IE45001B1

IE45001B1 - Ultrasound

Info

Publication number: IE45001B1
Application number: IE120377A
Authority: IE
Original assignee: Ti Group Services Ltd
Priority date: 1976-06-17
Filing date: 1977-06-13
Publication date: 1982-06-02
Also published as: IT1078302B; GB1565063A; FR2354825B1; NL7706731A; FR2354825A1; LU77567A1; BE855622A; DE2726827A1; IE45001L; DK265477A

Abstract

The invention relates to a method for generating and detecting ultrasonic waves in an electrically conductive solid body. Said method consists in using a transducer comprising a grid of parallel electrical conductors lying in the same plane and placed close to the surface of the body parallel to the latter, an alternating electrical signal being applied to all the conductors so that the current which circulates there generates ultrasonic waves in the body. The frequency of the alternating current is related to the spacing of the conductors and the waves generated by the current flowing in each conductor reacts with those generated by the adjacent conductors to produce an ultrasound beam oriented in a direction which is not not perpendicular to the body surface Application to the control of solid materials such as metals to detect defects or straws.

Description

This invention relates to the generation and detection of ultrasonic waves in an electrically conducting solid body, primarily, though not exclusively, for the purpose of detecting flaws in such .a body.

The normal ultrasonic testing of solid bodies involves beaming an ultrasonic pulse through the body and detecting that pulse, either on the opposite side / of the, body, or when -it is reflected from the opposite side and/or from a discontinuity within the body. The signal is generated electrically and applied, to a transducer, which may be electromagnetic or piezo-electric.

The transducer is in contact with a surface of the body (sometimes coupled with the help of a liquid or a semisolid material) and the direction of propagation is normal to that surface.

In many cases it is desired to beam the pulses in a particular direction, not necessarily perpendicular to the surface. The need for this arises for example in the inspection of thick-walled round tubes. Hitherto the only way of doing this has been to mount the transducer at an angle and couple it to the body by means of a wedge-shaped solid body or a liquid that has, as far as possible, the same .propagation characteristics as the body in question. This is sometimes inconvenient or difficult in' practice because it involves all the sealing problems associated with the handling of liquids, or the problems of making physical contact when the body is moving or is at high temperatures.

. An aim of the invention is to provide a way of propagating ultrasonic waves in an electrically conducting solid body in selected directions other than normal to the surface at which the transducer is mounted, yet without involving liquid coupling or other physical contact.

The invention makes use of the known fact that it is possible to generate ultrasonic waves in an electrically conducting body by inducing radio frequency eddy currents 2. 4S0Q1 in the body in the presence of a strong magnetic field so that the eddy currents and magnetic field interact to produce ultrasonic waves. The eddy currents are induced by an alternating current flowing in a conductor placed close to the surface of the body and the magnetic field has been a' steady externally applied field.

According to one aspect, the invention consists in a method of generating ultrasonic waves in an electrically conducting solid body using a transducer comprising a grid of parallel electric conductors, the conductors of the transducer' being located close to and parallel to the surface of the body, and the same alternating electric signal being connected to all of the conductors so that the current flowing therein generates ultrasonic waves in the body, the frequency of the alternating signal being related to the spacing between the conductors in such a manner that the waves generated by the current in each conductor interfere with the waves generated by the adjacent conductors and results in an ultrasonic beam produced by constructive interference in the required non-normal direction. > The behaviour of the transducer is analogous to that of a diffraction grating or a mattress-typo radar antenna, and results in abeam of ultrasonic waves being generated in the body at an angle to the normal at the surface where the transducer is located. The angle of inclination of the beam to the normal can be varied by varying the frequency of the alternating current energising the ί transducer.

The alternating current flowing in' the conductors produces its own magnetic field which will interact with the induced eddy currents and produce ultrasonic waves and ttesa waves may themselves he used to produce said beam. Preferably, however, a separate magnetic field is applied to the body to interact with tlie induced eddy currents.

According to another aspect, the invention consists in a method of detecting ultrasonic waves in an electrically .3. conducting solid body using a transducer comprising a grid of parallel electric conductors, the conductors of the transducer being located close to and parallel to the surface of the body, and a magnetic field being applied to the surface of the body so that ultrasonic waves in the body induce eddy currents in the conductors of the transducer, the spacing between the conductors being•related to the frequency of the ultrasonic waves to be detected so that only those waves propagating in a particular non-normal direction induce alternating currents in the conductors that are phased to reinforce one another.

