Composite end connections

1. An end connection comprising:

the end connection comprising:

a composite structural component made of a polymer matrix composite material and comprising an end portion, the end portion comprising an internal surface comprising a plurality of ridges and grooves defining a first engagement surface along an axial direction and an outer layer of circumferential fibre reinforcement;

a metal interface component comprising an outer surface comprising a plurality of ridges and grooves defining a second engagement surface along the axial direction and an inner surface defining a mounting surface for attachment of an end fitting;

wherein the metal interface component is engaged with the internal surface of the composite structural component by mating of the first and second engagement surfaces; and

an outer annular component fitted around the outer layer of circumferential fibre reinforcement of the end portion in axial alignment with the mating of the first and second engagement surfaces;

wherein the outer annular component forms an interference fit with the end portion so as to preload the end connection with compressive radial forces.

2. An end connection according to claim 1 , wherein the outer annular component forms an interference fit of about 0.1 mm with the end portion of the composite structural component.

3. An end connection according to claim 1 , wherein the ridges and grooves defining the first engagement surface extend at an angle θ relative to the axial direction of the first engagement surface, wherein θ≤20°.

4. An end connection according to claim 1 , wherein the outer surface of the metal interface component comprises a tapered portion and an engagement portion comprising the plurality of ridges and grooves, wherein the tapered portion tapers radially outwardly from an end of the metal interface component to the engagement portion.

5. An end connection according to claim 1 , further comprising an end fitting attached to the mounting surface of the metal interface component.

6. An end connection according to claim 1 , wherein the composite structural component comprises the end portion and a main portion;

wherein the end portion has a first internal diameter and the main portion has a second, smaller internal diameter that substantially matches the internal diameter of the metal interface component.

7. An end connection according to claim 6 , wherein the internal surface of the composite structural component tapers radially inwards from the end portion to the main portion.

8. An end connection according to claim 6 , wherein the outer surface of the metal interface component comprises a tapered portion and an engagement portion comprising the plurality of ridges and grooves, wherein the tapered portion tapers radially outwardly from an end of the metal interface component to the engagement portion.

9. An end connection according to claim 6 , further comprising an end fitting attached to the mounting surface of the metal interface component.

10. A method of forming an end connection for a composite structural component made of a polymer matrix composite material, the method comprising:

providing a metal interface component comprising an outer surface comprising a plurality of ridges and grooves defining an engagement surface and an inner surface defining a mounting surface for attachment of an end fitting;

forming an end portion of the composite structural component around the engagement surface of the metal interface component so that the metal interface component is engaged with an internal surface of the composite structural component at one end of the composite structural component;

circumferentially winding fibres around the end portion of the composite structural component in axial alignment with the engagement surface so as to form an outer layer of circumferential fibre reinforcement; and

fitting an outer annular component around the outer layer of circumferential fibre reinforcement of the end portion of the composite structural component in axial alignment with the engagement surface so as to form an interference fit with the end portion and preload the end connection with compressive radial forces.

11. A method according to claim 10 , wherein the polymer matrix composite material comprises fibre reinforced polymer, the method further comprising:

providing the metal interface component on a mandrel; and

winding glass or carbon fibres around the mandrel to form the composite structural component and embed the metal interface component in the end portion of the composite structural component.

12. A method according to claim 11 , wherein forming the composite structural component comprises winding glass or carbon fibres around the mandrel at a low angle θ between 8° and 15° and winding one or more layers of glass or carbon fibres around the mandrel at a high angle θ between 75° and 89°.

13. A method according to claim 10 , further comprising curing the composite structural component; and

removing at least some of the layer(s) of hoop fibre reinforcement before fitting the outer annular component.

14. A method according to claim 11 , further comprising attaching an end fitting to the mounting surface of the metal interface component.