WO2005057205A1

WO2005057205A1 - Ultrasound probe with deformable elastomeric coupling element

Info

Publication number: WO2005057205A1
Application number: PCT/GB2004/005180
Authority: WO
Inventors: Bruce Walton Drinkwater; Corin James Brotherhood
Original assignee: The University Of Bristol; Ndt Solutions Ltd
Priority date: 2003-12-12
Filing date: 2004-12-10
Publication date: 2005-06-23
Also published as: GB2409039A; GB0328899D0

Abstract

An ultrasound probe for use in non-destructive testing, and in particular for testing of spot welds, comprises an ultrasonic transducer and an ultrasonic coupling element. The coupling element comprises a substantially solid piece of deformable elastomeric material having low ultrasonic attenuation. The deformable coupling element conforms to any surface irregularities in the material under test.

Description

ULTRASOUND PROBE WITH DEFORMABLE ELASTOMERIC COUPLING ELEMENT

The present invention relates to an ultrasound probe for non-destructive investigation of materials and also to a coupling element for use with such a probe.

The standard ultrasonic testing involves the coupling of an ultrasonic transducer to a test structure via a coupling medium, which is commonly a liquid such as water or a coupling gel. The coupling medium allows the transmission of useful ultrasonic energy to and from the test structure. The coupling medium is required because of the large acoustic impedance mismatch between air and solid materials, which means that ultrasound is almost totally reflected at both a transducer-air boundary and at an air-test structure boundary. The coupling medium has a much greater acoustic impedance than air and so allows greatly increased transmission to and from the test structure.

The two most common ultrasonic testing procedures are immersion testing and manual inspection with contact transducers. In immersion testing the structure is fully immersed in a water bath, allowing coupling between the structure and the transducer via the water. The transducer can then be scanned over the structure with consistent coupling being maintained. This is clearly not practical for large structures or inspections in the field. Contact transducers, in which a layer of gel or grease is applied to the test structure to provide coupling, are useful for on-site inspection and the testing of awkward shapes. They are designed to make a series of point measurements, which can be done either manually or robotically, to build up an image of the structure under test.

One such application of contact transducers is in the testing of spot welds and adhesive joints in the automotive industry. The currently predominant method of manufacture of vehicles involves extensive use of spot welds to bond two layers of steel body panels together. Spot welds are used in preference to seam welding primarily because of the time savings involved in the manufacturing process. To maintain quality, periodic tests are made on representative samples to determine the quality of the spot welds being performed. To enable coupling into the irregular surface profile of a spot weld the current practise is to utilise a flexible membrane sack is attached to an ultrasonic sensor, with the membrane sack being filled with a liquid coupling medium, such as water. Although this provides a deformable contact point that allows the sensor to conform to any surface irregularities present at the test zone, this arrangement exhibits the major disadvantage that the liquid filled membrane sack regularly splits or tears on such irregularities, which in the case of spot welds are frequently formed of sharp metal sputters, and hence have to be regularly replaced. This means that the testing process must be conducted manually and is inevitably quite time consuming and therefore costly. An illustrative example of such a liquid filled membrane sack sensor is shown in Figure 1.

According to a first aspect of the present invention there is provided an ultrasound probe comprising an ultrasonic transducer and an ultrasonic coupling element, wherein the coupling element comprises a substantially deformable elastomeric polymer, such that in use the coupling element conforms to surface irregularities in a material under test.

The coupling element is preferably formed from an elastomeric polymer exhibiting low ultrasonic attenuation, typically less than 500 dB/m at 5MHz and 1000 dB/m at 10 MHz. The elastomeric polymer may be rubber based, although other elastomers having appropriate mechanical and ultrasonic properties may equally well suffice.

The use of an elastomeric polymer having low attenuation properties means that good ultrasonic coupling can be achieved between the transducer and the test piece without the use of any further coupling medium, such as gels or water. Furthermore, as the elastomer is substantially deformable, the probe will conform to any surface irregularities in the same manner as the prior art liquid filled membrane sack probes do, but without splitting or tearing.

