IL35091A - Metallic coupling for pipes and the like - Google Patents

Description

nonai rnnias'? 'nana asa Metallic coupling for pipes and the like RAYCHBM CORPORATION C: 33355 35ί>9.ΐ/2 This invention relates to hollow articles, for example couplings, especially pipes, couplings and similar fittings, and to processes for their manufacture and application.

Considerable effort has been expended over many years to provide simple and reliable means for joining the abutting ends of pipes and conduits. The method used should not interfere substantially with the material in the pipe, nor should flow be restricted to any significant extent. The joint between the two pipes should be capable of withstanding any mechanical or chemical misuse that the pipes themselves can withstand, without being too expensive or difficult to install, repair or replace.

With the great use of pipes in aerospace applications, for example in aircraft hydraulic lines, additional requirements of light weight and, in some cases, small size have to be met* Also, as aircraft are frequently take apart for servicing, the connections to hydraulic equipment in various components have to be broken and remade in the aircraft, ofte in confined conditions. These requirements militate against the use of welded or braeed connections, which would otherwise appear to be an obvious method of overcoming the problem, because welding and brazing require bulky equipment which must completely surround the pipes to be joined. Further, the resultin joints cannot readily be taken apart and remade* The present invention provides a tubular coupling member made from a memory metal and provided on its inner surface with one or more inwardly projecting teeth, which coupling member is capable of contracting radially on being heated above the transition temperature of the metal.

The present invention also provides method of mailing a connection, between two or more pipes which comprises positioning the pipes within said tubular coupling member and causing the coupling member to contract radially and grip the pipes by raising its temperature above the transition temperature. is The word "tubular" as used herein/not ¾«±»g limited to right cylindrical hollow members but also includes , members of irregular and/or varying cross-section as well as Y-shaped, T-shaped, X-shaped and similar members.

One of the most important applications of the coupling members of the presen invention is in the coupling of objects such as tubes and pipes, especially in hydraulic systems, and it has been found that the ability of the couplings to withstand very high pressure without pulling apart can greatly be enhanced by the location of teeth within the coupling. ibr this reason, the tubular coupling is provided with one or more inwardly projecting teeth on its inner surface, and, preferably, a series of axlally separated, continuous circumferential teeth. Preferably the teeth are formed from. plurality of tapered sections and the distance between ad acent teeth and the angle of tepering are such that expansion of the objects to be connected caused by the biting of the teeth into the object on shrinkage can be accommodated in the space between the teeth. Preferably the biting angle of each tooth is almost a right-angle and one edge of the biting angle advantageously lies approximately in a radial plane of the tubular coupling member. iubular coupling members according to the present invention are preferably provided with a recess of increased internal diameter about mid-way along their length so that the abutting ends of the objects to be connected do not come into contact with the internal surface of the coupling member.

Preferably the tubular coupling members have portions of reduced thickness at their ends and are preferably tapered or doubly tapered at these ends, so that in these regions only a slight positive pressure is exerted, on the objects being connected.

Especially when the tubular coupling members of the present invention are to be used in hydraulic systems it is preferred that the minimum wall thiclaaess is such that radial expansion of the coupling under a given internal pressure, e.g. a high hydraulic pressure, is slightly less than the radial expansion under the same internal pressure of the hydraulic pipes being coupled. This ensures that the connection remains tight under high hydraulic pressures.

As used, herein the term "memory metal" means a metallic material from which a heat-recoverable article can be made, that is, an article which can be deformed from an original heat stable con iguration to a different configuration in which it will remain until raised above a certain temperature (in the present case, the transition temperature of the memory metal) when it will return, or attempt to return, towards its original configuration. It will be understood that the heat recoverable article is capable of returning towards its original configuration without the further application of outside force. It will further be understood that by the term "capable of expanding (contracting)" there is meant that the hollow article will expand (contract) - unless, of course, it is forcibly restrained from doing so - without the further application of an external force.

