WO2017085512A1

WO2017085512A1 - Inspection device and method

Info

Publication number: WO2017085512A1
Application number: PCT/GB2016/053612
Authority: WO
Inventors: Colin Bird
Original assignee: Doosan Babcock Limited
Priority date: 2015-11-20
Filing date: 2016-11-21
Publication date: 2017-05-26
Also published as: GB201520488D0

Abstract

A device to facilitate the inspection of a material sample using sound is described. The device has a wedge-shaped body of a first solid acoustic medium shaped to define a first acoustic surface, and a second acoustic surface spaced from and angled relative to the first acoustic surface, and having a plurality of further surfaces to form a solid body; a barrier layer of a second solid acoustic medium disposed upon at least a substantial part of, and preferably the whole area of, each of the said further surfaces; wherein the first acoustic medium is selected such that the velocity of sound through the first acoustic medium is below 1500 ms"1 and such that the acoustic attenuation of the medium is below 0.3 dB mm"1 : and wherein the second acoustic medium is selected to have an acoustic impedance relative to the first acoustic medium that differs by no more than 5% and an acoustic attenuation such as to produce an attenuation at the interface between the first and the second acoustic medium of at least 20 dB. A system including a sound generator and a method of inspection of a material sample using sound are also described.

Description

INSPECTION DEVICE AND METHOD

The invention relates to a device to facilitate the inspection of a material sample and for example a pipe, and in particular the inspection for defects of butt weld regions of pipes, using sound and for example phased array ultrasound, and to a method of inspection of a material sample and for example a pipe, in particular to detect defects of butt weld regions of pipes, using such a device in conjunction with phased array ultrasound.

The invention in particular relates to a device and method suited to adapt established principles and phased array ultrasound probes to the inspection of pipes and, in particular butt weld regions of pipes fabricated from relatively high density and relatively high attenuation polymeric materials such as thermoplastic materials and in particular high density polyethylene (HDPE).

Plastics materials are finding increasing application as replacements for metal materials, such as cast iron and steel, for the fabrication of pipes used for the conveyance of fluids, whether under pressure or otherwise. Thermoplastic materials for pipework are becoming particularly widespread. Examples include ABS, PBC, PVDF and polyolefins. High density polyethylene (HDPE) in particular is an alternative material being adopted for the replacement of cast iron pipework. The attractiveness of HPDE pipework is related to its long-term structural integrity in that it does not corrode, is flexible, lightweight and a cost effective replacement. The invention is described herein in particular with reference to HDPE pipes, and to butt welds in such pipes, but it will be appreciated the principles of the invention may be considered for application in relation to similar problems which arise in fusion joints or welds in other polymeric pipe sections and other equivalent structures where similar geometric and material considerations, and in particular acoustic considerations, to those set out below apply.

In accordance with familiar principles, HDPE and other polymeric pipes are provided in sections to be joined together, usually in-situ, via a welding process that involves heating abutting portions of pipe sections. HDPE butt welds are manufactured in four stages: heating, heat soaking, heater plate removal and joining-cooling. The heating stage involves the two prepared pipe sections of HDPE being pressed against a heater plate under a specific pressure and being held there for a specific time (heat soaking). Thereafter, the heater plate is removed and the two ends are aligned exactly and brought together. The joining- cooling process involves the compression of the two faces together and being held for a specified duration in order to fuse the two ends. The excess material squeezed out during the butt fusion force forms a bead on the inner and outer surface of the weld leaving virtually no heat affected zone in the weld.

Installation and joining of HDPE pipework sections happens in-situ where environmental conditions may introduce impurities such as grease, dust, dirt and sand to both the heater plate and pipe ends. This type of contamination can lead to defects in the butt fusion weld. The size of the air pocket created by the contaminants can produce a lack of fusion where there is no bond or a partial bond between the faces of the sections. Kissing bonds or cold joints can occur when the contact that is made is not sufficient to form bonds capable of transmitting shear stress resulting in a lack of strength. Generally it is considered that such problems will occur perpendicular to the radial-axial plane. It is therefore desirable to develop a weld inspection system that is effective in that geometry and that is effective in detecting defects that occur perpendicular to the radial-axial plane.

