GB2123735A - Connectors - Google Patents

Abstract

A method of mounting a connector unit 3 on an elongate member 1, said connector unit comprising at one end a member having a tubular portion 4 having a bore dimensioned and sectioned to receive an end portion of said elongate member and having a further connecting portion 6, said method comprising the steps of providing the interior of said tubular portion and/or the exterior of the end portion of the elongate member with an adhesive, inserting the end portion of the elongate member into the bore of the tubular member and crimping the tubular portion so that the tubular portion firmly embraces the end portion of the elongate member, and so that a film of adhesive remains between the end portion of the elongate member and the embracing tubular portion, the crimping of the tubular portion being made along only part of the length of the region of that portion in which the elongate member is inserted, the end 21 of the tubular portion further from the further connecting portion 6 being uncrimped. Preferably, the tubular portion is aluminium and the elongate member is a fibre reinforced rod. <IMAGE>

Description

SPECIFICATION Connectors The present invention relates to connectors.

Rods, tubes and other sections made of fibre reinforced materials, containing fibres such as carbon fibres, glass fibres and polyaramid fibres and hybrids of them in matrices such as phenolic and polyester resin materials, are commercially available. These sections have a high axial strength, this axial strength being greater than or equal to that of steel. The carbon fibre reinforced sections also have a stiffness which is equal to the stiffness of steel, but the density of the sections is approximately one quarter the density of steel.

One problem that has been encountered with such sections is the problem of connecting the sections together in the formation of a framework or structure. It is to be appreciated that there are many possible applications and uses for such sections in civil and mechanical engineering, but in most such applications it is necessary to connect the sections together, or to connect the sections to other fixed points.

It has been proposed to utilise connecting fixtures involving fibre reinforced sections which incorporate bolts, holes being drilled or moulded in the ends of the sections to accommodate the bolts to secure the fixtures to the sections. However, when an axial force is applied to such a fitting the bolt tends to pull out of the end of the section, removing a plug of the material from the end of the section.

This applies particularly when the reinforcing fibres are oriented and arranged to lie primarily along the axis of the tube.

Fibre reinforced sections may be bonded together but the operation of bonding a closed section to another closed section is complicated because of the difficulty of applying the bonding agent uniformly over the areas to be glued. This can be achieved by having a very loose fit between the two components but the problem here is that on completion of the bonding operation a thick bond line is developed which is undesirable because of the weakness in the tensile strength of this thick bond line joint.

Published United Kingdom Patent Application No. GB 2053766A describes and claims the most suitable method which has been found to date for connecting elongate members such as rods and tubes of reinforced materials together or to elongate members of other materials. Connection is made by a connector unit comprising a member having a tubular portion mounted on at least one elongate member formed of reinforced plastics material. The tubular portion, which may for example be of aluminium alloy, is dimensioned and sectioned to receive one end of the elongate member and has a further connection portion. This portion, e.g. containing a hole for a bolt or containing a screw thread is for connection to another member, e.g. another fibre reinforced member having an elongate member having mounted on it a connector unit connecting with the first mentioned connector unit.The method of mounting the or each connector unit on its elongate member comprises the steps of providing the interior of said tubular portion and/or the exterior of the end of the elongate member with an adhesive, inserting the end portion of the elongate member into the bore of the tubular member and crimping the tubular portion so that the tubular portion firmly embraces the end of the elongate member, and so that a film of adhesive remains between the end portion of the elongate member and the embracing tubular portion. In contrast to a joint which is bonded only a crimped and bonded joint formed in this-way has a thin bond line and thus provides greater efficiency.

In the examples specifically described in the said Application the method of mounting the tubular portion of the connector unit on the fibre reinforced elongate member involves crimping the tubular portion of the connector unit fully along the entire length of the region of that portion in which the elongate member is inserted, i.e. fully along the ove#rlap region between the elongate member and the tubular portion.

We have now found that the resultant connection or joint obtained from the method of mounting specifically described in the examples of the said Application is not totally satisfactory in terms of its load bearing properties.

Accordingly, the purpose of the present invention in one form is to improve the load bearing properties of a connector unit mounted on an elongate member formed of reinforced plastics material in the manner described in UK Patent Application No. GB 2053766A.