The coupling element is preferably restrained in engagement with the ultrasonic transducer by means of a restraining collar that defines a recess adjacent to the ultrasonic transducer in which the coupling element is constrained. The restraining collar is preferably removably attached to the ultrasonic transducer to allow the replacement of the coupling element. The recess defined by the restraining collar is preferably dimensioned so as to provide free space for the coupling medium to deform into when in use. By providing free space within the constraining collar, the coupling element is free to deform without any constraint, thus imposing no constraints on the ability of the coupling element to conform to surface irregularities in the test piece. The probe may further comprise a restraining sleeve formed around the inner surface of the restraining collar and frictionally engaged with the coupling element, thereby holding the coupling element within the collar. The restraining sleeve may be constructed from an ultrasonically 'lossy' material such that unwanted internal reflections generated in the coupling element are diminished.

The ultrasonic coupling element preferably has a curved surface that, in use, contacts the surface of the material under inspection. More preferably the coupling element is substantially spherical.

In an alternative embodiment the ultrasonic coupling element is preferably substantially conical. In this embodiment, the restraining collar is preferably substantially frustoconical and substantially conforms to the outer surface of the coupling element over a substantial portion of the conical coupling element. A portion of the conical coupling element at the tip of the cone is preferably outside the restraining collar.

The conical shape of the coupling element in this embodiment enables inspection in difficult to access areas, for example around the neck and base of plastic bottles, whilst the conformable rubber tip provides good acoustic coupling on irregular surfaces. By providing the restraining collar in a corresponding frusto conical shape that conforms substantially to the outer surface of the coupling element, the restraining collar provides support to the coupling element over a substantial portion of its length, thus avoiding excessive and undesirable deformation of the coupling element when brought into contact with an object to be tested.

The conical coupling element preferably has at least one, and more preferably a series of, irregularities formed on its outer surface. These irregularities substantially reduce, and preferably eliminate, unwanted internal reflections within the probe assembly by scattering the ultrasonic waves that strike the side walls of the coupling element. The irregularities may comprise one or more ridges or depressions on the surface of the coupling element. One or both of the embodiments described above may additionally include an additional coupling medium provided at the interface between the ultrasonic transducer and the ultrasonic coupling element.

According to a second aspect of the present invention there is provided an ultrasonic probe comprising an ultrasonic transducer and an ultrasonic coupling element, wherein the coupling element comprises a deformable elastomeric polymer and is substantially spherical.

The spherical coupling element is preferably solid.

Embodiments of the present invention will now be described, by way of illustrative example only, with reference to the accompanying figures, of which:

Figure 1 schematically illustrates a liquid filled membrane sack transducer as known from the prior art;

Figure 2a schematically illustrates an ultrasound probe according to a first embodiment of the present invention;

Figure 2b schematically illustrates an alternative arrangement for a probe according to the present invention;

Figure 3 schematically illustrates an ultrasound probe according to a second embodiment of the present invention;

Figure 4a schematically illustrates a detailed view of the conical coupling element of the ultrasound probe shown in Figure 4;

Figure 4b schematically illustrates a detailed view of an alternative embodiment of the conical coupling element shown in Figure 4a; and Figure 5 is an illustrative graphical representation of an output signal from an ultrasound probe over a given period of time.

Figure 1 schematically illustrates a known prior art ultrasound probe. A flexible membrane 1 formed from latex or other similar material is fastened to an ultrasonic transducer 2 so as to form a flexible sack over the transducer face. A flange 3 may be provided around the periphery of the transducer 2 to assist in the fastening of the membrane 1 to the transducer. The sack formed by the membrane 1 is filled with water 4 or other predominantly fluid coupling medium. In use, the liquid filled membrane sack is brought into contact with a surface of a test piece 5, which in automotive applications is commonly formed from two separate layers of steel 6, 7 that have been bonded together by means of a spot weld 8. As illustrated in Figure 1, it is common place for the surface of the upper sheet of steel 6 to be deformed in the region of the weld 8. The flexible membrane 1 and its liquid contents readily deform to accommodate this surface irregularity. However, as previously mentioned, sharp protrusions formed during the spot welding process often tear or puncture the flexible membrane 1 , causing an operator to interrupt the testing process to replace and refill the membrane.

