This invention relates to an assembly of mechanically interconnected objects, and to a method of mechanically interconnecting two or more objects.

The subject matter disclosed in any document which is referred to below is incorporated in this specification by the reference to the document.

It is known to interconnect objects mechanically be means of shape memory alloy components, which can generate high forces capable of creating a connection which can withstand high pull- out forces, and which can provide a fluid tight seal between the objects. For example, US-4198081 discloses a coupling for tubes which is formed from a shape memory alloy and which is arranged to shrink when heat is applied to it. The coupling is provided on its internal surface with circumferentially extending teeth which, when the coupling shrinks, bite into the external surfaces of the underlying tubes. US-4469357 discloses a composite coupling which comprises a heat- recoverable sleeve of a shape memory alloy, and a member positioned in the direction of recovery of the sleeve; for example, the sleeve may be hollow and heat-shrinkable, and the member may be a hollow insert positioned within the sleeve. Application of heat to the sleeve causes it to recover and to deform the member so that it engages a substrate, for example, tubes may be positioned within a hollow insert and engaged by the insert when the sleeve shrinks. The surface of the member which engages the substrate may be provided with teeth which bite into the surface of the substrate when the sleeve recovers.

A heat-recoverable article is one whose dimensional configuration can be made to appreciably when subjected to heat. Usually the configuration of such an article changes towards one from which it had previously been deformed, but the article can also adopt a new configuration without having

previously been deformed. An article formed from a shape memory alloy is one type of article which can exhibit the property of heat-recovery.

Shape memory alloys exhibit the property of heat-recovery as a result of their ability to transform between martensitic and austenitic phases. The transformation may be caused by a change in temperature: for example, a shape memory alloy in the martensitic phase will begin to transform to the austenitic phase when its temperature increases to a temperature greater than A ₃ , and the transformation will be complete when the temperature is greater than A _f . The reverse transformation will begin when the temperature of the alloy decreases to a temperature less than M _s and will be complete when the temperature is less than _f . The temperatures M _s , M _f , A ₃ and _f define the thermal transformation hysteresis loop of a shape memory alloy. An article may be formed in a desired configuration while the alloy is in its austenitic phase. If the article is then cooled so that the alloy transforms to its martensitic phase, the article can then be deformed so as to obtain a strain on recovery of up to about 8%. The strain imparted to the article is recovered when the article is subsequently heated so that the alloy transforms back to the austenitic phase. Further information is available is available in an article by L. M. Schetky in Scientific

American, Volume 241, pages 68 to 76 (1979) entitled Shape Memory Alloys.

The provision of teeth in couplings of the types disclosed in US-4198081 and US-4469357 has the advantage that the strength of the joint between the coupling and the substrate, and between the coupled substrates, is enhanced significantly. Furthermore, the seal which can be formed between the coupling and the substrate, and between the coupled substrates, can withstand significantly higher pressures than would be the case in the absence of the teeth.

A problem arises when it is desired to form a connection to a substrate which is formed of a material which is harder than the material of the surface of the coupling with which it comes into contact when the shape memory alloy component of the coupling recovers, since teeth or other formations provided on the relevant surface of the coupling are unable to bite into the substrate. Indeed, the teeth themselves can be deformed by the substrate.

The present invention provides a technique for providing a fluid tight seal to an object which is provided with teeth which can engage another object, such as a coupling, of which at least a component is formed from a shape memory alloy.

In a first aspect, the invention provides an assembly of mechanically interconnected objects, comprising a heat- recoverable first object which comprises a shape memory alloy component which has been caused to recover, and a second object towards which the first object has recovered so that a surface of the first object engages a surface of the second object, the second object having at least one formation on its surface, so that, when the first object recovered, at least one of the surface of the first object and the formation on the second object was deformed.

In another aspect, the invention provides a method of mechanically interconnecting two or more objects, which comprises:

(a) positioning a heat-recoverable first object, which comprises a shape memory alloy component, relative to a second object such that the second object is positioned in the direction of recovery of the first object, the second object having at least one formation on the surface which faces the first object when the objects are so positioned;

(b) increasing the temperature of the first object to a temperature above the A ₃ temperature of the shape memory

alloy so that the first object recovers towards the surface of the second object, and at least one of the surface of the first object and the formation on the second object is deformed.