According to the present invention in a first aspect there is provided a method of mounting a connector unit on an elongate member, said connector unit comprising at one end a member having a tubular portion having a bore dimensioned and sectioned to receive an end portion of said elongate member and having a further connecting portion, said method comprising the steps of providing the interior of said tubular portion and/or the exterior of the end portion of the elongate member with an adhesive, inserting the end portion of the elongate member into the bore of the tubular member and crimping the tubular portion so that the tubular portion firmly embraces the end portion of the elongate member, and so that a film of adhesive remains between the end portion of the elongate member and the embracing tubular portion, the crimping of the tubular portion being made along only part of the length of the region of that portion in which the elongate member is inserted, the end of the tubular portion further from the further connecting portion being uncrimped.

We have discovered that by terminating the crimping of the tubular portion before the end of the tubular portion as specified above, in accordance with the method of the first aspect of the present invention, it is possible to improve significantly the load bearing properties of the resulting connection or joint. Examples of the improvement which may be obtained are described below.

Preferably, the crimping extends at least half way along the region of the tubular portion in which the elongate member is inserted, i.e. the overlap region, and preferably along between 70% and 95% of the overlap region, the uncrimped end forming between 5% and 30% of the length of that region.

Although an improvement in the load bearing properties will in general be obtained wherever the crimping is terminated there may be a point, along the length of that region of the tubular portion in which the elongate member is inserted, i.e. along the overlap region, where a maximum improvement may be obtained. The exact location of this point will depend on a number of factors including the dimensions and elastic moduli of the tubular portion and of the elongate member and the distance of the overlap between the two. However, it will be apparent to those skilled in the art that for a given joint construction the point of termination of the crimping on the tubular portion which gives best load bearing properties may be determined by experiment or by theoretical analysis, e.g. in the ways we have used as described below.

As the end part of the tubular portion remote from the further connecting portion is uncrimped it will flare out from the point at which the crimping is terminated. Adhesive will be squeezed out by the crimping to occupy this flared part of the tubular portion.

According to the present invention in a second aspect there is provided a connector unit mounted on an elongate member by the method of the first aspect, the connector unit comprising a tubular portion, the bore of which accommodates one end portion of the elongate member, there being a film of adhesive between the said end portion of the elongate member and the interior of the tubular portion, the tubular portion having been crimped firmly to engage the end portion of the elongate member, the crimping of the tubular portion extending along only part of the length of the region of that portion which accommodates the end portion of the elongate member, the end of the tubular portion remote from the further connecting portion being uncrimped.

As in the connection or joint of U.K. Patent Application No. GB 2053766A the elongate member used in the present invention is preferably a cylindrical member of circular section, although an elongate member of non-circular section, such as square, rectangular or hexagonal section may be used. The elongate member may be a solid rod, or may be hollow, thus comprising a tube. The elongate member may be of uniform section, although alternatively the elongate member may be of non-uniform section.

Thus the elongate member may taper or flare, or may even change the shape of its section. The elongate member may be formed of a fibre reinforced plastics material or other stiff, strong, lightweight material such as aluminium.

If the elongate member is formed from fibre reinforced plastics material the reinforcement fibres may be of any suitable known material, e.g. carbon, glass, polyaramid, polyamide, polyolefin or blends or hybrids of these, e.g. a glass-carbon hybrid.

The matrix material of the fibre reinforced material may be any suitable known material, e.g. a polyester.

Preferably the elongate member includes a pultruded rod or tube of the fibre reinforced material.

Advantageously the elongate member comprises a carbon fibre and/or glass fibre reinforced epoxy, phenolic or polyester material.

For an aluminium alloy tube having an outer diameter of 37 mm and a thickness of 3 mm and mounted on a glass fibre reinforced polyester tube having an outer diameter of 25 mm and a thickness of 2 mm by the method of the first aspect of the present invention, the overlap between the two tubes is preferably between 50 mm and 75 mm and the crimping on the alloy tube is preferably terminated approximately 10 mm from the end of the alloy tube (nearer the free end of the fibre reinforced tube), the remainder of the tube along the overlap distance being crimped.

As in the invention of UK Application No. GB 2053766A the said tubular portion of the connector unit preferably comprises an aluminium tube or aluminium alloy tube, or a steel or stainless steel tube, when a joint of high strength is required, or a titanium or titanium alloy tube. When carbon fibre reinforced members are to be inter-connected to form a structure that is to be used in a salty atmosphere e.g. in a sea borne vehicle such as a ship or hovercraft, the use of stainless steel and titanium may be preferred in order to minimise electrolytic corrosion. It is normally preferred, however, to utilise medium strength aluminium alloys such as those known under the designations "HT30", "HE30", "HT1 5TF" and "L105", which latter alloy is equivalent to "HT15TB".