Figure 2a shows a first embodiment of an ultrasound probe according to the present invention. The probe consists of an ultrasonic transducer 9 to which is secured by a restraining collar 10. The restraining collar 10 is fastened to one end of the transducer 9. In the case that the transducer is substantially cylindrical the restraining collar 10 may be attached to the transducer by means of a screwthread 11, as shown in Figure 2. However other attachment means may equally be employed, such as grub screws or a simple interference fit. Such alternative attachment means may be employed in the case that the transducer 9 is not cylindrical. This allows the securing collar 10 to be removed and refitted. The securing collar 10 extends away from the transducer 9 such that the side walls of the collar 10 define a recess 12 therein. Housed within the recess 12 is a substantially spherical, solid, ultrasonic coupling element 13. The coupling element 13 is retained within the recess 12 defined by the securing collar 10 by means of a flange 14 formed around the edge of the restraining collar 10 spaced away from the transducer 9. The flange 14 defines a substantially circular aperture that has a diameter less than the diameter of the spherical coupling element 13. As a consequence, the coupling element 13 is restrained between the flange 14 of the restraining collar 10 and the end face of the ultrasonic transducer 9. The probe is assembled by inserting the spherical coupling element 13 into the restraining collar 10 and subsequently attaching the ultrasonic transducer 9 to the collar.

The ultrasonic coupling element 13 is formed from a deformable material that exhibits low attenuation to ultrasound energy. The material is preferably an elastomeric polymer having an ultrasonic attenuation in the range of 2 to 20 MHz of approximately 500-1000 dB/m. The elastomeric material is produced by carefully controlling the amount of cross-linking agent used during the polymerisation process such that the material is substantially deformable and has a density substantially similar to that of water. The deformability of the coupling element 13 allows it to conform closely with any surface irregularities at an inspection area, in an analogous fashion to the liquid filled membrane probe from the prior art and illustrated in Figure 1. However, being formed from a solid sphere of elastomeric material, the coupling element 13 is not prone to tearing or splitting in the same manner as in the prior art and therefore has a substantially longer service life. This is a significant advantage as it provides for the possibility of automating the testing process, for example using robotic means. As will also be appreciated, geometries other than a sphere may be used in further embodiments of the present invention, such as an ellipse, without deviating from the advantageous feature of being able to conform to the surface under inspection.

The deformable characteristics of the elastomeric polymer forming the coupling element 13 in combination with the shape of the coupling element provide good ultrasonic coupling between the transducer 9 and the coupling element 13 and similarly between the coupling element and the surface of the item being tested, without the need for any further coupling medium such as water or gel. The probe is therefore ideal for use in applications such as the automotive industry where contamination from such liquid or gel coupling mediums is unacceptable or undesirable.

In addition to the inherent deformability of the coupling element 13, the space provided between the inner walls of the restraining collar 10 and the spherical surface of the coupling element 13 provides expansion space of the coupling element 13 to deform into whilst in use. Hence, the coupling element 13 is substantially free to deform into its maximum extent without being constrained by any housing means, in this case the restraining collar 10.

A further embodiment of the ultrasound probe is shown in Figure 2b, where like features have like reference numerals with respect to Figure 2a. In addition to the probe shown in Figure 2a, the probe shown in Figure 2b has a sleeve 20, or 'O' ring, on the internal wall of the restraining collar 10 and which in turn is in contact with the coupling element 13. The sleeve 20 functions to both hold the coupling element in place due to frictional forces between it and the coupling element and to limit or remove unwanted internal reflections from the sides of the coupling element, as the sleeve 20 is formed from an ultrasonically 'lossy' material, thus absorbing ultrasound waves that would otherwise be reflected from the sides of the coupling element.