The use of ultrasonic detectors for example employing phased array ultrasound transducers is well established for detecting defects in butt welds in cast iron and steel pipes. Cracks or other discontinuities perpendicular to the surface of a test piece, such as pipe weld discontinuities perpendicular to the radial-axial plane, may be invisible with straight beam test techniques because of their orientation with respect to the ultrasound beam. However, in metal pipes angle beam techniques are widely used, employing a system aligned so as to direct ultrasound energy into the pipe weld zone at a selected angle, and in particular at an angle acute to the horizontal.

Sound energy at ultrasonic frequencies is highly directional. Ultrasound energy that is incident upon the interface between one material and another may behave as shown in figure 1. Thus as an incident directional ultrasound signal at angle of incidence θι may be reflected forward at the same angle and/ or refracted in accordance with Snell's Law as, and in accordance with the formula: sin 9₁ / sin e₂ = V₁ / V₂ where

θι = angle of incidence (first material);

θ₂ = angle of refraction (second material);

V_! = velocity of sound in the first material;

V₂ = velocity of sound in the second material.

Figure 1 shows schematically an incident path I at angle of incidence θι with a reflected path R and a refracted shear path S and a refracted longitudinal path L both based on a scenario where V₂ > V_! (and hence θ₂ > θ^.

The relatively high longitudinal velocity of sound in iron and steel pipes (up to 6000 m/s or even above) can be exploited in angle beam ultrasound weld inspection in iron and steel pipes. An ultrasound transducer is used in conjunction with a plastic stand off shoe with a wedge shape with the ultrasonic transducer perpendicularly contacting a first wedge face and the second wedge face in contact with the specimen, to define an angle of incidence of the incident ultrasound to the specimen surface. A shoe is for example of Perspex® or Rexolite®. The much higher velocity of sound in the pipe means that a relatively small angle of incidence θι in the wedge can be used to generate a signal within the pipe with a much higher θ₂, producing a forward looking angle beam in the weld region (that is, an angle beam at a more acute angle to the horizontal) that is better able to detect defects perpendicular to the radial-axial plane.

The same acoustic considerations do not apply to HDPE. HDPE is a material with high density and high attenuation values. Shear waves are not supported in HDPE due to the material properties and hence longitudinal waves are the only possible inspection mode. The longitudinal velocity of sound in HDPE is approximately 2150 m/s. Furthermore, HDPE pipework is very attenuative (0.3 dB/mm using a 2.25 MHz probe) for example. Conventional Perspex® (2740 m/s) and Rexolite® (2350 m/s) shoes are inappropriate as the change in velocity into HDPE is not suitable to generate sufficiently forward looking angle beams. The challenge presented by high density high attenuation polymeric materials such as HDPE is to find a means to generate a sufficient forward steering angle.

The use of a stand off wedge using water as its acoustic medium has been proposed in order to produce a useful refracted angle within an HDPE or similar pipe (David MACLENNAN, Irene G PETTIGREW and Colin R BIRD, 18th World Conference on Nondestructive Testing, 16-20 April 2012, Durban, South Africa). However using a liquid as the acoustic medium has limited practicality in some applications, and presents containment and stability problems.

The invention aims to mitigate some or all of these disadvantages to develop a device and method suited to adapt established principles and phased array ultrasound probes to the inspection of pipes and, in particular butt weld regions of pipes fabricated from relatively high density and relatively high attenuation polymeric materials such as thermoplastic materials and in particular high density polyethylene (HDPE).

Thus in accordance with the invention in a first aspect a device to facilitate the inspection of a material sample using sound comprises:

a wedge-shaped body of a first solid acoustic medium shaped to define a first acoustic surface, and a second acoustic surface spaced from and angled relative to the first acoustic surface, and having a plurality of further surfaces to form a solid body;

a barrier layer of a second solid acoustic medium disposed upon at least a substantial part of, and preferably the whole area of, each of the said further surfaces;

wherein the first acoustic medium is selected such that the velocity of sound through the first acoustic medium is below 1500 ms^"1 and such that the acoustic attenuation of the medium is below 0.3 dB mm^"1 :

and wherein the second acoustic medium is selected to have an acoustic impedance relative to the first acoustic medium that differs by no more than 5% and an acoustic attenuation such as to produce an attenuation at the interface between the first and the second acoustic medium of at least 20 dB.