The connection technique provided by the present invention has the advantages associated with other techniques which use heat- recoverable shape memory alloy components, including a high connection force from small and light components, and the ability to form connections between objects to which access is restricted and to do so quickly. The deformation of one of the formation on the second object and the surface of the first object can allow a seal to be formed between the first and second objects.

In one embodiment, the material of the or each formation on the surface of the second object is harder than the material of the surface of the first object; recovery of the first object causes the formation to deform and to bite into the surface of the first object. Thus, the technique of the present invention enables connections to be made to objects formed from a material which is harder than the material of the object formed from shape memory alloy, which has not previously been possible in a convenient manner using a shape memory alloy based coupling technique. This embodiment also has the advantage that the second object is not deformed when the first object recovers, and therefore that connections to it can be made repeatedly.

In another embodiment, the material of the or each formation on the surface of the second object is softer than the material of the surface of the first object; recovery of the first object causes the formation to be deformed by the first object. The technique of the present invention therefore enables sealed connections to be made between objects, without the need to form formations in a surface of the heat-recoverable component, as disclosed in US-4198081 and US-4469357. This can be particularly advantageous when it is more attractive to provide

formations in the surface of the non-recoverable object than in the surface of the recoverable object, for example when the material of the heat-recoverable object makes difficult the provision of formations because it is relatively hard. This embodiment also has the advantage that the first object is not deformed when it recovers, and therefore that connections to it can be made repeatedly.

Preferably, one of the objects is positioned inside the other object. For example, the first object may be heat-shrinkable and hollow, and the second object may be positioned inside the first object. In this embodiment, the formation will generally be provided on the outer surface of the second object. This arrangement is particularly suitable for use in the coupling of tubes. In another embodiment, the second object may be hollow, and the first object may be heat-expansible and positioned inside the second object. In this embodiment, the formation will generally be provided on the inner surface of the second object.

Preferably, the or each formation on the second object extends around the entire perimeter of the second object, and is therefore similar to the circumferentially extending teeth disclosed in US-4198081 and US-4469357. This allows a fluid tight seal to be obtained when the shape memory alloy component recovers which can withstand particularly high pressures.

Preferably, the formation is tapered inwardly in a direction away from the surface of the. object on which it is provided. This can facilitate deformation of the surface of the first object by the -formation when the first object recovers. It is particularly preferred that more than one formation is provided on the surface of the second object. A pair of formations which extend around the entire perimeter of the second object, in the form of circumferentially extending teeth, define a groove between them, and a particularly effective seal can be formed between the objects if, on recovery of the first object, a circumferential portion of the second object is forced into

the groove in the surface of the first object, but preferably not to the extent that it contacts the base of the groove.

Other details of the design of suitable formations can be found in US-4226448.

The or each formation may be provided as a separate component from the second object, which is positioned appropriately on the surface of the second object between the first and second objects before recovery of the first object. It is preferred, however, that the formation is formed integrally as part of the second object, for example by machining or moulding.

The first object may include an insert in addition to the shape memory alloy component which can be positioned in the direction of recovery thereof, and which provides the surface of the first object which engages the surface of the second object. The insert may be formed from a gall-prone metal which can enhance the seal between the objects, as disclosed in US-4469357, or be provided with other features which are disclosed in that document.

The first object may be used to interconnect two or more second objects, by itself being connected to each of the second objects.

When the first object is hollow and heat-shrinkable, it will generally be positioned with the second object inside it. The second object may then be hollow, as in the case of a tube or a pipe, or it may be solid, as in the case of a rod or an electrical conductor. When the first object is heat- expansible, it will generally be connected to a hollow second object such as a tube or a pipe.

A support may be provided for the second object to help it to withstand the forces exerted by the first object when it recovers. For example, when the second object is hollow, and is positioned within a hollow heat-shrinkable first object, a sleeve may be positioned within the second object to ensure

cxidL it is not αerormed inwardly to an undesirable extent by the first object as it shrinks, and that, instead, the formations on the outer surface of the second object deform and bite into the surface of the first object.

