IE44575B1 - Ultrasonic inspection - Google Patents

Description

This invention relates to the subject of nondestructive ultrasonic testing of electrically conducting materials, primarily metals, and in particular to means for generating ultrasonic waves in a specimen under test and for detecting reflected ultrasonic waves.

It has been discovered that it is possible to * generate ultrasonic waves 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 strong magnetic field; the ultrasonic waves result from the interaction of the eddy currents, induced by the r.f. field, and the magnetic field.

This has led to an ultrasonic non-destructive testing technique 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 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, even where the r.f. coil is only of the order of a millimetre or less from the surface of the body the echo signal is very small, several orders of magnitude smaller than in the normal piezo-electric direct-contact method. The detecting circuit for the echo signal therefore has to be extremely sensitive.

The present technique in electromagnetic-acoustic testing (e.m.a. for short) is to apply the magnetic field with an electro-magnet and to use a single r.f. 44S7B coil, mounted in the neighbourhood of the poles of the magnet, both to transmit the r.f. signal and to pick up the echo. Experiments have shown that a pancake form is best, i.e. a spiral winding, as it keeps all the turns close to the surface of the body under test: The exact layout in relation to the poles of the magnet will depend on whether shear waves or compression waves are wanted, the production of these being dependent on the direction of the magnetic field in relation to the surface in question. In general the coil lies in the gap between one of the pole-pieces of the magnet and the work.

Using the same coil for transmitting and receiving means that the receiver front-end must be able to withstand without damage the (relatively) enormous transmitted signal yet the receiver must recover its high sensitivity in time to detect the echo. This creates problems in circuit design. It has been proposed to use separate windings, both of pancake form and would on a common axis, with their turns alternating but this still results in considerable coupling between the transmitter and receiver.

It is an aim of the invention to provide a layout suitable for e.m.a. ultrasonic testing of electrically conducting bodies in which the transmitted and received signals are separated as much as possible, and also giving flexibility in operation.

According to the present invention, electro-magnetic means for generating ultrasonic waves in a specimen under test and for detecting reflected ultrasonic waves comprises a magnet for producing a constant magnetic field in the specimen, a transmitting coil which is adapted for connection to a radio frequency transmitter and which is mounted in the neighbourhood of the poles of the magnet to induce in the specimen a radio frequency field that operates with said magnetic field to generate ultrasonic waves, and a receiving coil which is adapted for connection to a receiver and which is mounted in the neighbourhood of the poles of the magnet to receive ultrasonic signals from the specimen corresponding to reflections of said generated ultrasonic waves within the specimen, both coils being wound in a substantially flat form and being laterally spaced apart relative to one another.

Each coil is preferably mounted on its own respective pole-piece, the two pole pieces being movable independently of one another with respect to the pole with which they are associated. For example the pole may have two parallel bores in it, with a respective pole-piece slidable in each. Mounting the coils on separate pole-pieces cuts down very greatly not only the r.f. coupling between them but also, and this is the important thing, the acoustic wave coupling that would otherwise exist between them.

The use of separate transmitting and receiving coils allows the design of each to be optimised for its particular function, i.e. the transmitting coil can be made with a small number of turns of relatively thick wire, enabling a high current pulse to be generated, whereas the receiving coil can be made with a large number of turns to ensure that the maximum possible voltage is generated by the returning ultrasonic echo.

Also the separation of the transmitting and receiving coils in this way means that a large pulse can be transmitted without swamping the receiver. In fact it becomes possible to use a field effect transistor as a preamplifier stage for the receiver without it being destroyed by the transmitted pulse.

Arranging the coils side by side does, it is true, create problems in a layout intended for use with compression waves on strip, rectangular bar or other flat objects, as the echo pulse from the far side of the body, or from lamination type flaws, will go back to the transmitting coil and the signal at the receiving coil, spaced laterally from it, will be very small.