The first acoustic medium functions as a wedge-shaped standoff along similar principles to comparable prior art standoff wedges, acting as a wave guide for an input sound signal, and in particular for a directional ultrasound signal, to direct the signal towards a sample, such as a butt welded region of a pipe under test, at a desired predetermined incident angle to the surface of the sample.

In a typical mode of operation, in familiar manner, a suitable acoustic generator for example including a sound transducer such as an ultrasonic transducer is placed in association with the first acoustic surface to generate an acoustic signal, for example directed generally normally to the surface, which is thereby transmitted into the first acoustic medium with minimal reflection. The second acoustic surface is in contact with the sample, and as a result the transmitted signal reaches the surface of the sample at an angle of incidence relative to the surface of the sample which is predetermined by the geometry of the wedge.

The second acoustic medium ensures that the first acoustic medium operates effectively as a waveguide, by ensuring that stray signals are substantially absorbed in the second acoustic medium and that minimal reflection takes place at the interface. This is achieved by using a second acoustic medium which is much more highly acoustically attenuating, but acoustically impedance matched to the first acoustic medium.

The combined effect of the two acoustic mediums is to allow the device to serve as an acoustic waveguide in conjunction with a suitable acoustic generator to generate an angled incident acoustic signal, and for example an angled incident ultrasound beam, for examination of a sample such as the butt weld region of a pipe.

The general principles of such standoff wedges for ultrasound probes will be familiar from the field of ultrasound inspection of butt welds in steel and iron pipes. The invention differs from such standard wedges in that the first acoustic medium is selected such that the velocity of sound through the medium is below 1500 ms^"1. More preferably the first acoustic medium is selected such the velocity of sound in the medium is below 1 100 ms^"1 , and for example in the range 900 - 1 100 ms^"1.

As a result, the acoustic medium is better matched to the inspection of samples, and for example butt weld regions in pipes, fabricated from relatively high density and relatively high attenuation polymeric materials such as HDPE. The difference between the speed of sound in such polymeric materials and the speed of sound in the first acoustic material of the device of the invention is sufficient to produce a significant degree of refraction, according to the principles illustrated in figure 1 , so that a relatively small incident angle (desirable to minimise reflection at the interface) produces a relatively large angle of refraction, and consequently produces an effective forward probing beam.

The principles of the invention differ from the wedge in the paper referred to herein above by McLennan, Pettigrew and Bird in that the first acoustic medium is not water or another fluid but is selected to be a solid medium. Many of the practical difficulties associated with the use of water are thus eliminated. Water is difficult to handle and contain practically in a simply wedge structure because of its liquid nature. Practical examination of butt weld joins in-situ requires robust, field-resilient equipment, and the use of a liquid medium is not ideal in such a scenario. The acoustic properties of water are subject to variations over a typical range of operational and test temperatures, in particular in that it undergoes a liquid to solid phase transition over a normal range of in-field ambient operational temperatures. The use of a solid for the first acoustic material which has acoustic properties comparable to or better than that of water is consequently advantageous in several technical respects, conferring practical benefits to the device in addition to its acoustic properties that water does not offer.

Suitable materials for selection as the first acoustic medium might be considered for example from the field of medical or veterinary ultrasound diagnosis. Ultrasound examination finds use in medical and veterinary application for obtaining images of the human or animal body. In such scenarios the desire is not generally, as it is with angle beamed testing of material samples such as weld zones, to achieve a substantial mismatch between the acoustic properties of the test sample and the acoustic medium in immediate contact with its surface to ensure a high degree of refraction, but is usually instead to achieve a close match with human or animal tissue. Given the high water content of human or animal tissue, such acoustic materials are a useful category of material to be modified to serve as a solid acoustic materials in substitution for the water used in the prior art water-based wedge to solve the very different problem of providing a sufficient degree of refraction at the sample interface to get a sufficiently forward looking beam while overcoming the disadvantages associated with using a liquid acoustic medium.

Classes of material with suitable acoustic properties include natural and artificial elastomers, and in particular silicone rubbers. Preferably therefore, at least the first acoustic material, and for example both acoustic materials, may be elastomers such as silicone rubbers.

Technology for the modification of silicone rubbers to achieve desired low sound velocity and low acoustic attenuation for ultrasound application may be applicable to modification of at least the first acoustic material of the present invention. In particular, the first acoustic material may be a silicone rubber material modified by the provision of distributed dopant, for example in the form of distributed nanoparticles, and for example comprising metal oxide nanoparticle dopant. Such materials are likely to be particularly preferred for the first acoustic medium.

