GB2238363A

GB2238363A - Joint for composite tube

Info

Publication number: GB2238363A
Application number: GB8926089A
Authority: GB
Inventors: Andrew Taylor
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1989-11-17
Filing date: 1989-11-17
Publication date: 1991-05-29
Also published as: GB8926089D0

Abstract

In joint between a first tube formed from a composite material including fibres and a second tube an insert is placed within the composite tube with the fibres wrapped around it. Bolts link the second tube to the insert, securing the two tubes together. <IMAGE>

Description

COMPOSITE JOINT This invention relates to composite joints, that is joints between members at least one of which is formed from a composite material.

A composite material being defined herein as a material formed from at least two distinct materials.

At present composite joints are generally formed in the same way as joints between any other material.

For example a joint between two load bearing tubes in end-to-end abutment would be formed by forming flanges on the ends of both tubes, drilling holes through the flanges and then putting bolts through the holes to bolt the two tubes together, the load being transmitted through the bolts and flanges.

It has been found that in composite materials of the type where one of the materials is in the form of elongate elements, generally known as fibres, joints as described above are prone to premature failure due initially to delamination and fibre pull out and finally breakage. It has also been found that due to the visco-elastic nature of many composite materials the clamping load exerted by the bolts reduces over time, weakening the joint, which is of course undesirable in a load bearing joint.

This invention was intended to produce a composite joint overcoming these problems, at least in part.

This invention provides a composite joint between a first member and a second member, the first member being formed from a composite material comprising a plurality of elongate fibres embedded in a matrix, including an insert within the first member and securing means connecting the insert to the second member, at least some of the fibres being arranged to wrap around the insert the securing means passing between at least some of the wrapped-around fibre.

It has been realised that many of the problems encountered in the prior art are due to the fact that almost all of the load on the joint is carried by the relatively few fibres which are cut by the bolt holes.

This produces very high stress concentrations adjacent to then bolt holes, producing delamination in the flange, and puts very high loads on these cut fibres, causing fibre pull out and breakage. The use of an insert around which the fibres in the composite material pass allows the load on the joint to be initially applied to the insert and then to be spread evenly over all of the fibres. This reduces the stress concentrations found in the prior art and so allows the joint to withstand higher loads without failure.

Preferably the composite member has a flat portion and each fibre passes through the flat portion once before and once after it passes around the insert because this allows the composite member to be made thinner where it passes between the second member and the joint than its thickness elsewhere without cutting the fibres. As a result the rate at which clamping load reduces due to any visco-elasticity is reduced without losing the load bearing advantages of continuous fibres.

A joint embodying the invention will now be described by way of example only with reference to the accompanying diagrammatic figure 1 which shows a composite joint.

Referring to figure 1 a joint formed between a metal tube 1 and a composite tube 2 is shown.

The composite tube 2 is formed from a composite material comprising carbon fibres in a matrix of polyimide. The internal structure of the composite material is alternate layers of carbon fibres 3 and polyimide 4.

Each of the layers of carbon fibres 3 is formed by a woven mesh of individual carbon fibres arranged in different directions in a single plane. The woven carbon fibre mesh is impregnated with polyimide which fills up the gaps between the fibres, but for simplicity the mesh is shown as a continuous layer.

To form the wall 5 of the composite tube 1 each of the carbon fibre layers 3 is folded over so that each of the carbon fibre layers 3 passes along the wall 5 twice.

At the end of the composite tube 2, where the carbon fibre layers 3 are folded, they are wrapped around a titanium insert 6 in the shape of a ring. Thus each of the carbon fibre layers 3 passes up one side of the wall 5, around the insert 6 and down the other side of the wall 5. The insert 6 is made from titanium because it has a very similar coefficient of thermal expansion to the composite material, so minimising the thermal stress generated within the composite tube 2 in use.

The corners of the insert 6 are rounded to allow a large enough radius of curvature of the composite material around the corners that there is no danger of fibre breakage during manufacture.

Where the two sets of the carbon fibre layers 3 come into contact below the insert 6 a void 12 is formed, this is filled in with short lengths 13 of carbon fibre and polyimide 4.

The end of the metal tube 1 bears a flange 7. The two tubes 1 and 2 are held together by a plurality of bolts 8 positioned at equal intervals around the circumference of the two tubes 1 and 2. Each of the bolts 8 passes through a hole 9 in the flange 7, through the carbon fibre and ployimide layers 3 and 4 and into an internally threaded bore 10 in the insert 6.

The carbon fibre and polyimide layers 3 and 4 are formed as continuous layers around the insert 6 and holes 14 are cut through them to allow the bolt 8 to pass through.

A resilient protective layer 11 is placed between the two tubes 1 and 2 to prevent scratches being produced on the outer surface of the composite tube 2 by contact with the flange 7, because scratches would provide stress concentrations and initiation points for cracking of the composite tube 2.

The load on the joint is initially carried by the insert 6. The carbon fibre layers 3 are wrapped around the insert 6 and as a result the load is spread across all of the fibres in the layers 3 by the insert 6.

The thickness of the composite material between the insert 6 and the metal tube 1 is half that of the wall 5 of the composite tube 2. The rate at which the clamping load generated by the bolts 8 is reduced over time due to the visco-elastic properties of the polyimide is related to the thickness of polyimide clamped by the bolts 8. As a result the clamping load generated by the bolts 8 reduces over time at a lower rate than if the thickness were the same between the insert 6 and metal tube 1 as in the wall 5.

The bolts 8 are placed at an angle to the walls of the tubes 1 and 2 in order to reduce the bending moment acting on the wall 5 of the composite tube 2.

The use of bolts to secure the joint is not essential, other securing means such as studs or rivets could be used.

This invention is applicable to composite materials other than a carbon fibre and polymide combination.

The fibres could of course be looped at both ends of the composite tube 2, allowing each fibre layer 3 to be woven from a single fibre or a small number of fibres.

The carbon fibre and polyimide layers 3 and 4 could be formed with access holes 14 for the bolts 8 instead of the holes 14 being cut as a seperate operation.

Although in the example shown the bolts 8 are equally spaced around the circumference of the tubes 1 and 2 they could be non-equally spaced if required, for example they could be placed closer together in regions which were expected to be subject to high levels of stress.

Claims

ClAIMS

1 A composite joint between a first member and a second member, the first member being formed from a composite material comprising a plurality of elongate fibres embedded in a matrix, including an insert within the first member and securing means connecting the insert to the second member, at least some of the fibres being arranged to wrap around the insert the securring means passing between at least some of the wrapped-around fibre.

2 A composite joint as claimed in claim I where all of the fibres pass around the insert.

3 A composite joint as claimed in claim 1 or claim 2 where the first member has a substantially flat portion and each of the fibres which passes around the insert passes through the flat portion twice, once before and once after it passes around the insert.

4 A composite joint as claimed in claim 3 where the thickness of the first member between the insert and the second member is approximately half of its thickness in the flat portion.

5 A composite joint as claimed in any preceding claim where the fibres are carbon fibres.

6 A composite joint as claimed in claim 5 where the insert is made of titanium.

7 A composite joint as claimed in any preceding claim where the flat portion of the first member forms a sidewall of a tube.

8 A composite joint as claimed in any preceding claim in which the securing means are straight and are not parallel with the flat portion of the first member.

9 A composite joint substantially as shown in or as described with reference to the accompanying drawing.