It has also been found that the spherical arrangement of the coupling element 13 shown in Figures 2a and 2b greatly reduces the criticality of sensor alignment that is a common feature of the membrane probe shown in Figure 1. Typically, for prior art membrane probes, the probe should be normal to the surface of the specimen, with only a small misalignment from this normal being permissible. Incorrect alignment of the probe causes a reduction in the signal amplitude. Where the surface of the specimen is locally deformed it may be difficult to align the probe adequately. In contrast, use of a probe according to embodiments of the present invention allows a larger misalignment whilst still providing an acoustic signal containing enough information to be able to discriminate a good/bad weld simply from the analysis of the ultrasonic echoes. This further improves the applicability of the probe to robotic operations, or requires less skill for manual testing, allowing testing to be performed more quickly. The spheres also allow a greater degree of insonification into the spot weld (insonification is to ultrasound what illumination is to light).. It has been found that optimum diameters for the spherical coupling element 13 are 6, 8, 10 and 12 mm, when used in conjunction with standard high frequency sensor probes of 2 to 20 MHz. However, coupling elements 13 of other diameters may also be used depending upon the frequency of the ultrasonic transducer and the intended application. Figure 3 shows a second embodiment of an ultrasound probe according to the present invention. As with Figures 2a and 2b, an ultrasonic coupling element 15 is held in engagement with an ultrasonic transducer 16 by means of a restraining collar 17 removably secured to the transducer. However, in the embodiment shown in Figure 3, the coupling element 15 is substantially conical in shape, with the base of the conical coupling element 15 being adjacent to the end face of the transducer 16. The restraining collar 17 is also substantially frusto conical and conforms closely with the shape and dimensions of the coupling element 15. However, the restraining collar 17 does not extend fully to the tip of the conical coupling element 15, such that the tip of the coupling element protrudes from the restraining collar 17. The amount of protrusion is preferably approximately 2 mm. The coupling element 15 is again preferably formed from the same, or similar, rubber based elastomeric polymer as the coupling element of the first embodiment described with reference to Figure 2. Consequently, the tip of the coupling element 15 protruding from the securing collar 17 is substantially deformable and can therefore conform to surface irregularities in the same fashion as described with reference to Figures 2a and 2b. The conical shape of the coupling element 15 enables inspection in difficult to access areas, for example around the neck and base of plastic bottles, whilst the conformable elastomeric tip provides enhanced dry coupling between the probe and the material under test. The restraining collar 17 extends over a substantial portion of the coupling element 15 to provide support to the conical coupling element 15, without which the coupling element 15 is excessively flexible and tends to deform bodily away from the material surface to be tested when pressed against the surface at an angle.

The conical shape of the coupling element 15 can cause internal reflections to occur that lead to an additional echo being produced before the expected back wall echo is received from the test sample. This additional echo may also interfere with the desired signal from the sample. The reflections are caused by the smooth surface of the conical coupling element 15. To therefore minimise and preferably eliminate, these reflections ridges 16 are formed on the surface of the coupling element 15, as shown in Figure 4. In other embodiments the ridges may be longitudinally formed on the surface of the coupling element, as illustrated in Figure 4b. Equally, the ridges may be replaced with corresponding depressions formed in the surface of the coupling element. In fact, and as will be appreciated by those skilled in the art, virtually any pattern, regular or random, of roughness formed on the surface of the coupling element will tend to have the desired effect of dampening the unwanted internal reflections. In further embodiments an additional layer of ultrasonically lossy material may be formed over the surface of the coupling element. Rather than reflecting internally from the outer surface of the coupling element, the lossy layer allows the ultrasonic waves to pass into it, where they are substantially absorbed (attenuated).

In both the embodiments described with reference to Figures 2, 3 and 4, an additional coupling medium, such as a coupling gel, may be provided between the ultrasonic transducer 9, 16 and the face of the coupling element 13, 15 in engagement with transducer.