But the invention is primarily applicable to the inspection of round-section rod, bar or tube, in which case, according to a further feature of the invention, the two coils are spaced apart in the direction of the 4457B circumference of the body and are preferably shaped to follow its curvature. The waves emanating from the surface immediately beneath the transmitter coil then bounce back not only to the transmitter but also to the received coil. In fact it can be shown that,· apart from echoes coming from flaws very close to the transmitter, the amplitude of the echo signal picked up at the receiver does not show the normal falling off with distance but is substantially independent of distance at least within a limited range of rod or tube diameter.

.The invention will now be described by way of example with reference to the accompanying drawings in which:Flgure 1 is a section through one embodiment of the invention designed for testing round section bar, Figure 2 is a schematic circuit diagram of the transmitting and receiving coils of Figure 1 together with the associated radio frequency transmitter and receiver, and Figure 3 is a polar plot of the cross-sectional beam pattern obtained in the embodiment of Figures 1 and 2.

The magnet means is an electro-magnet comprising a central cylindrical pole 1, an outer annular pole 2, a ring-shaped yoke 3 joining the two poles 1, 2 at their upper ends, and an energising coil 4 wound around the central pole 1. The lower end of the outer pole 2 is shaped so as to match the curvature of a cylindrical bar 5 to be tested. The central pole 1 is formed with 44»? » •5 two cylindrical holes 6 through it with axes parallel to the axis of the pole and egui-spaced on opposite sides of the pole axis so that they lie in a plane perpendicular to the axis of bar 5. The two holes 6 are then spaced apart around. the circumference of the bar. Each hole 6 contains a cylindrical rod-like polepiece 7 which carries on its lower end a substantially flat spirally wound coil 8, the lower end of each polepiece and the coil itself both being shaped to the Ιθ curvature of the surface of the bar 5. The pole-pieces 7 aremovable independently of one another in the holes 6 to allow easy and accurate setting of the positions of the respective coils 8 in relation to the bar 5, and each can be withdrawn completely, together with its coil. A single screw (not shown) clamps each polepiece in place.

Each coil 8 is surrounded by a copper screen 9 which is slotted to its full depth to reduce eddy currents.

The outer end of each coil is connected to its associated screen 9. Further, each coil is connected via a co-axial 2o connecting lead 10 to a pair of connecting terminals 11 at Its upper end, the lead 10 passing through an axial hole 12 in the pole-piece 7 and having an outer braided copper cover 13 to reduce eddy currents. The coils 8 may have ceramic facings and backings, this being 25 necessary when testing rod 5 at a high temperature.

The design of the individual coils 8 is different, one being designed as a transmitting coil for connection to a transmitter 14 which supplies a radio frequency signal, and the other being designed as a receiving coil 2q for connection to a receiver comprising a pre-amplifier 15 and amplifier 16. The transmitting coil has just a few turns of relatively thick wire to suit a high current pulse from the transmitter, whereas the receiving coil has a large number of turns of thinner wire to maximise the voltage generated by the reflected ultrasonic pulse from the bar 5. Typically, the transmitting coil has an outer diameter of 16 m.m. and consists of 6¾ turns .6. 44878 of 22 S.W.G. enamelled copper wire, and the receiving coil has an outer diameter of 22 m.m. and consists of many closely wound turns of 43 S.W.G. enamelled copper wire.

The two coils 8 are tuned to their operating frequency by capacitors 17. For a typical operating frequency of 2 MHz using the coils specified above, a total capacitance of 11,000 pf is provided across the transmitting coil using high voltage ceramic capacitors, and a total capacitance of 100 pf is provided across the receiving coil using a mica capacitor. A 10 Kohm resistor in the pre-amplifier 15 damps the response of the receiving coil to give a 2 MHz band width with a centre frequency of 2 MHz.

The transmitter 14 consists of a charging circuit which charges a pulse capacitor, and a trigger circuit which periodically discharges the capacitor through the transmitting coil. The capacitor typically has a capacitance of 9,000 pf and is charged to a voltage of 6 KV. The trigger circuit may comprise a tetrode thyratron triggered from a lumped constant delay line operated by a silicon controlled rectifier. Typically^, the capacitor is discharged at a pulse repetition frequency of 110 Hz, The pre-amplifier 15 is of the FET type comprising a FET first stage with clipping diodes to protect it from the transmitted pulse, which despite the screens 9, can still induce up to 1 KV across the receiving coil.