The transition temperature of the memory metal may be a temperature range and, as hysteresis usually occurs, the or lowered, Furthermore the transition temperature is a function of the stress applied to the material, the temperature rising with increasing stress* Amongst the metallic materials which can "be used in the present invention are, for example, the alloys disclosed in U.S. Patents Nos. 3, 012,882 and 3, 174, 35 and Belgian Patent No. 703,6 9, the disclosures of which are incorporated "by reference. As is made clear in these patents, these alloys undergo a transition at a certain temperature which in the case of the gold-cadmium and silver-gold-cadmium alloys described in U.S. Patent No. 3, 012,832 is simply referred to as a phase change. The other patents describe the transition which takes place in the disclosed alloys as one between austenitic (or high temperature) and martensitic (low temperature) forms of the material.

It is to be understood that the alloys disclosed in the references cited above are only exemplary of the memory metals which may be used in the present invention, and it is to be understood that the scope of the invention is in no way limited to these materials, nor to the materials specifically set forth as examples in this specification. Each of the materials tested has been found capable of being subjected to a secondary deformation sufficient to give useful spontaneous expansion to an article constructed in accordance with the invention.

A preferred article constructed in accordance with the present invention is a fitting, especially a coupling, for use in hydraulic systems, especially in aircraft. Present aircraft require hydraulic equipment to be capable of operating at temperatures within the range of from about -55°C to 125°C, but it is desirable to have equipment capable of standing over 500°C. For reasons which will he apparent, a heat recoverable metallic material for use in hydraulic line fittings must therefore have a transition temperature somewhat below -75°C.

Liquid nitrogen, boiling at -196°C, offers a convenient means of reducing temperatures of articles to its boiling point. Temperatures lower than -196°C\ can, of course, be obtained by other liquefied gases, but nitrogen offers the advantages of ready availability and comparative safety.

Preferred metals for use in this aspect of the present invention are, therefore, those which have a transition temperature within the range of about -196°C and -75°0· In addition to satisfying the environment requirements, the use of such a metal at normal temperatures assures that it is in its austenite form where it has greater strength.

Among metals suitable for the present invention there may be mentioned, by way of example, the following alloys, in which the percentages are atomic percentages and are approximate: No. 1 [Titanium 50 Nickel 47 Iron 3 No. 2 Titanium 9.25 to .00 Nickel 0 Aluminium 0.75-1 -00 No. J Titanium 48. 5 Nickel . 5 No. 4 Titanium 50 Nickel 48 Manganese 2 These alloys each have a transition temperature within the desired range. It will be appreciated that the alloys may contain incidental impurities provided that the impurities do not adversely affect the mechanical and physical properties of the material nor alter the transition temperature to such an extent that the article is rendered unsuitable for its intended purpose.

These titanium-nickel alloys (whether for a hydraulic making titanium alloys, either from their constituent elements or from alloys or compounds of the constituents in other proportions. Among such methods there may be mentioned by way of example consumable and non-consumable arc melting, vacuum induction melting in graphite crucibles, electron beam melting, powder metallurgical methods and floating zone melting.

The resulting alloy can readily be formed into suitable blanks for final manufacture of the articles by, for e ample, hot forming, swaging, hot extrusion, hot rod-rolling or hot forging, or, to a lesser extent, by cold forming, whatever method of shaping is adopted, the alloys are desirably annealed to ensure reproducibility of properties in the resulting article.

Aircraft hydraulic systems contain many lengths of pipe which have to be joined by couplings, tees, crossovers and the like and which terminate in a variety of pumps, valves, meters and operational equipment. At each such junction or termination, a fitting of some type will be required. For simplicity of description, a coupling for a simple butt joint between two pipes of the identical material and diameter will, now be described in more detail, but. it will be understood that the present invention provides fittings of configurations other than such a coupling.

A tube of a metallic material of the type referred to is expanded radially, i.e. is distorted at a temperature below the transition temperature of the material, for example, by forcing through the tube a mandrel having a diameter greater than the original internal diameter of the tube.

The degree of expansion preferably is great enough so that both thermally recoverable and secondary deformation (as explained hereinafter) take place. The tube is then approximately similar diameter. The tube will fit tightly on the mandrel or keeper because it will attempt to recover and contract radially toward its original configuration* At the appropriate time, the tube is again cooled to below its transition temperature* When the tube reaches the transition temperature, spontaneous expansio occurs, increasing the internal diameter of the tube and resulting in the tube being easily removable from the mandrel or keeper.

