GB2106247A - Ultrasonic testing of cylindrical test piece - Google Patents

Abstract

An ultrasonic probe comprises a mount 24 with inclined side surfaces, and a curved base surface for application to a cylindrical test piece. Transducers 14, 16 on the side surfaces of the mount direct signals towards the test piece, the principal axes of the signals intersecting points 18, 19 on a line 20 lying in the base surface and extending parallel to the axis of curvature of this surface. <IMAGE>

Description

SPECIFICATION Improvements in the ultrasonic testing of cylindrical test pieces The ultrasonic testing of cylindrical test pieces, and in particular pipes, is known. For convenience, all such test pieces will be referred to herein as pipes. Testing may be carried out using a probe which is applied to the outer surface of the pipe. A signal, in the form of transverse ultrasonic waves, emitted by a transducer incorporated in the probe is directed into the pipe and undergoes multiple internal reflections therein so that the signal follows a zig zag course around the pipe wall.

Echoes received back by the transducer indicate the presence of flaws in the pipe. A probe is also known from Krautkramer, Ultrasonic Testing of Materials, 2nd Edition, at Page 442 in which the probe incorporates a pair of transducers, each of which directs a respective signal into the test piece. One signal travels around the pipe in the clockwise direction, while the other travels counterclockwise. Both signals impinge with equal but opposite angles of incidence at a location diametrically opposite to the probe and provide reference echoes from this location. An advantage of a probe of this type is that a reference echo is continually available for monitoring the coupling between the probe and the pipe. A disadvantage of this design, however, is that a separate probe must be constructed for each combination of pipe diameter and thickness which may need to be tested.Krautkramer supra, also described at page 44, a probe which can adapt to different pipe diameters. A disadvantage of this arrangement is the extensive region which is located between the two transducers of the probe and which cannot itself be tested, the excessive size of the probe itself, and difficulty which is encountered in manipulating it.

The probe described hereinafter is of the type which includes a pair of transducers but has several advantages compared with the previously proposed probes of this type. The new probe may be used with pipes of different diameters since, in use, ultrasonic signals are emitted which impinge on the pipe at constant angles of incidence irrespective of the diameter of the pipe. Echoes received from a location diametrically opposite to the probe are used as reference echoes and allow the continuous monitoring of the probe/pipe coupling during testing.

Briefly, the probe proposed herein comprises two separate transducers carried by a support which has a curved base surface for application to the pipe. The support may be in the form of a single mount which supports both transducers, or in the form of two separate mounts. The transducers are arranged in respective planes so inclined that the principal axes of ultrasonic signals emitted by them are directed at a single straight line disposed on the base surface and extending parallel to the axis of curvature of this surface. The principal axes of the ultrasonic signals intercept this line at axially spaced points. In use, the signals from both transducers enterthe pipe along this line.The angles of incidence and refraction of the signals as they pass from the probe to the pipe remain constant, regardless of whether the diameter of the pipe corresponds to that of the probe base surface or is less than the diameter of the probe base surface. Of course, the probe may be used with test pieces other than pipes, and it may therefore be used to test the walls of cylinders, tubes and rods for example.

In the drawings: Figure 1 is a diagrammatic cross-section through a pipe under test using a known probe of the type described in Krautkramer, supra, Figure 2 is a section through a pipe under test using a probe in accordance with the present proposal, Figure 3 is a plan view of the probe shown in Figure 2, Figure 4 is a perspective exploded view of a probe similar to that shown in Figure 2, and Figure 5 is a perspective view of a second embodiment of probe.

