WO1980002074A1

WO1980002074A1 - Ultrasonic probe

Info

Publication number: WO1980002074A1
Application number: PCT/GB1980/000053
Authority: WO
Inventors: J Dickson
Original assignee: Balteau Sonatest Ltd; J Dickson
Priority date: 1979-03-23
Filing date: 1980-03-24
Publication date: 1980-10-02
Also published as: GB2044929B; BE882394A; EP0046747A1; GB2044929A

Abstract

A wheel probe will detect flaws in workpieces over which it travels without the use of a coupling medium between the tyre and the test material. The tyre (20) is filled with oil under pressure and houses a transducer (14). When pressed against the test material the tyre elongates to bring the internal surface of the tyre into acoustically coupled contact with the transducer. The transducer (14) is biassed towards the internal surface of the tyre (20).

Description

ULTRASONIC PROBE

DESCRIPTION

This invention relates generally to ultrasonic probes, and is concerned with wheel probes which incorporate a rolling element to traverse a material under test.

Conventionally, when using wheel probes to detect flaws in workpieces over which the probe travels, one uses a coupling fluid on the surface of the workpiece to provide the necessary acoustic coupling between the workpiece and the tyre, i.e. the rolling element, of the probe. However, with certain materials it is not possible to use an oily coupling medium on the surface of the material. This is the case for example when testing for faults in friction materials, such as for brake linings, where one is testing for delaminations, unresinated areas and faulty bonding of the friction material to its backing. A further problem which arises from the use of a liquid coupling medium between the probe and test object is the difficulty of maintaining the thickness of the liquid layer constant. During testing it is necessary to maintain the liquid layer at a constant thickness and as thin as possible in order to obtain reasonably consistent results.

It is an object of the present invention to provide a wheel probe which does not require the use of a coupling medium between the surface of the rollingelement and the material under test.

In accordance with the present invention there is provided an ultrasonic probe arranged to traverse a material undertest with rolling contact therebetween, the probe comprising a rotatable rolling element which is filled with an acoustic coupling medium, and trans ducer means within said rolling element, the rolling element being arranged to deform in shape when pressed against a material under test, thereby to bring the internal surface of the rolling element into acoustically coupled contact with a face of the transducer means.

The rolling element is preferably a tyre with the transducer means mounted within the contour of the tyre so that there is an acoustically good interface between the transducer means and the internal surface of the tyre as the probe traverses the material under test.

There is thus provided what may be referred to as a "bellows" probe, in which the contour of the rolling surface is designed to change when the wheel is pressed into contact with a material under test, this change in shape, in a preferred embodiment, both causing an axial elongation of the wheel and also causing the internal surface of the tyre to contact a face of the transducer means.

Preferably, the rolling element is filled with a thin oil at a pressure of about 14 kN/m².

Preferably, the transducer means is spring-urged towards the internal surface of the rolling element to provide a loading of said face of the transducer means against the rolling element. One embodiment of probe in accordance with the invention will now be described by way of example and with reference to the accompanying drawing, in which: Fig. 1 is a side elevation of the probe; and, Fig. 2 is a front elevation of the probe. As shown in the drawing, the wheel probe comprises a centre axle 10 which carries a housing 12 for a transducer 14 which includes a piezoelectric crystal. The emitting/receiving face of the crystal is positioned at the underside of the transducer 14. The transducer housing 12 is spring-loaded in a downward direction away from the axle 10 with a suitable constant spring force. One end of the axle 10 is journalled in a downwardly extending leg of a connection box 16 into which passes a coaxial cable 18 for taking signals to and from the probe transducer.

The rolling element of the probe is a tyre 20 which is hollow and comprises a membrane which at rest has a circumferential surface which is curved convexly. The tyre 20 may be a moulded element of rubber or equivalent material which is resilient and deformable. One side of the tyre is fitted with a sealing disc 22, and axially outwardly of this disc there is a non- friction bearing 24 which permits rotation around the centre axle 10. The other side of the tyre 20 likewise is fitted with a sealing disc 26 which has fitted into it a non-friction bearing seal 28. This non- friction bearing seal is designed so that it will still provide a seal between the membrane of the tyre and the axle even when the bearing seal moves axially along the length of the axle 10.