As long as the temperature of the tube is kept below the transition temperature, it will retain this new internal diameter, enabling the coupling to be placed in position over the pipes or other objects to be connected. These objects, . of course, must have an outside diameter smaller than the expanded diameter of the coupling, and slightly larger than the original internal diameter of the coupling.

After the coupling has been installed on the objects to be connected, the joint is heated to above the transition temperature of the metal of the coupling. The coupling being heat recoverable, it shrinks towards its original heat-stable configuration until it engages the objects and is restrained from further recovery. Because the recovery forces are substantial, the coupling makes an extremely tight fit on the objects so long as the joint is maintained above the transition temperature. The restraining action of the objects on the coupling, which prevents complete recovery of the latter, introduces further secondary deformation stresses (as explained hereinafter) into the coupling material and when the coupling is again cooled to the transition temperature it will undergo spontaneous expansion allowing the objects to be removed.

As is known, it is necessary mechanically to deform articles made from memory metals in order to realize useful heating and cooling a memory metal through its transition temperature without prior deformation will not give useful dimensional changes, \7e have further found that it is preferable to deform the article so that it cannot completely recover its original configuration, i.e. that it is advantageous to introduce a small amount of secondai'y deformation into the articles* Without in any way intending to restrict the scope of the present invention it is believed that the small amount of secondary deformation introduces stresses into the article which stresses can be relieved when the article is passed downwardly through its transition temperature. It is further believed that the release of the internal stresses in some way actuates the normal changes which occur at the transition temperature and thus makes them spontaneous and "snappier" and, consequently, more useful. Furthermore, it is believed that it is the internal stresses that cause the surprising reversibility of the articles of the present invention in that they are capable of expansion after contraction and vice versa.

Secondary deformation can be introduced in a number of ways, especially by expanding the article below the transition temperature and then heating it causing it to recover and contract about an object having a dimension greater than the corresponding dimension of the original high temperature form of the article.

Other methods which may be used include imposing a force on the article which exceeds that sufficient to impart the maximum thermally-recoverable deformation, and working the article above the transition temperature of the memory metal so that only secondary deformation is introduced.

Furthermore, in some cases intentional deformation may of the article* This may be the case, for example, if the manufactured article was not annealed. In general, however, annealing is preferred since this removes random residual stresses and ensures satisfactory results from subsequent working.

The introduction of secondary deformation, when, for example, a tubular coupling is caused to contract over a keeper, a mandrel or the pipes which are, to be connected, prevents complete recovery of the coupling thereafter. That is to say if the total deformatio is 9 units, 2 of which are secondary, the coupling can only recover 7 units if the secondary deformation is permanent i.e. completely non-recoverable. This means that the new heat stable configuration is effectively 2 units greater than the original configuration and therefore that the objects which are to be coupled must be dimensionally greater (by at least 2 units) than the original coupling.

In order to make the articles of the present invention as versatile as possible, i.e. for example to maximize the range of diameter of the pipes which can be coupled, it is obviously preferable that as great a possible percentage of the total deformation is recoverable, consistent with the need for at least some permanent deformation.

Y/e have found that the elasticity of most of the metals used in the present invention results in the loss of recoverable deformation being less than might have been expected.

Thus when a coupling is caused to contract about a mandrel so that secondary deformation is introduced, it has been found that generally the new heat-stable configuration lies somewhere between that defined by the mandrel and the original configuration. In other words, before permanent deformation restraining force is removed* (In the titanium-nickel alloys discussed above, the elasticity is about 2% so that the objects to be connected can have diaiaeters that much smaller than the diameter of the mandrel or keeper) , By properly selecting the material of which the mandrel or keeper is made, or its configuration, the total recovery, i.e. elastic plus thermal, of the coupling can be made to approach that which was originall3r built into it upon expansion.

The distance through which the coupling can be recovered can be increased, for example, by constructing the mandrel or keeper of a material having substantial elasticity. In such a case, the elastic limit of the coupling must be exceeded before permanent deformation takes place in the coupling. For this purpose, the keeper cotild be constructed of the same material as is the coupling, or of another material having substantial elasticity, for example, beryllium copper.