Advantageously the adhesive is a structural adhesive comprising an adhesive having a minimum shear strength of 20 mega Pascals, as defined herein, and advantageously the adhesive has a shear strength of approximately 30 mega Pascals as defined herein as follows.

The shear strength of an adhesive is measured by adhering together the ends of two strips of metal, each strip being 2.54 centimetres wide, the ends being adhered together so that there is a 1.27 centimetre overlap. When the adhesive is fully cured the two strips are pulled apart so that the joint is placed under shear strain, and the tension that must be applied before the joint breaks divided by the bonded area is utilised at the measure of shear strength of the adhesive.Shear strength measured in this way is termed "shear strength as defined herein." For the bonding of an aluminium tube (part crimped along its length) to a fibre reinforced plastics member the adhesive that is used should preferably be a cold or warm curing (setting) adhesive, that is to say an adhesive curing or setting at a temperature of less than about 700C, rather than a hot curing adhesive which may cure at about 1 200C to 1 500C.

It will be appreciated by those skilled in the art that it is preferred to avoid the use of high temperature curing adhesive when bonding light aluminium alloys to fibre reinforced members, since the large coefficient of expansion of such alloys would lead to glue line stresses or subsequent cooling to ambient temperature after the adhesive has been cured. However, if the connector unit is formed of a material having a low co-efficient of thermal expansion, such as steel or titanium, it may then be preferable to use a hot curing adhesive, such as "Hysol EA9312" as sold by Hysol-Dexter of the United States of America, since such an adhesive may provide a stronger bond than a cold curing adhesive.

In one embodiment of the present invention the said further connecting portion of the connector unit comprises a second tubular portion which protrudes beyond the tubular portion which protrudes beyond the tubular portion that is crimped, which second tubular portion is flattened after the crimping step to form a connecting portion that may accommodate bolts or the like. However, in alternative embodiments of the present invention the further connecting portion of the connector unit may comprise a second tubular portion having a bore dimensioned to receive the end of a second elongate member and adapted to be crimped, with adhesive, in a corresponding manner, on to said second elongate member. The connector unit thus serves to connect two elongate members together.

In further embodiments of the present invention the further connecting portion may be threaded and may comprise an externally or an internally threaded boss.

In a further embodiment of the present invention, in which the said elongate member comprises a tube, a rod is additionally provided, the rod being dimensioned to have one end inserted into the tube, the exterior of the rod and/or the interior of the tube comprising the elongate member being coated with adhesive and the said one end of the rod being inserted into the end of the tube, the exterior of the tube and/or the interior of the tubular portion of the connector unit being coated with adhesive, the end of the tube comprising the elongate member being inserted into the tubular portion of the connector unit, the said tubular portion of the connector unit subsequently being crimped.Advantageously, a tubular end portion of the connector unit may subsequently be flattened to form said further connecting portion, the rod extending through this flattened portion and also being at least partially flattened. Preferably the other end of the said rod is at least partially flattened before the said one end of the rod is inserted in the tube, this said other end of the rod extending through the end portion of the tubular member that is flattened.

Preferably where a glass fibre and/or carbon fibre reinforced thermosetting material is utilised to form the elongate member, the end of the elongate member to which the connector unit is to be secured is initially abraded and cleaned. Preferably the end of the member is abraded with silicon carbide paper, the end of the member being abraded under wet conditions to minimise the creation of dust, since the dust generated when abrading glass fibre and/or carbon fibre reinforced epoxy resin material under dry conditions might be injurious to the health of personnel.

Preferably the cleaning of the material is performed by utilising acetone or other ketonic solvent, and the end of the elongate may be wiped with a tissue that has been soaked in acetone until no further dirty marks are added to the tissue.

Where an aluminium or aluminium alloy tubular portion is to be crimped or flattened it is preferred that the method comprises the step of initially solution heat treating and quenching the tubular portion.