Figure 1 depicts at 1 the probe illustrated on page 442 of Krautkramer supra, for testing a pipe 4. The probe includes a pair of transducers 2 and 3 which transmit respective ultrasonic signals 5 and 6 into the pipe 4 and receive back echoes which are converted into electrical pulses for display purposes. The signal 5 from the transducer 2 travels around the wall of the pipe in the clockwise direction, whereas the signal 6 from the transducer 3 travels around the wall of the pipe in the counterclockwise direction. Both signals undergo multiple internal reflections and therefore follow a generally zig zag course within the pipe wall. The transducers are arranged at a distance from each other such that both signals are reflected within the pipe from a location on the pipe outer surface diametrically opposite to the probe.The Figure also illustrates the display on the screen of a cathode ray tube of various pulses received back by both transducers during testing, including the input pulse 7 received as the signals from both transducers enter the pipe and the echo 8 received from the location diametrically opposite to the probe mentioned above. The echo 8 is used as a reference echo and serves the same purpose as a back wall echo in the testing of flat specimens, i.e. to show that the probe is operating correctly and that it is coupled with the pipe, even if no flaw echoes are present.

It will be appreciated that the reference echoes caused by both signals are received back by the respective transducers after equal lapses of time.

Consequently, two echoes 10 and 11 are received from a flaw 9 and appear to be displayed symmetically relative to the reference echo 8.

Referring now to Figures 2 and 3, a probe in accordance with the present proposal includes a housing 12, a mount 13 bearing a first transducer 14 and a mount 1 5 bearing a second transducer 1 6. The mounts may be separate components arranged adjacent to each other or fixed together, or a single component. The mounts may be made from any suitable material, for example plexiglass.

The base surface 17 of the probe, formed by the undersurfaces of the mounts or the undersurface of the housing if the latter encloses the mounts on the underside, has a radius of curvature conforming to that of the largest diameter of pipe which it is expected to test, so that its axis 14 will coincide with the axis of this pipe. The centres of the active surfaces of the transducers 14 and 16 are spaced apart considered in the direction parallel to the axis 40 so that the principal axes of the ultrasound beams or waves intersect the base surface 17 at axially spaced points 18, 19. More particularly, the projections of the transducers onto a plane which includes the axis 40 and with respect to which the transducers are disposed symmetrically, do not overlap.As illustrated, the transverse edges of the transducers 141, 161 as projected onto this plane are separated by a distance "a". The faces of the mounts 1 3 and 1 5 opposite to their undersides are bevelled or oblique so as to have inclined plantar portions on which the transducers are fixed. The angles of inclination of these surfaces of the mounts relative to the reference plane passing through the axis 40 are equal and opposite. Consequently, the principal axes of the ultrasound beams or waves where they intersect the base surface 1 7 at axially spaced points 1 8 and 1 9 have equal but opposite angles of incidence 21 and 22 on the base surface, and hence the pipe, measured relative to the reference plane.Also, the points 1 8 and 1 9 lie on a single staight line 20 parallel to the axis 40, and are on the respective apex straight lines when the probe base is curved and adapted to the pipe surface. The angle 21 and angle 22 may, for example, each be equal to 36 . It will be appreciated that these angles also represent the angles of incidence on the pipe itself, and that they do not alter if the probe is used on a pipe having a lesser diameter from that of the base surface, so that a reliable and exact coupling remains possible. The angles of incidence also remain constant if, according to manufacturing specifications, the base surface 17 is reground to adapt the probe to a pipe of larger diameter.

The principal angles of refraction of the ultrasound beams or waves as they pass into the pipe, as indicated at 211 and 221, are also equal.

and likewise remain constant regardless of the diameter of the pipe.

The distance "a" between the adjacent transverse edges 141 and 161 of the transducers as projected onto the reference plane, the dimensions of the transducers, the delay distances 142 and 1 62 (i.e. the distances between the transducers and the points 18 and 1 9) and the angles of incidence 21 and 22 of the principal axes of the sound waves are so chosen that, on account of the divergence of the waves reaching the pipe, when the transducer 14 is energised, the reference echo can be received by the transducer 16. The same requirements apply to the energisation of the transducer 1 6 and the reception of the reference echo by the transducer 14.