Alternatively, the keeper may be constructed of a material exhibiting a negative coefficient of thermal expansion. In this case, the keeper's diameter would contract as it was raised in temperature, allowing the coupling to recover to a greater degree than if the diameter of the keeper remained constant. To the same result the keeper could be constructed in the form of a cylindrical spring such as a conventional roll pin that could be collapsed by recovery of the coupling on it. Other alternative methods of providing the keeper with sufficient elasticity for the purposes described will be apparent to those skilled in the art and are intended to be included within the scope of the invention. Prom the foregoing description it can be seen that the present invention provides a metallic article useful for many temperatures and are quick and easy to install or replace, yet can withstand extreme temperature cycling.

Various forms of the present invention will now "be described in more detail, by way of example only, with reference to the accompanying drawings, in which Figure 1 is a cross-sectional view of one form of coupling according to the present invention; Figure 2 is an end view of the coupling of Fig. 1 ; Figure J is a cross-sectional view of the coupling of Fig. 1 positioned over a pair of pipes ; Figure is a cross-sectional view of the coupling of Fig. 1 after installation on the pipes; Figure 5 is a cross-sectional view of a second form of coupling according to the present invention; .

Figure 6 is a cross-sectional view of the coupling of Fi o 5 positioned over a pair of pipes; Figure 7 is a cross-sectional view of the coupling of Fig. 5 after installation on the pipes; and Figure 8 is a cross-sectional view of a length of pipe with a band of metal shrunk about its exterior* Referring to the drawings and particularly Figure , a coupling indicated generally by the reference numeral 10 was formed as described below. The outside surface of the coupling 10 comprises a central portion 12 of constant cross-section and two end portions 14 and 16 each tapering toward their respective ends 18, 20 of the coupling.

The inner surface, formed by the bore 22, comprises a central portion 24 of constant cross-section and flared end portions 26 and 28. Between each end portion and the central between the end portion 28 and the central portion 24). The teeth are each generally of saw tooth shape, the radial portion of each saw tooth facing the centre of the coupling* It has been discovered that the pull-out strength of such a coupling is substantially increased by proper spacing of the teeth along the interior wall of the coupling. The pull-out strength of the coupling is further enhanced when the tubing material to be joined is itself deformed by the coupling. Figures 5 through 8 illustrate such a coupling.

In Figure 5j a coupling 50 fabricated from a heat-recoverable metal has an outer wall having a cylindrical portion 51 and tapering portions 52 and 53 at each end. The interior wall has a series of spaced teeth 54— 6 and 58'-60 and a recess 57· Figure 6 shows the coupling 50 in an expanded (cooled) condition with tubes 61 and 62 inserted therein. The space between tubes 61 and 62 is centrally positioned in the recess 57.

Figure 7 shows the coupling after recovery on tubes 61 and 62. As can be seen, the tubes 61 and 62 are partially deformed by the teeth of the coupling. It should be understood that the illustrated deformation is exaggerated for purposes of clarity; in normal situations the deformation is considerably less than that illustrated. Typical deformation is, however, sufficient to significantly increase the pull-out strength of the coupling.

The optimum spacing "a" and minimum depth "b" of the teeth depend on the properties of tubes 61 and 62. One method of determining the dimensions "a" and "b" is shown in Figure 8. A ring 63 of the memory metal from which the coupling 50 is fabricated, and having the same recovered diameter, is shrunk around a length of tubing 64. Tubing 6 is fabricated of the movement of some of its material, the maximum outward deformation being indicated by "b" in Figure 8. The axial extent of the radial deformation is indicated by "a" in Figure 8 which thus is the distance from the point where radial deformation begins to the point where it ceases. This distance "a" is then used as the spacing between adjacent teeth as shown in Figure 6. The minimum depth "b" of the teeth in Figure 6 is preferably selected to be equal to or greater than the dimension "b" of Figure 8· The minimum coupling wall thickness "c" along the cylindrical portion 51 °f "the coupling, that is, the thickness or the wall in the area of the recess 57» is determined by the maximum expected pressure in the hydraulic line and by the relative elasticity of the material of tubings 61 and 62 and that of the coupling. The thickness "c" is preferably sufficient to withstand the maximum hydraulic pressure and to resist bulging of the coupling, and to withstand metal fatigue under operating conditions. Further, the thickness "c" is preferably such that the coupling 0 will tend to expand elastically under pressure less than tubing 61 and 62· Otherwise, when the joint is subjected to high pressure, the greater expansion of the coupling would cause leakage. By so designing the minimum wall thickness, coupling weight may be minimized.