Then invention relies upon selection of suitable acoustic materials to produce the desired properties. The first acoustic medium is selected such that the velocity of sound through the first acoustic medium is below 1500 ms^"1 and such that the acoustic attenuation of the medium is below 0.3 dB mm^"1. Preferably the velocity of sound through the first acoustic medium is below 1 100 ms^"1. For example the velocity of sound through the first acoustic medium is in the range 900 - 1100 ms^"1. Preferably the acoustic attenuation of the first acoustic medium is below 0.2 dB mm^"1.

The second acoustic medium is selected to have an acoustic impedance relative to the first acoustic medium that differs by no more than 5% and preferably no more than 1 % to minimize reflection at the interface and an acoustic attenuation such as to produce an attenuation in the second acoustic medium of at least 20 dB to maximize attenuation of stray signal. The acoustic attenuation of the second acoustic medium is selected accordingly.

All values of physical and material parameters given to define the material components of the invention will be presumed, except where context necessarily requires otherwise, to be the values at 25°C and 100 kPa pressure and to be the values applicable to a longitudinal ultrasound wave at 1 MHz.

However it is an advantage of using first and second solid acoustic media in the device of the invention that the physical and acoustic properties may be relatively invariant over a range of conditions and especially a range of temperatures of likely operation, in particular when compared with systems using water or other fluids that undergo a liquid to solid phase transition over a range of temperatures of likely operation.

Preferably, at least the first and more preferably each of the first and second solid acoustic media is selected to be a phase stable solid at least throughout the range from -25°C to 60°C. Preferably at least the first and more preferably each of the first and second solid acoustic media is selected to have relatively invariant acoustic properties over this temperature range. For example for at least the first and more preferably each of the first and second solid acoustic media none of the listed acoustic properties varies over this temperature range by more than 20%.

The device in the invention comprises a wedge-shaped body of the first solid acoustic medium. References to a wedge-shape should be understood generally as a requirement that the body is shaped to define a first acoustic surface and a second acoustic surface spaced from and angled relative to the first acoustic surface so as to serve as a standoff by means of which an ultrasound beam generally normally incident on the first acoustic surface will form a predetermined non-normal incident angle emerging from the second acoustic surface and in contact with a sample in use. Subject to this, no particular geometric shape is required or should be inferred.

The wedge-shaped body will typically comprise a prismatic shape, in which the first and second acoustic surfaces are extended into a third dimension to provide a three dimensional solid shape. The wedge-shaped body may comprise a triangular prism, or a truncated triangular prism with the apex truncated, or other trapezoidal or more complex structures without departing from the principles of the invention.

The second acoustic surface makes contact with a sample under test in use. Typically in a preferred embodiment the second acoustic surface may be planar. Alternatively for certain applications the second acoustic surface may be shaped for engagement with a shaped sample. The first acoustic surface serves as an engagement surface in use for a sound generator, in particular disposed to generate a substantially normal sound beam, and thereby a sound beam that reaches the second acoustic surface and thereby the sample surface at the predetermined angle of incidence. The first acoustic surface is preferably generally planar. Alternatively, for particular applications where desired, the first acoustic surface may define two or more planar portions with different angles relative to the second acoustic surface to provide the option of two or more angles of incidence at the second acoustic surface and/or may comprise a none planar curved surface defining a continuously variable angle relative to the second acoustic surface to provide the option of a continuously variable angle of incidence at the second acoustic surface.

Each of the further surfaces not being the first and second acoustic surfaces of the wedge-shaped body has disposed upon it a barrier layer of a second solid acoustic medium. Conveniently, each such further surface is a planar surface. Conveniently, the barrier layer on each of said surfaces is of constant thickness. Each barrier layer conveniently comprises a layer of the second acoustic material with an inner surface in intimate contact with a said other surface of the wedge-shaped body and an outer surface opposed to the inner surface. A housing may be provided at least to cover the said outer surfaces of the second acoustic medium, for example for environmental protection.

Such a housing must necessarily expose at least a portion of the first acoustic surface and at least a portion of the second acoustic surface for the transmission of a sound signal. For example the housing may define a window extending over at least a part of the first acoustic surface and a window extending over at least part of the second acoustic surface. Each of the first and second acoustic surfaces, or if applicable at least the window regions of each of the said first and second acoustic surfaces, may be provided with an additional protective layer of a substantially acoustically transparent material. A part of the housing in the vicinity of the first acoustic surface may be adapted for engagement with a suitable sound generator such as a suitable ultrasonic probe.