It will be appreciated that this method of detecting ultrasonic waves is simply the converse of said method of generating ultrasonic waves according to the invention, except that an applied magnetic field has to be provided in the case of detection to produce the required eddy currents. That is, the process whereby the energising alternating current in the transducers produces a directed ultrasonic beam when generating ultrasonic wave:.·, is reversible, and allows a directed ultrasonic beam to produce a coherent alternating current in the conductors when detecting ultrasonic waves.

• The invention will now be described by way of example with reference to the accompanying drawings in which:Figure 1 is a schematic plan view of a transducer for use according to the invention, Figure 2 is a schematic side view of the transducer of Figure 1, Figure 3 is a diagram illustrating how a nonnormal beam is generated by the transducer of Figures 1 and 2, Figure 4 is a schematic diagram showing apparatus for measuring the beam pattern of the transducer of Figures 1 and 2, .4 4&QOl Figure 5 is a polar diagram produced using the apparatus of Figure 4, Figure 6 is a schematic plan view of an alternative transducer for use according to the invention, and Figure 7 is a schematic side view of yet another alternative transducer’for use according to the invention.

The transducer illustrated in Figures 1 and 2 comprises a single wire 1 arranged in a zig-zag path so that successive sections of the wire extend parallel to one another and form a grid of parallel, equi-spaced conductors. A current passing through the wire flows in opposite directions in alternate conductors, as shown by the arrows in Figure 1.

In use, the wire grid 1 is placed close to, and parallel with, the surface of the electrically conducting body 2 in which ultrasonic waves are to be generated, and an electro-magnet 3 applies a steady magnetic field to the surface of the body 2 in the locality of the grid 1 so that the field extends parallel to the surface of the body and substantially perpendicular to the parallel conductors, An alternating current is passed through the wire and this induces a pattern of eddy currents in the surface layer of tha body, which pattern comprises equal and opposite currents at alternate regions in 'and parallel to the surface. These eddy currents interact with the steady magnetic field to produce Lorenz forces, as shown by the arrows in Figure 2, which are alternately in antiphase. The region beneath each of the parallel conductors can therefore be regarded as a source of ultrasonic waves at the frequency of the alternating current, alternate sources being in antiphase. Because the waves from these sources propagate into the body over a wide arcuate front, interference occurs between them, and .5 this phenomenon is used according to the invention to produce a beam of waves in a particular non-normal direction by arranging for there to be constructive interference in this direction. This is achieved by relating the frequency of the alternating current to the spacing of the parallel conductors as illustrated in Figure 3.

If the frequency of the alternating current were such that the wavelength λ of sound in the body were equal to the spacing d of the parallel conductors, a beam would be produced normal to the surface of the body, but by selecting a slightly different frequency one produces destructive interference in the normal direction and constructive interference (i.e. a beam) in a direction, inclined at an angle 0° to the normal. The relationship between the parameters, λ, d and Θ is conventionally expressed as 2d sin 6 = ηλ, where n is 1 for first order interference. · Second or higher order interference with n=2 or a higher integer is only significant at shorter wavelengths. This relationship is represented in Figure 3 by the condition that, the, path difference between alternate, in phase, parallel conductors 1 should be an integral number of wavelengths λ for constructive interference in the direction inclined at the angle 0 to the normal. Figure 3 also shows that there will be two such beams, one on either side of the normal, but generally only one of these beams will be used, the other being ignored.

In order to demonstrate that a beam of ultrasonic waves can be generated at an angle to the normal, and that its direction varies with the frequency as described above, beam pattern measurements for a transducer such as illustrated in Figures 1 and 2 were made using the apparatus shown in Figure 4. The transducer was located centrally adjacent the flat face of a hemi-cylindrical block 2 of aluminium alloy (that sold under the trade name DURALUMIN) so as to produce longitudinal bulk waves therein, and a receiving transducer 4 was scanned over the cylindrical surface of the block to measure the intensity of the waves reaching this surface.

The spacing of the parallel conductors of the transducer was 3 m.m. and the transducer was energised with a pulsed sine-wave r.f. generator operating at a frequency, variously 0.77 MH^, 1.2 MH,, 1.7 MHz and 2.0 MHZ, a separate beam pattern being measured for each frequency. The electro-magnet 3 produced a field of 0.4 T across a gap of 2.5 cm. The diameter of the block 2 was 7 cm., and the receiving transducer 4 was a highly damped piezoelectric longitudinal wave transducer operating below its natural resonant frequency.