Where the connector element comprises aluminium or an aluminium alloy it is preferred that the aluminium or aluminium alloy is initially treated to provide a surface that will strongly bond to the adhesive, by vapour degreasing, subsequently etching in a mixture of chromic and sulphuric acid, washing a plurality of times in running water and finally washing with de-ionised water to provide a surface comprising aluminium oxide.

The tubular portion or portions may be partially crimped along their length by placing the part requiring crimping between two matching dies each having an arcuate recess having a diameter equal slightly less than the outer diameter of the tubular portion after crimping. The part of the tubular portion to be uncrimped overlaps the end of the dies at the point where the crimping is to be terminated. A press is then used to press together the two dies to provide the crimping of part of the tubular portion.

In order that the present invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of a tube on which a connector is to be mounted, and a tubular connector unit; Figure 2 is a perspective view of a completed connector formed from the components shown in Figure 1; Figure 3 is a cross sectional view taken on the lines Ill-Ill in Figure 2; Figure 4 is a partly cross-sectional view on the lines lV-IV in Figure 3; Figure 5 is a perspective view of a second embodiment of a connector element mounted by a method in accordance with the invention;; Figure 6 is a perspective view of a third embodiment of a connector element mounted by a method in accordance with the invention; Figure 7 is a perspective view of a fourth embodiment of a connector element mounted by a method in accordance with the invention, and Figure 8 is a second perspective view, partly cut away, of the embodiment shown in Figure 7.

Figure 9 is a graph showing normalised shear stress distribution curves for three types of joint A, B and C (one embodying the invention) having an alloy tube mounted on a fibre reinforced member by crimping and bonding.

Referring to Figure 1 of the accompanying drawings a member 1 on which a connector unit is to be mounted comprises a cylindrical elongate member 1 formed of carbon fibre reinforced epoxy resin.

The elongate member 1 is a circular cross-section tube. The end of the member 1 is first abraded with wet silicon carbide "wet and dry" abrasive paper. The paper is kept wet to minimise the creation of dust.

The abraded end of the rod is then cleaned of dust by wiping the end of the rod with tissue soaked in acetone. The wiping is repeated until no further black marks are added to the tissue on subsequent wiping of the rod.

A tube 2 is utilised to form the connector unit, the tube being formed of an aluminium alloy. If a connector of medium strength is to be provided the alloy is "HT30" alloy, whereas if a high strength connector is to be made the alloy is "L105" alloy which is equivalent to "HT" 1 5 Tb" alloy.

Since the aluminium alloy tube 2 is to be crimped or flattened, as will be described, preferably the tube 2 is solution heat-treated to make the tube relatively soft, the tube being heated to 5300C, if the low strength aluminium alloy is used, and to 5050C if the high strength aluminium alloy is used the tube 2 then immediately being quenched in cold water.

The tube 2 is then prepared by Method O of United Kingdom Defence Standard 03-2. That is to say the aluminium alloy tube 2 is initially subjected to a vapour de-greasing process and then the aluminium alloy is etched in a mixture of chromic and sulphuric acid, the etching bath being maintained at the relatively low temperature of approximately 550C. The aluminium alloy is then subjected to two running water washes and a final de-ionised water wash. The aluminium alloy then has a stable surface of aluminium oxide to which adhesive will bond well.

The exterior of the fibre reinforced member 1 and the interior of the aluminium alloy tube 2 are then coated with a high strength epoxy resin e.g. the resin sold by CIBA-Geigy under the Code No. 2005, which sets at room temperature.

Referring to Figures 2 and 4, after the end of the fibre reinfoced member 1 and the interior of the aluminium alloy tube 2 have been coated with adhesive the member 1 is inserted inside the tube 2 with an overlap distance of at least 50 mm.

The bore of the alloy tube 2, is, of course, dimensioned to receive the end of the fibre reinforced member 1 freely, the interior diameter of the fibre reinforced member 1.

Subsequently the alloy tube 2 is crimped onto the fibre reinforced member 1 along part of the overlap distance between the two the alloy tube 2 being crimped by two dies (not shown) each having an arcuate recess therein, the radius of curvature of each arcuate recess being slightly less than the exterior radius of the uncrimped alloy tube 2. In order to leave an uncrimped region at the end of the alloy tube 2 a region 21 is allowed to protrude from the ends of the dies.