Preferably, the first object has a closed cross-section. This allows high forces to be generated by the first object when it recovers. For some applications, however, it may be preferred that the cross-section of the first object not be closed since this can allow a greater displacement to be obtained when it recovers. The shape of the first object, when viewed in cross- secton, will generally be determined by the shape of the second object to which it is to be connected. Generally, both the first and second objects will be circular when viewed in cross- section. The shape of the first object, when viewed in plan, will generally be determined by the number and orientation of second objects to which it is to be connected. For example, when the first object is to be connected to two cylindrical second objects having a circular cross-sectron so as to interconnect them, and the second objects are aligned, the first will be a cylinder also having a circular cross-section. When the first object is to be connected to three second objects, it may be for example T- or Y-shaped. When the first object is to be connected to four second objects, it may be for example X-shaped.

The alloy will generally be a nickel-titanium based alloy, which may include additional elements for example to modify its transformation temperatures or its strength characteristics. For example, the alloy may be a binary alloy consisting essentially of nickel and titanium, for example 50.8 atomic percent nickel and 49.2 atomic percent titanium, or it may contain a quantity of a third element such as copper, iron or niobium. Ternary nickel-titanium based shape memory alloys are disclosed in US-3753700, US-4337090, US-4565589 and US-4770725. Copper or iron based alloys may also be used, for example alloys consisting essentially of copper, aluminum and nickel, copper, aluminum and zinc, copper and zinc, or iron, manganese

and silicon. Copper based alloys are disclosed in US-4144104, US-4146392 and US-4166739.

The material of the surface of the second object will be selected according to the use which it will be required to perform. It may, but need not necessarily, be a metal, for example, it may be provided by a polymeric material. When the material of the surface is polymeric, it will generally be softer than the material of the surface of the first object. The second object may be a composite article which comprises a body of a first material which is provided with a layer of a second material, of appropriate hardness.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawing in which:

Figure 1 is a cross-section through a first assembly;

Figure 2 is a cross-section through a second assembly; and

Figure 3 is a cross-section through a third assembly.

-Referring to the drawing. Figure 1 shows an assembly of three interconnected objects 1, 3. One of the objects 1 comprises a hollow heat-shrinkable driver 5 which is formed from a nickel- titanium shape memory alloy, and an insert 7 formed of aluminum positioned within the driver. The other two objects 3 are tubes formed of a hardened stainless steel, and each has two outwardly extending formations 9 on its outer surface in the manner of circumferential teeth.

The objects are connected to one another by positioning the ends 11 of the tubes 3 within the driver 5 and the insert 7 while the driver is in its martensitic phase and is shrinkable. The temperature of the driver is increased to a temperature above the A _a temperature of the shape memory alloy, to cause the driver to shrink radially, and to deform the insert

inwardly, in a direction towards the tubes. The material of the insert is less hard than the material of the tubes, so the teeth 9 deform and bite into the internal surface 13 of the insert 7 when the driver 5 shrinks.

Figure 2 shows an assembly of three interconnected objects 21, 23. One of the objects 21 comprises a heat-expansible driver which is formed from a nickel-titanium shape memory alloy. The other two objects 23 are tubes formed from a hardened stainless steel, and each has two inwardly extending formations 29 on its inner surface in the manner of circumferential teeth. A tubular sleeve 30 surrounds the ends 31 of the tubes to support them against the force exerted by the driver when it recovers.

The objects are connected to one another by positioning the driver 21 within the ends 31 of the tubes 23 while the driver is in its martensitic phase and is expansible. The temperature of the driver is increased to a temperature above the A _s temperature of the shape memory alloy, to cause the driver to expand radially in a direction towards the tubes. The material of the driver is less hard than the material of the tubes, so the teeth 29 deform and bite into the external surface 33 of the driver 25 when it expands.

Figure 3 shows an assembly of three interconnected objects 41, 43. One of the objects 41 comprises a heat-shrinkable driver which is formed from a nickel-titanium shape memory alloy. The other two objects 43 are tubes formed from an ultra high molecular weight polyethylene, and each has two outwardly extending formations 45 on its outer surface in the manner of circumferential teeth.

The objects are connected to one another by positioning the ends 47 of the tubes 43 within the ends of the driver 41 while the driver is in its martensitic phase and is shrinkable. The temperature of the driver is increased to a temperature above the A _s temperature of the shape memory alloy, to cause the driver to shrink radially in a direction towards the tubes.

The material of the formations is less har _d than t _h e mater _i a _l o ^f t ^h e ^d river, so t ^h e formations are deforme _{d b} y the _d river as ⁱ t shrinks, resulting in a seal between the _d river an _d the tubes.