The results ootained are shown on a polar diagram in Figure 5, only half of the beam pattern being shown, the other half simply being a mirror image of that shown.

The results clearly indicate that a well defined beam of ultrasonic waves can be produced by the transducer at an angle to the normal, and that this beam can c* be moved over an angular range of 29' by varying the frequency of the alternating current from 1.2 to 2.0 MH . z Figure 5 also shows the existence of a secondary beam which is shifted angularly towards the normal from the primary beam. This beam is produced by waves at a frequency double that of the energising current, these waves being generated as a result of the interaction between the eddy currents induced by the transducer and the magnetic field produced by the energising current itself. The frequency doubling effect is caused by the fact that the eddy currents and the alternating magnetic field are always in phase and therefore the resulting Lorenz forces always act in the same direction away from the wire. Comparing this situation with that shown in Figure 2 for a steady magnetic field, it will be readily appreciated that the frequency of the generated waves is doubled compared with that caused by the steady magnetic field. .7. 43001 In the apparatus of Figure 4, the presence of the double frequency beam is igriored, but in other apparatus, for example, when the magnet 3 is not used, this double frequency beam may be used as the effective beam.

Figure 5 does not show any second or third order beams.. However, these will appear at higher frequencies at larger angles Θ than the first order beam. Generally, the smallest angle 0 that can be achieved with the first order beam is about 30°. At the other extreme the largest angle 0 that can be achieved is about 60°.

The results shown in Figure 5 were obtained by making measurements of the beam pattern at a distance of 7 cm. from the transducer. If this distance is reduced, it is generally found that the beam pattern becomes less distinct, the peak intensity being reduced and the angular spread of the beam being increased. Narrowing the overall width of the grid (i.e. the distance measured perpendicular to the conductors) counteracts this effect, but .the frequency of the energising current then has to be increased. In practice, the minimum useful working range at which the ultrasonic beam is still reasonably sharp is about twice the width of the grid.

. The grid must be placed close to the surface of the body 2 for efficient generation of ultrasonic waves, and the maximum acceptable grid to surface spacing decreases with closer spacing of the parallel conductors. Generally, the spacing between the grid and the surface of the body should be less than one sixth the spacing between the conductors.

When the body is made of non-magnetic material, as in Figure 4, this has no effect on the form of tlie applied magnetic field, but this will not be the case if the body is made of magnetic material, such as for example, steel. In this case, it may be preferable to replace the steady-field electro-magnet 3 with a coil fed with a pulsed current so as to produce the magnetic field. .8. 4-3 0 0 j A particular advantage of the invention when used to generate ultrasonic waves in mild steel is that a strong beam of longitudinal waves can be generated, especially at an angle of about 40° fco the normal, whereas it is difficult using the known electro-magnetic techniques to generate any longitudinal waves in mild steel at all.

The transducer illustrated in Figures 1 and 2 has a single zig-zag wire conductor 1. In an alternative embodiment, the wire conductor 1 may be bent around this same zig-zag path a plurality of times in an analogous manner to a multi-turn coil, as shown in Figure 6.

In another alternative embodiment, the wire conductor 1 may be replaced by a similar zig-zag conductor etched on a printed circuit board.

Further, -where the body to be examined is a tube or bar and the beam is to be directed towards the axis but at an angle to a plane that is normal to the axis, the grid of parallel conductors may be formed by a bifilar winding around the tube or rod, with the current flowing in opposite directions in the two wires, as shown in Figure 7, successive turns of the winding forming successive parallel conductors.

In other embodiments of the invention, the spacing between the parallel conductors of the grid is nonuniform, the spacing between successive conductors decreasing in one direction, in such a manner as to iiring the signal to a focus at a selected point within the specimen. This has the further advantage that such a layout propagates a beam in that one direction, i.e. towards the focus, but suppresses the (generally unwanted) other beam on the opposite side of the normal.

When tbe focussed version is used on a round-section bar and is in the form of a bifilar winding around the bar, as in Figure 7, it could be arranged to focus the signal at the centre of the bar. .9 4S001 For the continuous examination of a moving bar on a production-line basis there could be practical difficulties in threading the bar through a closely fitting coil, and so instead one could use two, three or more individual grids arranged symmetrically around the bar to achieve the same result. The grids could be mounted on a linkage that moves them all towards and away from the axis to allow for insertion of the bar and possibly to cater for bars of different diameters.