The two dies are forced towards each other, thus crimping the alloy tube 2 on to the fibre reinforced tube 1 as shown at 3 in Figure 2, the alloy tube 2 thus forming two lateral seams 4. The pressure that is applied during the crimping is merely sufficient to deform alloy tube 2 so that the alloy tube 2 is in intimate contact with the adhesive coated fibre reinforced tube 1, and preferably the pressure that is applied is such that the thickness of the adhesive 5 between the fibre reinforced tube 1 and the crimped alloy tube 2 in between 100 and 200 ztm.

As the crimping terminates at the point where the end of the alloy tube 2 protrudes from the dies, which point is labelled 20 in Figures 2 and 4, the uncrimped region labelled 21 flares out towards the end of the tube 2 from the point 20.

During the crimping stage excess adhesive is squeezed out towards the ends of the alloy tube 2 and occupies the space between the flared, uncrimped region 21 and the fibre reinforced tube 1.

Excessive adhesive squeezed out of the ends of the alloy tube 2 is removed. The adhesive is indicated by reference numeral 5 in Figures 2 to 4. The adhesive 5 is then allowed to set or cure.

The end of the alloy tube 2 that protrudes beyond the crimped region 3 at the end distant from the Region 21 is stamped flat to form a further connector portion 6. The stamping is preferably preformed so that the resultant connector unit has a gradual change of section from the substantially circular cross section of the portion embracing the fibre reinforced tube 1 to the flattened portion 6 to avoid any undesirable stress concentrations when the connector is in use.

The flat connector portion 6 may be drilled to provide a hole (not shown) that may accommodate a bolt or the like (not shown).

Turning now to Figure 5 of the drawings it is to be appreciated that the invention also relates to a connection between two rods or tubes and Figure 5 illustrates two fibre reinforced tubes 1, 7 that have been connected together by a connector comprising a tube, the two reinforced tubes 1, 7 being prepared and inserted with appropriate use of adhesive 5 into the two opposed ends of an alloy tube prepared as described above, the two ends of the alloy tube 2 being crimped on to the respective fibre reinforced tubes 1, 7 to form crimped regions 3,3'.

The regions 3, 3' do not extend to the free ends of the alloy tube 2 but terminate at points 20, 20' to form flared regions 21, ' respectively at the ends in the manner described above for the region 21 shown in Figures 2 and 4.

Figure 6 illustrates a further embodiment of the invention where the connector unit comprises a tubularportion that is crimped on to the fibre reinforced tube 1 has, instead of the flat portion 6, an externally screw threaded boss 8, which may be a solid boss or a hollow tubular boss.

The flared region 21 at the end of the crimped region 3 of the alloy tube 2 is formed in the manner described above for the region 21 shown in Figures 2 and 4.

In a further embodiment, instead of the boss 8 shown in Figure 6, there may be a hollow internally threaded boss (not shown) provided on the alloy tube 2.

Figures 7 and 8 illustrates a further embodiment of the invention where the connector is to be utilised in connection with a tube 10 of carbon fibre reinforced epoxy resin material.

As can be seen in Figures 7 and 8, in the illustrated connector a rod 11 made of aluminium alloy HE30 having a diameter slightly less than. the interior diameter of the tube 10 of carbon fibre reinforced material is provided, this alloy rod 11 being coated with adhesive 14 and being inserted into the tube 10 of carbon fibre reinforced material.

Subsequently an aluminium alloy tube 33 containing a tubular portion 13 located in position in a manner corresponding to that described with reference to Figure 2, there being adhesive 14 between the exterior surface of the carbon fibre reinfoced tube 1 0 and the interior of the tubular portion 13 of the alloy tube 33. Subsequently the tubular portion 13 is subjected to the crimping process as described above to form seams 1 5 which terminate at a point 34 on the portion 13 to form a flared region 35 in the same manner as the region 21 shown in Figures 2 and 4.

The end of the tube 13 distant from the flared region 35 is then substantially flattened, to form a flattened portion 16, this flattening process also flattening the part of the central rod 11, which extends through the flattened portion 16. It is preferred that the entire space within the flattened portion 16 surrounding therod 11 is filled with adhesive 17. It is also preferred to flatten the portion of the rod 11 that is to be located within the flattened portion 16. It is to be appreciated that the central rod 11 will provide an extra load bearing glue line area, thus significantly increasing the strength of the bond between the connector unit and the fibre reinforced tube 10.Also the flattened portion of the rod 11 may be penetrated by a bore for a connecting bolt so that the connecting bolt is firmly connected in a load bearing relationship not only to the alloy tube 2 but also to the alloy rod 11.