In accordance with the invention in a second more complete aspect a system for the inspection of a material sample using sound comprises a device in accordance with the first aspect of the invention provided in combination with an acoustic generator adapted in use to engage upon the first acoustic surface of and transmit an acoustic signal into a device in accordance with the first aspect of the invention.

The acoustic generator is for example an ultrasound generator adapted to generate ultrasound, for example in at least a part of the range 500 kHz to 10 MHz the range.

The acoustic generator is for example an acoustic transducer and for example an ultrasound transducer. The acoustic generator is for example an piezoelectric transducer.

The acoustic generator is for example a phased array transducer.

The acoustic generator may be mechanically coupled to the first acoustic surface of the device.

The acoustic generator may be adapted in use to engage upon and in the preferred case mechanically couple to the first acoustic surface of the device in accordance with the first aspect of the invention in a generally perpendicular alignment to the first surface, to minimize reflection and maximize transmission of the acoustic signal into the first acoustic medium of the device.

The acoustic generator may have an engagement surface portion adapted in use to effect engagement with the first service of the device, which comprises an acoustic medium closely impedance matched to the impedance of the first acoustic medium of the device, to minimize reflection and maximize transmission of the acoustic signal into the first acoustic medium of the device.

The acoustic generator may be integrally formed with the device in accordance with the first aspect of the invention.

The device of the second aspect of the invention conveniently further comprises an acoustic detector to detect sound reflected from within a sample under test during use.

In accordance with the invention in a third aspect, a method of inspection of a material sample and for example a pipe, and in particular the inspection for defects of butt weld regions of pipes, using sound and for example phased array ultrasound, comprises the steps of:

disposing a device on the surface of the sample comprising:

and wherein the second acoustic medium is selected to have an acoustic impedance relative to the first acoustic medium that differs by no more than 5% and an acoustic attenuation such as to produce an attenuation at the interface between the first and the second acoustic medium of at least 20 dB such that the second acoustic surface seats against the surface of the sample;

causing an acoustic signal to impinge upon the first acoustic surface, for example generally normally, so as to be transmitted through the sample surface into the sample at a desired incident angle;

collecting sound reflected from within the sample at a suitable detector. The acoustic signal is for example ultrasound, for example in at least a part of the range 500 kHz to 10 MHz the range. The acoustic signal is for example phased array ultrasound. The method is in particular a method for the detection of defects in a sample perpendicular to a transverse sample direction and for example of defects in a pipe perpendicular to the radial-axial plane.

The method is in particular a method for the inspection of a pipe, in particular to detect defects of butt weld regions of pipes.

The method is in particular a method of using a device of the first or second aspect of the invention and preferred features will be understood by analogy. The invention will now be described by way of example only with reference to figures 1 to 6 of the accompanying drawings, in which:

Figure 1 illustrates the principles of refraction of an incident ultrasound beam;

Figure 2 is a perspective view from below of a device in accordance with an embodiment of a first aspect of the invention;

Figure 3 is a further perspective view from below of the device of figure 2;

Figure 4 is a perspective view from above of the device of figure 2;

Figure 5 is a longitudinal cross-section of the device of figure 2 showing (in part) operation in conjunction with a suitable ultrasound transducer;

Figure 6 is a transverse section along A-A of the device of figure 5 with a schematic representation of the position of a suitable sound generator to form an embodiment of the second aspect of the invention;

Figure 7 is a representation of a more complete system showing the device of figure 5 in use inspecting the weld zone of a pipe butt weld. Figure 1 illustrates the principles of Snell's Law which are exploited both in prior art wedge-shaped standoff waveguides and in accordance with the principles of the invention. An ultrasound transducer, typically a phased array transducer comprising a phased array of ultrasound elements which are for example piezoelectric elements, is provided with a suitable acoustic medium defining an ultrasound signal delivery end. This delivery end is engaged with a first surface of a wedge-shaped shoe, and for example integrally includes such a shoe, so that an ultrasound beam generated by the transducer is directed towards a lower surface of the wedge-shaped body as an angled incident beam (I in figure 1). The second surface of the shoe is in contact with a sample, and the beam I in consequence has an effective angle of incidence of θι and is in consequence refracted at the interface in the manner shown in figure 1.