So far, the invention has only been described in relation to the generation of ultrasonic waves in an electrically conducted body. However, it will be appreciated that all of the aforesaid transducers including a magnet to apply a magnetic field can also be used to receive or detect a beam of ultrasonic waves inclined to the normal to the surface of the body at the transducer. This fact can be most easily appreciated from Figure 3 if the generated beam is considered as a received beam. The condition 2d sin 0 = λ is now necessary for the received waves to be in phase at alternate conductors and in antiphase at neighbouring conductors so that currents are induced in these conductors in the directions shown by the arrows in Figure 2 and reinforce one another. Thus, the presence of a received ultrasonic beam of a wavelength λ = 2d sin 0, at an angle 0 to the normal, is detected by the flow of a current of a frequency in the grid of conductors.

The transducers described above can be used to generate and detect angled ultrasonic beams in electrically conducting bodies so as to test for flaws therein. The beams used may be directed in a fixed direction relative to the body during testing or may have its direction switched or scanned during testing by changing the frequency of the energising or detected current.

Claims

1. A method of generating ultrasonic waves in an electrically conducting solid body using a transducer comprising a grid of parallel electric conductors, the conductors of the transducer being located close to and parallel to the surface of the body, and che same alternating electric signs?, being connected to all of the conductors so that ths current flowing therein generates ultrasonic waves in the 'body, the frequency of the alternating signal being related to the spacing between 'the conductors in such a manner that the waves generated by the current in each conductor interfere with the waves generated by the adjacent conductors and results in an ultrasonic beam produced by constructive interference in the required non-normal direction.

2. A method as claimed in Claim 1 in which a magnet applies a magnetic field to the body opposite the grid.

3. Zi method of detecting ultrasonic waves in an electrically conducting solid body using a transducer comprising a grid of parallel electric conductors, the dfc-nduetoxs of tha transducer being located close to and parallel to the surface of tha body, and a magnetic field being applied to the surface of the body so that ultrasonic ( waves in ths body induce eddy currents therein which in turn induce electric currents in the conductors of the transducer, the spacing between the conductors being related to the frequency of the ultrasonic waves to be detected so that only those waves propagating in a particular non-normal direction induce alternating currents in the conductors that are phased to reinforce one another.

4. A method as claimed in any one of the preceding claims in which the grid is such that successive parallel conductors carry the same current in opposite directions. .11.

5. A method as claimed in Claim 4 in which the parallel conductors comprise successive neighbouring sections of a single conductor arranged in a zig-zag path.

6. A method as claimed in Claim 5 in which the single conductor runs once over said zig-zag path.

7. A method as claimed in Claim 5 in which the single conductor runs over substantially the same zig-zag path a plurality of times so that each of said parallel conductors comprises a sub-group of parallel conductors which all carry said alternating current in the same direction.

8. A method as claimed in any one of the preceding claims in which the parallel conductors are formed of wire.

9. A method as claimed in any one of Claims 1 to 7 in which the parallel conductors are formed on a printed circuit board.

10. A method as claimed in any one of the preceding claims in which the parallel conductors are substantially straight.

11. · A method as claimed in any one of Claims 1 to 4 In which each of the parallel conductors consists of a turn of a helically coiled conductor which generates surface acoustic waves in an elongate electrically conducting body which is received within the turns of the coiled conductor.

12. A method as claimed in Claim 11 as dependent on Claim 4 in which the parallel conductors are formed by the turns of two helically coiled conductors that have their respective ends at one end of the coil connected together and their respective ends at the other end of the coil adapted for connection to the alternating current source or detector. .12 43ϋ Οί

13. A method as claimed in any one of the preceding claims in which the parallel conductors are egui-spaoed.

14. A method, as claimed in any one of Claims 1 to 12 in which the spacing between the parallel conductors is non-uniform, the spacing 'between successive conductors decreasing in one direction in such a manner as to bring the beam to a focus at a selected point within the body.

15. , A method of generating ultrasonic waves in an electrically conducting solid body substantially as herein described with reference to Figures 1 to 5 or Figure 6 cr Figure 7 of the accompanying drawings.

16. , A method of detecting ultrasonic waves in an electrically conducting solid body substantially as herein described with reference to Figures 1 to 5 or Figure 3 or Figure 7 cf the accompanying drawings.