It is to be noted that it has been found desirable to crimp a tube forming a connector as described above before flattening the end thereof to form a further connector portion.

In all the above described embodiments of the invention the fibre reinforced tube used may be replaced by a fibre reinforced rod. However, this may make it desirable to use stronger materials for the connector unit so the connector unit will have a strength that closely matches the strength of the rod.

Thus, in such an application, the use of steel, titanium or titanium alloy tube may be desirable for use to form the connector unit.

It is to be appreciated that in utilising a tube, rather than a rod, of fibre reinforced material in the above described embodiments of the invention, there is a slight possiblility that the fibre reinforced tube may be damaged during the crimping process since, during the crimping process, a certain pressure will be transmitted to the tube on the interior of the crimped portion of the connector. However, using the construction of the embodiment illustrated in Figures 7 and 8 may help avoid this.

It has been found that using the above described methods light and strong connections can readily be made with fibre reinforced plastics tubes and rods.

Whilst the invention has been specifically described with reference to embodiments in which the elongate member of fibre reinforced material is a tube or a rod of circular section, the invention may be used with tubes or rods of sections which are non-circular, e.g. square, rectangular or hexagonal, although other shapes may be equally useful. Also it is envisaged that elongate fibre reinforced members of non-uniform cross section may be used. Thus, for example the cross section of the elongate member may taper or flare, or the cross section may change shape. Thus for example a circular section rod may have a square sectioned end region, the square sectional region and a portion of the circular sectional region being embraced by the connector unit.In each case the connector unit will have a tubular portion dimensioned and sectioned to receive the end of the elongate member so that the tubular portion may subsequently be crimped to mount the connector unit on the elongate member, the crimping terminating before its end to form a flared region on the elongate member.

Using the threaded external boss construction shown in Figure 6 we have investigated the improvement which may be obtained in providing the flared region 21 instead of crimping to the end of the alloy tube 2 (in the crimping and bonding process) as described in UK Application No GB 2053766A. In our investigation the elongate members were tubes of the trade name Hypul manufactured by BTR Permali Ltd of Gloucester, England. These are manufactured from E glass fibre and polyester resin. They have a fibre/matrix weight ratio of 60/40, an outside diameter of 25 mm and a wall thickness of 2 mm.

The tubes used to form the connecting portions were of drawn aluminium alloy HT 30 TF to British Standard No 1471. They had an outside diameter of 37 mm and a wall thickness of 3 mm. Prior to the crimping operation the aluminium alloy tubes were solution treated at 5300C for one hour and then quenched in cold water. This operation prevented cracking of the material in the regions of high deformation during the crimping process. A wet and dry abrasive clean of the alloy was followed by a degrease soak in 1,1,1 -trichloroethane for 15 minutes.

The adhesive used was that sold by CIBA-Geigy under Code No. 2005.

The fibre reinforced tubes were roughened before application of the adhesive to their surface.

Two identical aluminium alloy tubes were each mounted on two identical adhesive coated fibre reinforced tubes in the manner described with reference to Figure 2 above to provide two experimental samples of joint. In each case the overlap distance between alloy tube and fibre reinforced tube was 60 mm. The alloy tubes were then crimped. One was crimped along the entire overlap distance as is known in the prior art. The other was crimped for 50 mm along the overlap distance, the 10 mm at the end of the alloy tube on the fibre reinforced tube being left uncrimped and allowed to flare. The adhesive was allowed to cure and bond the tubes of each sample joint.

The failure loads of the two joints so formed were then measured under identical conditions.

The two experimental joint samples were placed in turn in a tensile testing machine and a tensile load was applied at the rate of 0.10 m/min to both the joints until failure of the joints or their fibre reinforced tube occurred.

We found that the sample joint having a fully crimped overlap portion in accordance with the prior art failed at 36 kN whereas the sample joint embodying the present invention which had an uncrimped and flared region on the end of the aluminium tube mounted on the fibre reinforced tube failed at 42 kN.

Up to a maximum load at which the joint components fail we have found that the failure load increased roughly linearly with overlap distance between aluminium alloy tube and fibre reinforced tube.

Short overlap distances (e.g. > 50 mm) are therefore preferably avoided although in general for each overlap distance the breakdown load is higher if the end of the aluminium alloy tube is uncrimped and flared.