For examination of iron and steel pipes, of Perspex® or Rexolite® shoes may be used. The respective speeds of sound in such materials, 2740 ms^"1 and 2350 ms^"1 are substantially lower than the typical speed in the metal pipe sample. In consequence, the effect of refraction at the interface is to produce a forward looking angled beam for angled beam analysis of the sample, which is for example the heat affected zone of a butt weld.

The invention is directed at employing similar principles in association with high density high attenuation polymeric pipes, such as for example HDPE pipes. Conventional Perspex® and Rexolite®. shoes are inappropriate, given that the longitudinal velocity of sound in HDPE is approximately 2150 ms^"1 , so the change in velocity is not suitable to generate forward looking angle beams. Accordingly, an alternative embodiment of wedge structure, attempting to adapt the general principles illustrated in figure 1 to the particular problems associated with such high density high attenuation polymeric pipes where the velocity of sound is relatively lower, is illustrated with reference to figures 2 to 7. A general schematic of a standalone wedge structure is illustrated by the three views of figures 2 to 4, and by the relevant parts of the respective longitudinal and transverse cross-sections 5 and 6. Where applicable, like numerals have been used to illustrate like components. The wedge structure generally designated as 1 comprises a wedge body 3 of a first acoustic material. The wedge body 3 is surrounded by a barrier layer 5 of a second acoustic material in such a way as to leave first and second planar faces uncovered, respectively to constitute acoustic surfaces 15 and 10. Optionally, a third acoustically transparent material may be applied as a protective layer over these uncovered faces. The first acoustic material is selected to have the desired compositional properties and acoustic properties. The second acoustic material is selected to have the desired compositional properties and acoustic properties.

The whole assembly is contained within a casing 7, which has a protective rather than an acoustic function.

Figure 3 illustrates a lower face of the wedge structure 1 intended to engage upon a sample surface, for example on the butt weld zone of HDPE pipework, in use. The lower part includes a wear surface 9 forming a part of the casing 7, and defining a window 11 exposing the surface 10 of the first acoustic medium forming the second or lower acoustic surface that contacts the pipe in use.

An upper surface of the wedge structure 1 is shown in figure 4. An upper portion of the casing 13 again defines a window 14 exposing an area of the first acoustic material not covered by the second acoustic material and forming the first or upper acoustic surface 15 that receives the signal from the sound generator in use.

Accordingly, if an ultrasound signal is introduced by a suitable ultrasound transducer into the surface 15, in particular a directional signal applied in a direction generally normal to the surface 15, the wedge 1 will act as a waveguide.

This is illustrated schematically in figure 5, where the wedge of figures 2 to 4 is shown in longitudinal cross-section in conjunction with an ultrasound delivery end 21 of an ultrasound transducer which may be engaged upon the surface 15, coupled to it mechanically or integrally formed with it. This is further illustrated in use by the more complete system of figure 7, shown applied in situ for inspecting the weld zone 31 of a pipe butt weld between two pipe sections 30a, 30b. In figure 7 a wedge structure device 1 in accordance with the invention is placed with a surface (being the acoustic surface 10 in the other figures) in contact with the surface of the weld zone 31. An acoustic generator 25 in the form of a phased array ultrasound piezoelectric transducer acoustically couples to the other surface (being the acoustic surface 15 in the other figures) via the ultrasound delivery end 21 of the ultrasound transducer which may be engaged upon the surface 15, coupled to it mechanically or integrally formed with it. Operation of the ultrasound transducer generates directional ultrasound in the direction U, which is transmitted into the first acoustic medium 3 to form an incident beam in the direction I. As a result of the shape of the wedge structure 1 , this incident beam reaches the lower surface 10, and consequently approaches the interface with the HDPE pipework (not itself shown in the figures) at an incident angle away from the normal. Because the first acoustic material 3 has been specifically selected as a material with a longitudinal sound velocity of substantially less than that of the material of the pipe (for example below 1500 ms^"1) refraction of the incident beam occurs to produce a more forward looking refracted beam in the manner illustrated in figure 1. The purpose of the second acoustic material 5, which provides a layer that entirely surrounds the first acoustic material block 3 except where for the first and second acoustic surfaces, is to maintain directionality of the acoustic beam I by absorbing ultrasound signals that are off direction, and is accordingly selected to be a second acoustic material with closely matched impedance properties to minimise reflection at the interface and substantially greater attenuation properties to attenuate stray ultrasound signals.