When the wheel probe is assembled, the hollow interior of the tyre membrane is filled with an acoustic coupling medium, such as a very thin oil, to a pressure of about 14 kN/m². When the wheel probe is pressed into contact with a material under test,or when a load is otherwise applied to the underside of the tyre, the membrane deforms under this pressure so that the surface of the tyre which is in contact with the test material forms a flat region beneath the transducer, as indicated by the broken line in the drawing. At the same time, the bearing seal 28 is displaced axially outwards along the axle 10 due to the resilience and deformability of the tyre material. As a result, the face of the transducer 14 which carries the piezoelectric crystal comes into contact with the internal surface of the membrane. The pressure of the oil within the tyre, taken in conjunction with the bias force on the transducer, is a critical factor. If the pressure is too high, one does not achieve the proper displacement, i.e. there is an excessive and possibly irregular thickness of oil between transducer and tyre, and side reflections can arise. If the pressure is too low, then there will be insufficient acoustic coupling and also the generation of mechanical noise. The spring loading of the transducer housing maintains a constant loading of the crystal face against the tyre membrane on the one hand, while on the other hand the couplant which fills the tyre acts as a load bearing between the crystal and the internal surface of the membrane to maintain a constant acoustic coupling between these two surfaces. Because of the spring pressure and the couplant pressure there is a constantly maintained film of coupling fluid between the crystal and the membrane, thus ensuring a constant path length for the ultrasonic waves while the wheel probe is in motion. This constant path length acts as a near field stand-off, thus eliminating any reflection echoes from the internal surface of the membrane. The membrane thickness then acts as the transducer stand-off. One thus achieves a constant thickness stand-off in good acoustic contact with the crystal face by virtue of the film of couplant between the membrane and the transducer. The deformation of the tyre whereby the membrane surface is brought into contact under load with the transducer ensures that one does not have a varying thickness of fluid between the internal surface of the membrane and the crystal face.

In one particular embodiment of wheel probe as described above, with a tyre of 25 mm diameter and a tyre membrane having a thickness of 0.635 mm, good results have been achieved when the transducer housing is spring loaded with a fixed loading of 225 grams and the oil pressure within the tyre is 14 kN/m².

Although in the example illustrated in the drawing the axial displacement of the tyre, i.e. the bellowseffect, is to one side of the tyre only, one could arrange for the tyre to deform in both axial directions by the use of appropriate bearing seals. Again, the acoustic couplant within the tyre and the pressure under which it is loaded will vary depending upon the particular application and mode of use of the probe. Furthermore, although the probe is shown as having only a single transducer within the tyre, the invention is to be understood as extending also to the use of more than one transducer, for example arranged side-by-side.

The rolling element probe of the present invention can be used at relatively high speeds of rotation on a variety of surfaces. It is particularly suited to the detection of laminations within various types of material without the use of any coupling liquid between the probe and the material under test.

Claims

CLAIMS :

1. An ultrasonic probe arranged to traverse a material under test with rolling contact therebetween the probe comprising a rotatable rolling element and transducer means within said rolling element, characterised in that the rotatable rolling element is filled with an acoustic coupling medium and is arranged to deform in shape when pressed against a material under test, thereby to bring the internal surface of the rolling element into acoustically coupled contact with a face of the transducer means.

2. A probe as claimed in claim 1, characterised in that the rolling element is a tyre with said transducer means mounted within the contour of the tyre.

3. A probe as claimed in claim 1 or 2,characterised in that the rolling element is arranged to elongate axially along its axis of rotation under pressure.

4. A probe as claimed in claim 3, characterised in that the rolling element is arranged to elongate in only one axial direction under pressure.

5. A probe as claimed in any preceding claim, characterised in that the acoustic coupling medium is an oil at a pressure of the order of 14 kN/m².

6. A probe as claimed in any preceding claim, characterised in that the transducer means is biassed towards the internal surface of the rolling element to provide a loading of said face of the transducer means against the rolling element.

7. A probe as claimed in any preceding claim, characterised in that the rolling element comprises a tyre having a convex circumferential surface, and sealing and bearing means at each side of the tyre, said bearing means being co-operable with an axle extending centrally through the tyre.

8. A probe as claimed in claim 6, characterised in that the pressure of the acoustic coupling medium and the biassing pressure, are such that. a. film of coupling medium is maintained between the rolling element and said face of the transducer means, said film providing a constant path length for ultrasonic waves when the probe is in rolling motion.