For a joint having the crimped region extending to The end of the aluminium tube we have found that the overlap distance needs to be about 75 mm to obtain a failure load figure of 42 kN (which is equal to the maximum failure load), the failure load obtained with the sample joint embodying the present invention.

Thus, use of the invention allows the same failure load as the prior art but with a considerably shorter overlap distance, thus providing a considerable saving in materials.

We have also investigated crimped and bonded joints by theoretical analysis to see whether load bearing properties should be improved by using methods embodying the present invention.

The theory we used is the so-called "finite element" theory, a method which has been used previously by those skilled in the art to undertake parameter investigations in tubular jointing problems.

According to this theory the joint is analysed as an axisymmetric body. The general finite element method has been fully described in the following references and will not be repeated in detail, here. The references are: "Concepts and Applications of Finite Element Analysis" by R D Cook, published by J Wiley 8 Sons Inc (1973); and "The Finite Element Method" by O C Zienkiewicz, published by McGraw Hill Book Company (1977).

An eight node quadrilateral element as a solid of revolution is used to model the axisymmetric body. The element stiffness formulation is similar to a two dimensional plane problem except that the volume integration is over the solid of revolution.

The finite element method reduces a structure to an assemblage of individual elements each connected at nodal points. Body forces and displacements within the loaded structure are transmitted through this network of nodal points, stress and strain distribution being obtained from the element nodal displacements.

For a joint comprising a crimped and bonded aluminium alloy tube mounted on a fibre reinforced elongate member the alloy tube may be externally threaded as described above with reference to Figure 6. In the case of skeletal structures made using one or more such joints, it is necessary, in order to investigate stress distributions, to model the load transfer through the connection but this is complicated by the threaded joint on the alloy tube. In order to simplify matters a linear constraint is introduced and the inner and outer components of the threaded joint are treated as separate finite element structures.

The stress analysis assumes a linear material stress strain relationship and geometric load deflection behaviour. That is, adherend or adhesive yielding does not occur and the joint deformations are assumed to be negligibly small. These assumptions will not be valid if a prediction of ultimate joint strength is required; however, the linear analysis does indicate joint efficiency and gives the magnitude and positions of stress concentrations, before yielding of components.

Three variations of crimped and bonded joints were investigated in the analysis; all three have a threaded aluminium alloy tube: type A: an aluminium alloy outer tube crimped and bonded to a pultruded glass fibre reinforced rod or tube the outer tube being crimped and bonded (in accordance with the prior art) along the entire overlap length, taken as 70 mm, between the fibre reinforced rod and the alloy tube; type B: a constructon as in type A but with a pultruded hybrid glass/carbon fibre reinforced rod instead of the glass fibre reinforced rod; type C: a construction as in type A using a glass fibre reinforced rod but in which the crimping on the alloy tube terminates 10 mm from the unthreaded end of that tube to provide a free end.

The known material properties which were assumed for our analysis are as listed in Table 1 as follows.

TABLE 1 MECHANICAL PROPERTIES OF COMPONENTS OF JOINTS

Pultruded Puitruded Mechanical Glass Fibre Glass Carbon Fibre Adhesive Property reinforced hybrid reinforced (Epoxy Parameters Aluminium tube tube resin) E11N/mm2 69000 5000 10000 3000 E22 N/mm2 69000 21500 65000 3000 G12N/mm2 26136 10192 28846 1111 s11 0.32 0.3 0.3 0.35 v22 0.32 0.3 0.3 1 0.35 where E represents modulus of elasticity G represents modulus of rigidity v represents Poisson's ratio and the suffixes 1. 2 denote upper (alloy tube) and lower (reinforced rod) adhered respectively.

The glue line thickness of the adhesive in the crimped joints is not constant but has a maximum value where the crimped flanges are formed and a minimum one at +90 (around the circumference) to these flanges. As the system was assumed to be axisymmetric it was necessary to use a constant glue line thickness and a value of 0.15 mm was considered: this value was measured in the experimental tests. It has been shown experimentally that variations in glue line thickness has negligible effect on joint performance as failure usually occurs, at the joint in the composite adherend, as a fibre/matrix interface failure.

The relatively long overlap length of 70 mm was used in the investigation as it represented the typical length required to approach the ultimate load capacity for the pultruded tube. A large overlap was necessary because of the weak interlaminar shear strength of the pultruded tubes.