The inside diameter of recess 57 is preferably sufficiently great so that the tubing ends do not touch the coupling at any point, particularly when subjected to vibration as fretting corrosion of the coupling by the tubing might occur resulting in a weakening of the coupling and ultimately its failure. The tapered end portions 5 and 53 of the coupling comprise another important feature of the The purpose of providing the taper to the inner surface 65 is to reduce the radial force exerted on the tubing at the ends of the coupling upon its recovery, A high radial force on the tubing at the ends of the coupling would tend to weaken the tubing at those points, particularly if the tubing was subjected to bending or vibrations. A. zero radial force would be theoretically ideal. However, it has been found that if the coupling ends are not tight around the tubing a rubbing can occur with vibration and this can lead to fretting corrosion and failure. Thus, the inside diameter at the coupling ends should advantageously be such that when the coupling shrinks about the tubing there will be a slight-positive pressure on the tubing but the pressure v;ill be as low as possible. The provision of the taper to the outer surfaces 66 further helps keep the pressure as low as possible because a thinner metal cross-section will exert less force than a thicker section. The tapered outer surface imparts flexibility to the coupling which tends to protect the tubing from the effects of bending or vibrations. This is parties ularly true in the case of titanium tubing. Advantageously, this reduction in outside diameter should be a substantial reduction. It has been found that an edge thickness on the order of one-fourth or one-fifth of the minimum wall thickness "c" of the coupling at the recess 57 is particularly effective.

It has been found that the teeth - 6, 58-60 are most effective when their biting edges approximate a right angle, preferably as close to radial as possible. These circumferential teeth need not be formed by tapered sections, however, and could be formed merely by forming a series of V-shaped or "ϋ" shaped ridges or the like in the interior of the coupling.

A coupling as shown in Figure 1 was constructed as of nickel (International Nickel 270), titanium- (Titanium Metals Corporation 25A) and iron (99·9% pure). The strips were cleaned to remove any dirt or grease, weighed and assembled in bundles such that the elements were in the ratio of alloy No, 1 above at each cross-section through the longitudinal axis of the bundle. The bundle was then hung in the chamber of a Lepel HCP-F floating zone unit. The chamber was evacuated, then filled with high purity argon to a pressure of 1 atmosphere; this procedure was repeated twice; after the third filling a pressure of +3 p,s.i. gauge was established and maintained during the melting to minimize air influx.

The lower end of the sample was heated by a single-turn induction coil attached to the secondary winding of a 12:1 load matching step down transformer, the primary being powered by a Lepel high frequency induction heating unit (Model T-10-3-DF-E-H) operating in the kilohertz range. Rapid melting resulted from the combination of induction heating and the heat of formation of the intermetallic compound TiNiQ ^ FeQ 0g. The falling droplets of alloy were collected in a cold copper mould, the bundle being fed into the induction coil until it had all been melted and collected in the mould. After cooling, the copper mould and drip-cast ingot were removed from the chamber, and the mould was stripped.

The dripped ingot, which was a semi-compact cylinder, was returned to the chamber and an argon atmosphere established as before, A molten zone was passed along the ingot from bottom to top at a rate of about 0,5 cm/minute using the floating zone technique to avoid possible contamination by a crucible. The product was a homogeneous, void-free bar of allojr about 2 cm. diameter, 12 cm, long.

An axial hole was drilled in the end of the bar of the of the central portion 12, and the blank cut off the end of the bar* The tapered end portion 26 was then formed on a lathe, using a lathe taper attachment. The attachment was then reset and a radial cut made, to form the outer tooth JO. A second taper was then started, and a second radial cut made, to form the inner tooth 32. The cut was then continued parallel to the axis to form half the central portion 2 . The coupling was then turned round, and the tapered portion 28, teeth 36 and 4- formed, and the remaining half of the central portion bored to size. The tapered portions 14- and 16 on the outer surface were then formed, the surfaces polished and de-burred.