It has been found that the distance "a" between the projection onto the reference plane of the edges 141 and 161 of the transducers should be between about 2 to about 5 mm. The delay distances 142 and 1 62 should be substantially equal to the near field length of the transducers in order to reduce the interfering echoes produced in the probe by reflection from the base surface 1 7.

In one practical embodiment, use was made of transducers 14 and 1 6 emitting ultrasonic waves having a frequency of 2 MHz and each having a square active surface with an area of 225 mm2.

The transducers had plexiglass mounts with the wedge shape illustrated, so dimensioned as to provide a delay length of substantially 56 mm, longitudinal waves being excited in the test piece.

The angles of incidence 21 and 22 were so chosed that the angles 211 and 221 were 370C in one steel pipe, and 45 in a second steel pipe. The probe was successfully used with pipes having diameters between 100 and 400 mm. The signals from both transducers impinged with equal and opposite angles of incidence on a location diametrically opposite to the probe and so provided a continuous reference echo, comparable to the echo 8 shown in Figure 1. The reference echo received from all pipes within this range was of adequate amplitude.

Separate mounts for the two transducers may be employed as shown in an exploded view in Figure 4. Possibly, a gap may be present between the mounts in use, the mounts being held in place by the housing. Alternatively, a single mount 24 may be used as shown in Figure 5, the mount having a constant generally truncated triangular cross-section throughout its length, with a transducer carried by each inclined side surface so arranged that the points 1 8 and 1 9 at which the predetermined axes of the ultrasound intersect the base, are separated by a distance 23.

The housing need not enclose the base surface of the mount or mounts. Transducers of other shapes may be employed, the distance "a" then being determined as the shotest distance between the projection onto the reference plane of lines perpendicular to this plane and tangential to the transducers.

Claims (11)

1. A method of ultrasonicaliy testing a test piece, particularly in the form of a cylinder, pipe or rod, wherein there is applied to the test piece a probe including two transducers arranged in respective planes such that the principal axes of ultrasonic signals emitted by the transducers and directed towards the test piece intersect a common straight line extending parallel to the axis of the test piece and impinge after multiple internal reflection within the test piece at a location diametrically opposite from the probe; the angle of incidence of the signal from one transducer being equal and opposite to that of the signal from the other transducer, and the projected inter-transducer spacing "a", the dimensions of the transducers, the delay distances, and the angle of incidence of the ultrasonic signals on the test piece of said line are such that, as a result of the divergence of the ultrasonic signals reaching the test piece, each transducer is capable of receiving from the test piece a reference echo of the signal transmitted by the other transducer.

2. A method according to claim 1 wherein the distance "a" is between about 2 and about 5 mm.

3. A method according to claim 1 or claim 2, wherein the delay distance is approximately equal to the near field length of the transducers.

4. A method according to any preceding claim, wherein the transducers generate ultrasonic waves having a frequency of 2 MHz and have a square active surface with an area of 225 mm2 the angle of incidence of the signal directed by each transducer to the test piece impinging thereon such that the angle of refraction in steel is 37O or 45ops

5. A method according to any preceding claim, wherein the probe has a base surface conformed to the surface curvature of the test piece and the straight line is an apex line.

6. An ultrasonic test probe comprising two separate transducers carried by a mount, or by respective mounts having a concavely curved base surface, the transducers being arranged in respective planes such that the principal axes of the ultrasonic signals emitted by the transducers are directed at axially spaced points on a straight line on the curved base surface and extending parallel to the axis of curvature thereof.

7. A probe as claimed in claim 6, wherein the transducers are carried on a plexiglass mount.

8. A probe according to claim 6, wherein two conventional angle probes are disposed one behind the other considered in the direction parallel to said straight line.

9. A probe according to any of claims 6 to 8, wherein the transducers are adapted to generate ultrasonic waves having a frequency of 2 MHz, and each transducer has an active service with an area of 225 mm2.

10. A method substantially as hereinbefore described with reference to the drawings.

11. A probe substantially as hereinbefore described with reference to and as illustrated in the drawings.