The whole assembly provides a compact design of acoustic wedge which can be used as a waveguide in conjunction with a range conventional ultrasound transducers, and in particular phased array transducers, either as a stand alone item, an item specifically adapted to be coupled with the sound delivery end of such a transducer probe, or an item integrally formed with such a probe, to exploit the principles of Snell's Law set out in figure 1 in a manner which is compatible with producing forward looking refracted ultrasound beams in pipes with the acoustic characteristics of materials such as HDPE for angled beam ultrasound NDT examination of weld zones in particular for the detection of defects tending to be in a direction perpendicular to the radial-axial plane. Of course, the device is applicable to other equivalent structures where similar geometric and material considerations apply. This is achieved by means of a self contained solid unit which avoids the practical difficulties encountered either by attempting to use a water standoff or by attempting to include water as one of the materials within a fabricated wedge.

Claims

1. A device to facilitate the inspection of a material sample using sound comprising:

2. A device in accordance with claim 1 wherein the first acoustic medium is selected such that the velocity of sound through the medium is below 1500 ms^"1.

3. A device in accordance with claim 1 or 2 wherein the first acoustic medium is selected such that the velocity of sound through the medium is in the range

900 - 1 100 ms^"1.

4. A device in accordance with any preceding claim wherein one or both of the first acoustic medium and the second acoustic medium is an elastomer.

5. A device in accordance with any preceding claim wherein one or both of the first acoustic medium and the second acoustic medium is a silicone rubber.

6. A device in accordance with any preceding claim wherein the first acoustic medium is a silicone rubber material modified by the provision of distributed dopant in the form of distributed nanoparticles.

7. A device in accordance with claim 6 wherein the first acoustic medium comprises a metal oxide nanoparticle dopant.

8. A device in accordance with any preceding claim wherein the first acoustic medium is selected such that the acoustic attenuation of the medium is below 0.3 dB mm^"1.

9. A device in accordance with claim 8 wherein the first acoustic medium is selected such that the acoustic attenuation of the medium is below 0.2 dB mm^"1.

10. A device in accordance with any preceding claim wherein each of the first and second solid acoustic media is selected to be a phase stable solid at least throughout the range from -25°C to 60°C.

1 1. A device in accordance with any preceding claim wherein the wedge-shaped body is shaped to define a first acoustic surface and a second acoustic surface spaced from and angled relative to the first acoustic surface, and each of the further surfaces not being the first and second acoustic surfaces of the wedge-shaped body has disposed upon it a barrier layer of a second solid acoustic medium.

12. A device in accordance with any preceding claim further comprising a housing to cover at least the second acoustic medium.

13. A system for the inspection of a material sample using sound comprising a device in accordance with any preceding claim provided in combination with an acoustic generator adapted in use to engage upon the first acoustic surface of and transmit an acoustic signal into a device in accordance with the first aspect of the invention.

14. A system in accordance with claim 13 wherein the acoustic generator is an ultrasound generator adapted to generate ultrasound in at least a part of the range 500 kHz to 10 MHz.

15. A system in accordance with claim 14 wherein the acoustic generator is a phased array piezoelectric transducer.

16. A method of inspection of a material sample and for example a pipe, and in particular the inspection for defects of butt weld regions of pipes, using sound and for example phased array ultrasound, comprises the steps of:

disposing a device on the surface of the sample comprising:

wherein the first acoustic medium is selected such that the velocity of sound through the first acoustic medium is below 1500 ms^"1 and such that the acoustic attenuation of the medium is below 0.3 dB mm^"1 : and wherein the second acoustic medium is selected to have an acoustic impedance relative to the first acoustic medium that differs by no more than 5% and an acoustic attenuation such as to produce an attenuation at the interface between the first and the second acoustic medium of at least 20 dB

such that the second acoustic surface seats against the surface of the sample;

collecting sound reflected from within the sample at a suitable detector.

17. A method in accordance with claim 16 wherein the acoustic signal is ultrasound in at least a part of the range 500 kHz to 10 MHz.

18. A method in accordance with claim 17 wherein the acoustic signal is phased array ultrasound.