The machined coupling was the heated in an inert atmosphere at 950°C for 0 minutes, cooled rapidly to 270°C to 300°C and maintained at that temperature for 2 hours, still in an inert atmosphere. The coupling could alternatively be treated initially for a longer period at a somewhat lower temperature, e.g. 2 hours at 850°C, and the second heating period can be at any temperature between 250°C and 300°C.

This treatment anneals the coupling and removes the internal stresses therefrom.

The coupling was then immersed in liquid nitrogen and a tapered mandrel, previously cooled to liquid nitrogen temperature, forced through the bore 22 of the coupling to expand it radially by 8.3 , enough to provide both thermally recoverable and secondary deformation of the coupling. The mandrel was removed, and a previously cooled rod or keeper having a diameter slightly less than the maximum diameter of the mandrel inserted in the bore. The coupling and its keeper were then allowed to return to room temperature, the coupling tightening onto the keeper as its temperature rose The ends of two stainless steel hydraulic pipes were cleaned and prepared for connection, a mark on each pipe indicating the correct extent of insertion into the coupling. The coupling was then removed from the liquid nitrogen, the keeper "being readily removed from its bore* As shown i Figure 3» the ends of the pipes 40 and 42 were then inserted into the coupling up to the indicated marks, and the assembly was left to warm up to room temperature. As shown in Figure 4, a firm junction "between the pipe ends was obtained, the teeth of the coupling biting into the pipes 40 and 42 and being slightly deformed in the process* The extent of the deformation depends on the relative hardness of the coupling compared to that of the pipes to be formed, (It is to be understood that the showing of Figure 4 is for the purpose of clarity and is not necessarily intended to be to scale.) By a procedure similar to that described above a coupling of Alloy No* 3 above was manufactured and applied to form a butt joint between two stainless steel hydraulic pipes of outside diameter 0,25 inch (0,655 cm,). The pipes were filled with petroleum base hydraulic fluid (conforming to MIL-H-5606B) , and subjected to the following tests, 1. The pressure in the system was maintained at 6,000 O p p.s,i» (4,14 x 10 dyne/cm ) for 5 minutes, then at 12,000 o p p.s.i, (0,28 x 10 dyne/cm ) for 5 minutes. No leak was observed at the end of either period. 2, The coupling was then subjected to the rotating beam test (conforming to LHL-P-18280B) with the hydraulic fluid at 3,000 p.s.i. (2,0? x 108 dyne/cm2) , being cycles at approximately 2,000 r.p.m. for 26,2 x 10 cycles. No leaks were observed, nor was any damage to the coupling visible. 4-, The system was tested in a mass spectrometer, with helium at atmospheric pressure outside the coupling, the inside of the system being evacuated. No helium was observed inside the system; the sensitivity of the spectrometer was 9. 5 x 10 1 std.atm.cc/sec· Couplings of Alloy Wo, 5 were applied to pairs of hydraulic pipes as described above. The following tests were carried out.

· One system was filled with hydraulic fluid at a o pressure of 6, 000 p.s.i. (4.14-) x 10 dyne/cm ). The temperature of the system was cycled between -55°C and +175°C 29 times, each cycle occupying about 2 hours. No leak was observed. 6. The other systems were tested to destruction by increasing the internal pressure of hydraulic fluid. In each case, the stainless steel tubing burst before any leak took place at the coupling, the pressures varying between 15> 000 and 25, 000 p.s.i. (1.04 x 109 and 1.72 x 109 dyne/cm2).

All the tests described above show that the invention provides a reliable lealcproof hydraulic coupling which can be stored and transported at normal temperatures and which can be quickly and easily installed at conveniently obtainable low temperatures without the use of bulky and complex equipment and by people of relatively little skill. The strength of the coupling can be further increased by suitable selection of the materials and by the incorporation of advantageous design features.