From the analysis we carried out in the way indicated above we found firstly that the most important and critical feature of the three types of joint studied i.e. types A, B and C as specified above, is the shear stress distributions in the adhesive along the length of overlap between the fibre reinforced rod and the outer alloy tube; that secondly there is a high stress concentration near the free (unthreaded) end of the alloy tube (which is uncrimped and flared in the case of the type C joint); and that thirdly this stress concentration is reduced significantly by changing from the type A joint to the type B joint and then to the type C joint in turn. Figure 9 is the shear stress distribution graph which we produced by the theoretical analysis. The vertical scale represents shear stress normaiised with respect to uniform stress in the fibre reinforced tube. The units are arbitrary.The horizontal scale represents the distance along the overlap between the alloy tube and the fibre reinforced member. X represents the end of the overlap region distant from the free end of the fibre reinforced member and Y represents the end into which the fibre reinforced member is first inserted when the joint is made, (the end which is uncrimped and flared in the case of the type C joint). As is shown in Figure 9, the maximum shear stress near the end Y may be reduced by a factor of more than two by providing the uncrimped and flared end on the alloy tube in type C (and as described in the above embodiments) instead of the fully crimped overlap region as in type A.

The analytical result shown in Figure 9 confirms the results we have obtained experimentally, namely that the joint of type C, which embodies the present invention, is much less prone to failure than the prior art joints of types A and B, particularly type A, owing to the lower maximum shear stress of type C.

Claims (10)

1. A method of mounting a connector unit on an elongate member, said connector unit comprising at one end a member having a tubular portion having a bore dimensioned and sectioned to receive an end portion of said elongate member and having a further connecting portion, said method comprising the steps of providing the interior of said tubular portion and/or the exterior of the end portion of the elongate member with an adhesive, inserting the end portion of the elongate member into the bore of the tubular member and crimping the tubular portion so that the tubular portion firmly embraces the end portion of the elongate member, and so that a film of adhesive remains between the end portion of the elongate member and the embracing tubular portion, the crimping of the tubular portion being made along only part of the length of the region of that portion in which the elongate member is inserted, the end of the tubular portion further from the further connecting portion being uncrimped.

2. A method as claimed in claim 1 and wherein the crimping extends at least half way along the overlap region of the tubular portion in which the elongate member is inserted.

3. A method as claimed in claim 2 and wherein the crimping extends along between 70% and 95% of the said overlap region, the uncrimped end forming between 30% and 50% of the length of that region.

4. A connector unit mounted on an elongate member by the method claimed in claim 1, the connector unit comprising a tubular portion, the bore of which accommodates one end portion of the elongate member and a further connecting portion, there being a film of adhesive between the said end portion of the elongate member and the interior of the tubular portion, the tubular portion having been crimped firmly to engage the end portion of the elongate member, the crimping of the tubular portion extending along only part of the length of the region of that portion which accommodates the end portion of the elongate member, the end of the tubular portion remote from the further connecting portion being uncrimped.

5. A connector unit as claimed in claim 4 and wherein the elongate member is a rod formed of fibre reinforced material and the tubular portion is formed of aluminium.

6. A connector unit as claimed in claim 4 or claim 5 and wherein the said further connecting portion of the connector unit comprises a second tubular portion which protrudes beyond the tubular portion that is crimped, which second tubular portion has been flattened after the crimping step to form a connecting portion that may accommodate bolts or the like.

7. A connector unit as claimed in claim 4 or claim 5 and wherein the further connecting portion of the connector unit comprises a second tubular portion having a bore dimensioned to receive the end of a second elongate member and crimped, with adhesive on to said second elongate member, the connector unit serving to connect two elongate members together.

8. A method as claimed in claim 1 , claim 2 or claim 3 and wherein the tubular portion is partially crimped along its length by placing the part requiring crimping between two matching dies each having an arcuate recess having a diameter equal slightly less than the outer diameter of the tubular portion after crimping, the part of the tubular portion to be uncrimped overlapping the end of the dies at the point where the crimping is to be terminated and pressing together the two dies to provide the crimping of part of the tubular portion.

9. A method as claimed in claim 1, claim 2, claim 3 or claim 8 and substantially the same as any one of the specific methods described hereinbefore.

10. A connector unit produced by the method claimed in claim 9.