the second embodiment of Figure 4 is that it allows improved performance in testing smaller diameter rods because it avoids any null effect in the centre of the beam pattern that might be produced by the coaxial coil arrangement of Figures 1 to 3.

In yet other embodiments of the invention, the magnet may be constructed so that the free end is removable (say along the broken line 25 in Figure 1) so as to expose a flat face for the testing of flat bodies, a new pair of transmitting and receiving coils being provided on the flat pole-face of the central pole 1.

Claims

1. Electromagnetic-acoustic testing apparatus comprising a magnet with a central pole surrounded by an outer tubular pole and transmitting and receiving coils adjacent the pole-face of the central pole, the pole-face of the outer tubular pole lying in a curved cylindrical surface with the greater part of its area concentrated in regions lying at opposite ends of a diameter of the pole that is parallel to the axis of the cylindrical surface f so that when the pole-face is located adjacent the surface of a body being tested the magnetic flux is concentrated along that diameter.

2. Apparatus as claimed in Claim 1 in which the outer tubular pole has a side wall with portions at the ends of a diameter perpendicular to the firstmentioned diameter that are of reduced thickness so as to concentrate the greater part of the area of the pole-face along the first-mentioned diameter.

3. Apparatus as claimed in Claim 2 in which said portions of reduced thickness comprise portions that extend the axial length of the pole.

4. Apparatus as claimed in Claim 2 or 3 in which said portions of reduced thickness comprise chamfers between the pole-face and the outer surface of the side wall of the pole.

5. Apparatus as claimed in any one of Claims 2 to 4 in which the outer tubular pole is cylindrical and said portions comprise flats or chamfers formed on the cylindrical side wall of that pole. ι;

6. Apparatus as claimed in any one of the preceding claims in which the transmitting and receiving coils are arranged co-axially on the pole-face of the central pole.

7. Apparatus as claimed in any one of Claims 1 to 6 in which the transmitting and receiving coils are spaced apart along said first-mentioned diameter.

8. Apparatus as claimed in Claim 6 and 7 in which the transmitting and receiving coils are elongated along said first-mentioned diameter.

9. Apparatus as claimed in one of Claims 6 to 8 in which the transmitting and receiving coils are each mounted on respective portions of the central pole that are adjustable relative to one another along the central axis of the magnet.

10. Apparatus as claimed in any one of the preceding claims in which the magnet is an electromagnet having a winding located between the central and outer poles within an enclosed space defined by the outer pole and an inner sleeve that receives the central pole and a yoke joining the two poles and a ring-shaped plate opposite the yoke, fluid connections being provided for the flow of cooling fluid through said enclosed space.

11. Electromagnet-acoustic testing apparatus substantially as herein described with reference to Figures 1 to 3 or Figure 4 of the accompanying drawings.