A further advantage of the low attenuation coupling elements is that they also function as an integrated "delay line". Typically an ultrasonic sensor has a "dead zone" around it in which no measurements can be made. This occurs because during a short time period immediately after an ultrasonic pulse has been emitted by the sensor a strong signal is immediately received back, which is a combination of the reflection signal from the interface between the transducer and adjacent coupling element/medium and an inherent artefact produced by the transducer itself. This is illustrated in Figure 5 which schematically shows a plot of received signal amplitude versus time. The time period labelled DZ on Figure 5 represents the period over which this unwanted signal, represented by the trace 17, is received. Consequently, any object placed close enough to the transducer such that the time of flight for the ultrasonic pulse to reach the object and return to the sensor is less than the time period DZ in which the unwanted signal occurs, will not be detected. This distance is therefore the "dead zone" of the sensor. An ultrasonic delay line reduces this dead zone as it effectively increases the distance over which any ultrasonic energy has to traverse, therefore providing sufficient time for the unwanted ultrasound echo to decay away before echoes from objects of interest are received. The elastomer used in the coupling elements of the embodiments of the present invention described herein is particularly suitable for use in ultrasonic delay lines as its attenuation properties are particularly good. The elastomer may also have the added benefit of operating at temperatures of the order of 180°C. This makes it particularly suitable for use as a contact delay for applications where a sensor may be exposed to harmfully high temperatures. The coupling element acts as a thermal insulator and protects the sensor. Additionally, the elastomer is also preferably resistant to contamination from oil and other similar substances found in industrial environments.

Embodiments of the present invention therefore provide for an ultrasonic probe having excellent dry -coupling properties, both in terms of ultrasonic attenuation and conformability with surface irregularities.

Claims

1. An ultrasound probe for use in non-destructive testing comprising an ultrasonic transducer and an ultrasonic coupling element, wherein the ultrasonic coupling element comprises a substantially solid portion of deformable elastomeric polymer, such that in use the ultrasonic coupling element conforms to surface irregularities in a material under test.

2. An ultrasound probe according to claim 1, wherein the elastomeric polymer has ultrasonic attenuation of less than 500dB/m at 5MHz and less than 1000 dB/m at 10MHz.

3. An ultrasonic probe according to claim 1 or 2, wherein the elastomeric polymer is rubber based.

4. An ultrasonic probe according to any preceding claim comprising a restraining element for maintaining the coupling element in engagement with the ultrasonic transducer.

5. An ultrasonic probe according to claim 4, wherein the restraining element is removably attached to the probe.

6. An ultrasonic probe according to claim 4 or 5, wherein the restraining element defines a recess that the coupling element deforms into in use.

7. An ultrasonic probe according to claim 4 or 5, wherein the restraining element has an inner surface in engagement with a portion of the outer surface of the ultrasonic coupling element, the inner surface of the restraining element substantially corresponding to the outer surface of the portion of the coupling element.

8. An ultrasonic probe according to any one of claims 4 to 6, wherein the restraining element includes a coupling element gripping portion arranged to frictionally engage with the coupling element.

9. An ultrasonic probe according to claim 8, wherein the coupling element gripping portion has higher ultrasonic attenuation than the coupling element.

10. An ultrasonic probe according to any one of claims 1 to 5, wherein the ultrasonic coupling element is substantially conical.

11. An ultrasonic probe according to claim 10, wherein the coupling element comprises at least one surface irregularity formed on the outer surface thereof.

12. An ultrasonic probe according to claim 11, wherein the or each surface irregularity comprises a depression format in the outer surface of the coupling element.

13. An ultrasonic probe according to claim 11, wherein the or each surface irregularity comprises a ridge formed on the outer surface of the coupling element.

14. An ultrasonic probe according to any one of claims 1 to 6, wherein the ultrasonic coupling element is substantially spherical.

15. An ultrasonic probe substantially as shown in, and/or as described with reference to, the accompanying figures.