Although the couplings will in general be stored on keepers, as described above, it is also possible to store them without keepers, relying on their expansion on cooling to obtain sufficient clearance to install them on the articles,

Claims (2)

1. 35091 /2 CLAIMS : 1 o A tubular coupling member made from a memory metal and provided on its inner surface with one or more inwardly proj ec teeth, which coupling member is capable of contracting radiall on being heated above the transition temperature of the metal.

2. A tubular coupling mes¼er as claimed in claim 1 , which is provided with a series of axially separated continuous circumferential teeth. 3· A tubular coupling member as claimed in claim 2, where! the teeth are formed from a plurality of tapered sections. 4. A tubular coupling member as claimed in any one of claims 1 to 3t wherein the biting angle of the or each tooth is almost a right angle* 5· A tubular coupling member as claimed in any one of claims 1 to 4» wherein one edge of the biting angle lies approximately in a radial plane of the coupling member. 6 o A tubular coupling member as claimed in any one of claims 1 to 5» which is provided with a recess of increased internal diameter about mid-way along its length. 7. A tubular coupling member as claimed in any one of claims 1 to 6, which is provided with a portion of decreased external diameter at each of its ends. 8» A tubular coupling member as claimed in claim 7, which tapered at each of its ends. 9. A tubular coupling member as claimed in claim 8, which doubly tapered at each of its ends. 10. A tubular coupling member as claimed in any one of claims 1 to 9» wherein the wall thickness about mid- way alcng its length is from 4 to 5 times greater than the wall thic-s e; < . ■ . 35091/2 1 1 . A tubular coupling member as claimed in any one of claims 1 to 10 , which is being maintained in its low temperature form by immersion in liquid nitrogen. 12. A tubular coupling member as claimed in any one of claims 1 to 1 1 , wherein the transition temperature of the memory metal lies in the range of from -196° C to -75°C. 13· A tubular coupling member as claimed in any one of claims 1 to 12, wherein the memory metal is an alloy of about 50 atom percent Ti, about 47 atom percent Ni and about 3 aton percent Fe. 14. A tubular coupling member as claimed in any one of claims 1 to 12, wherein the memory metal is an alloy of about 50 atom percent Ni, about 0.75 to about 1 .0 atom percent Al and correspondingly about 49.25 to about 49 atom percent Ti. 15. A tubular coupling member as claimed in any one of claims 1 to 12, wherein the memory metal is an alloy of about 51 .5 atom percent Hi and about 48.5 atom percent Ti. 16. A tubular coupling member as claimed in any one of claims 1 to 12, wherein the memory metal is an alloy of about 2 atom percent Mn, about 48 atom percent and about 50 aton percent Ti. 17o A tubular coupling member as claimed in claim 1 , substantially as described herein with reference to , and as illustrated in, the accompanying drawings. 18. A method of making a connection between two or more *" pipes which comprises positioning the pipes within a tubular coupling member 'as claimed in any one of claims 1 to 17/ and causing the coupling member to contract radially and grip the pipes by raising its temperature above the transition t emperature. 39051/2 19· A method as claimed in claim 18, wherein the coupling member has ends of reduced thickness and exerts only a slight positive pressure on the pipes at its ends. 20. A method as claimed in claim 18 or claim 19, wherein the coupling member has a recess of increased internal diameter about mid-way along its length, and the connection is made BO that the abutting ends of the pipes meet within this recess so that the coupling member does not contact the abutting ends in this region. 21. A method as claimed in any one of claims 18 to 20, wherein the coupling member is provided with teeth formed from a pluralit of tapered sections, the angle of each tapered section being sufficient to permit and accommodate all the free natural expansion of the pipes caused by the contracting of the coupling member about the pipes. 22. A method as claimed in any one of claims 18 to 21 , wherein the distance between two adj acent teeth of the coupling member is chosen to be approximately equal to the axial deformation of the pipes caused by one of the teeth upon the contracting of the coupling member about the pipes. 23. A method as claimed in any one of claims 18 to 22, wherein the minimum wall thickness of the coupling member is chosen so that under a given internal pressure the radial expansion of the coupling member will be slightly less than the radial expansion of the pipes. 24. A connection between two or more pipes made by a method as claimed in any one of claims 18 to 23· For the Applicants DR. REINHOLO COHN AND PARTNERS