A current-limiting resistor is connected in series with the receiving coil and the 10 Kohm clamping resistor is connected between the gate and ground terminals of the FET. This FET is part of a feedback pair which typically is tuned to have a frequency response from 0.3 MH to 3.5 MHz at the 3 db points and a high rate of roll off. A wide band feedback pair follows this and feeds via a 75 ohm co-axial cable to the main amplifier 16. The mid-band gain of the pre-amplifier is 38 db. 457 3 The main amplifier 16 is based on wide-band feedback pairs 18 and the frequency response is controlled by a third order Butterworth band-pass filter 19 typically with a frequency response from 0.9 MHz to 7.4 MHz at the 3 db points and a 60 db/decade roll off. The mid-band gain is 62 φ and a 0-40 db attenuator 20 between the first two stages controls the gain.

All time constants in both amplifier 15 and 16 are kept as' short as possible in order to reduce the time during which the receiver is swamped by the transmitted pulse.

The main amplifier 16 has a detector circuit 21 which produces an output signal which is used to produce a visual display in any Of the known display systems, for example Ά* scan, the form normally presented on a cathode ray oscillograph; 'B' scan, which presents an ultrasonic cross-section of the bar under examination or 'polar', which indicates the variation in Ultrasonic reflectivity of defects in the bar with respect to their 2o angular positions. A combination of these displays enables the location arid nature of flaws to be determined visually with the minimum of skill.

Typically, the coils 8 are positioned 2 m.m. from the surface of the bar. The strength of the reflected signals falls of at a rate of 8 db/m.m. for clearances of up to 4 rum. from the bar with the coils described above operating at a frequency of 2 MHz.

Polar plots of the cross-sectional beam pattern are shown in Figure 3, the solid and broken line plots representing contours of uniform ultrasonic intensity for 1.4 and 2 MHz beams, respectively, as measured at the 3db points, reveal that the transmitted and reflected ultrasonic beams are narrow and overlap with the result that attenuation of the reflected beam with distance is substantially cancelled out. For a given defect, therefore, the same peak echo signal will be obtained along 8. the maximum^intensity line of all but the first fifth of an Ά' scan display. Resolution is such that a defect 5 mm. in front of the back face of bar 5 will be separated from the back echo on a PPI scan which also traces the bar outline showing surface flats.

The width of the beam may be changed by varying the ratio of pole piece diameter to the ultrasonic frequency.

Because the intensity distribution in front of the transmitting coil is not uniform a double-lobed beam results. This effect cannot be eliminated entirely but since it occurs over a known region and produces lobes on the polar scan whose included angle is accurately defined it is not a major problem.

At the centre of the transmitting coil there will 15 be no horizontal component of the radio frequency field and hence along its axis there exists an intensity 'null'. Similarly for the receiving coil, any signal incident at its centre cannot induce a voltage in the coil. Due to scatter and beam overlap this effect only appears to any 2o marked degree on the far side of the bar over a narrow range, and decreases with lowering of frequency.

A triple peak occurs along the length of the bar, but the intensity of the two side peaks is some 20 db down and so can be disregarded.

If the surface of the bar is covered with scale which contains magnetite then magnetostrictive generation and reception of ultrasound takes place which can give up to 30 db increase in signal strength. The increase in signal depends on scale thickness, percentage magnetite and field strength and will vary around the bar if these properties are not constant. On simple bars these properties are reasonably constant but variations in the signal due to scale thickness variation may be overcome by monitoring back surface echo.

The operating frequency quoted above is 2 MHz, which is currently the most popular frequency for conventional testing using longitudinal waves. However, the ultrasonic waves produced in bar 5 are shear waves 44875 rather than longitudinal waves, this being due to the orientation of the magnetic field perpendicular to the surface of the bar which is easier to achieve than to arrange the magnetic field parallel to the surface of the bar for the production of longitudinal waves.

Shear waves travel at approximately half the velocity of longitudinal waves and thus the wavelength of the transverse waves is only about a half that of the longitudinal waves in conventional ultrasonic j_0 testing systems. Reducing the operating frequency increases the wavelength and is found to produce a wider beam, an increased signal level and reduced attenuation with distance. The Operating frequency can be decreased by increasing the capacitance of capacitors 16. In this way an operating frequency of 1.4 MHZ can be obtained which gives an increase of 2 db in the signal level and causes 6 back echoes to appear on the display instead of 4 at 2 MHz due to less attenuation with distance. An operating frequency of 1 MHz is best obtained by replacing the 2MHz transmitting coil with one of higher inductance. 44Β7Ι»

Claims (17)

1. CIAIMS:

1. Electro-magnetic means for generating ultrasonic waves in a specimen under test and for detecting reflected ultrasonic waves comprising a magnet for producing a constant magnetic field in the specimen, a transmitting coil which is adapted for connection to a radio frequency transmitter and is mounted in the neighbourhood of the poles of the magnet to induce in the specimen a radio frequency field that co-operates with said magnetic field to generate ultrasonic waves, and a receiving coil which is adapted for connection to a receiver and is mounted in the neighborhood of the poles of the magnet to receive ultrasonic signals from the Specimen corresponding to reflections of said generated ultrasonic waves within the specimen, both coils being wound in a substantially flat form and being laterally spaced apart relative to one another.

2. Means as claimed in Claim 1 in which both coils are mounted closely adjacent to the same pole of the magnet which produces a magnetic field substantially normal to the specimen surface.

3. Means as claimed in Claim 2 in which the magnet comprises a central pole and an outer pole which surrounds the central pole, the ends of both poles being adapted to lie closely adjacent the specimen surface and the coils being mounted closely adjacent to the end of the central pole.

4. Means as claimed in Claim 3 in which the ends of the outer pole is shaped to match a curved outer surface of the specimen.

5. Means as claimed in Claim 4 in which the end of the inner pole is shaped to match said curved outer surface of the specimen. 11 44B’« 5

6. Means as claimed in Claim 4 or 5 in which the coils are spaced apart laterally around the circumference of the surface of curvature of the specimen.

7. Means as claimed in any one of Claims 2 to 5 in which the coils are secured to the end of said same pole and lie in the plane of the end surface.

8. Means as claimed in Claim 7 in which said same pole comprises two pole-pieces and in which a respective one of said coils is mounted on the end surface of each pole-piece.

9. Means as claimed in Claim 8 in which the two polepieces are each separately movable relative to a fixed portion of the pole.

10. O Means as claimed in Claim 9 in which each pole-piece is movable longitudinally in a respective hole through the fixed portion of the pole so as to move the respective coil on the end of the pole-piece towards and away from the specimen.

11. Means as claimed in any one of Claims 8 to 10 in which each coil is surrounded at its outer edge by an electrical screen which is mounted on the respective pole-piece and connected to the outer end of the coil.

12. Means as claimed in any one of Claims 8 to 11 in which each coil is connected to connection terminals at the opposite end of the respective pole-piece by a co-axial cable which passes -through a hole in the pole'-piece.

13. Means as claimed in Claim 12 in which the cable has a screen applied around its outer surface.

14. Means as claimed in any one of the preceding claims in which the transmitting coil comprises a few turns of relatively thick wire and the receiving coil comprises a much larger number of turns of relatively thin wire. -.12 »457»

15. Means as claimed in any one of the preceding ciat ma which further comprises a receiver which is connected to the receiving coil and comprises a FET pre-amplifier.

16. Means as claimed in any one of the preceding claims 5 which further comprises a radio frequency transmitter which is connected to the transmitting coil.

17. Electro-magnetic means for generating ultrasonic waves in a specimen under test and for detecting reflected ultrasonic waves substantially as herein described with 10 reference to the accompanying drawings. Dated this the 23rd day of December, 1976. F. R. KELLY & CO.