Lightning strike protection material

1. A lightning strike protection material for an aircraft, the lightning strike protection material comprising:

an electrically-conductive grid comprising:

grid-forming members; and

nodes, which are formed where grid-forming members overlap or intersect each other;

wherein a plurality of the grid-forming members each comprise an outward-facing surface that is at least partially a concave; and/or

wherein a plurality of the nodes each comprise an outward-facing surface that is at least partially concave.

2. The lightning strike protection material according to claim 1 , wherein the grid is flexible.

3. The lightning strike protection material according to claim 1 , wherein:

the grid-forming members are elongate and have a mean aspect ratio of at least 3:1 and no more than 20:1; and

the grid comprises an expanded mesh.

4. The lightning strike protection material according to claim 1 , wherein the grid-forming members;

are elongate;

have a mean aspect ratio of at least 50:1 and no more than 30,000:1; and

comprise interwoven wires.

5. The lightning strike protection material according to claim 1 , wherein the grid-forming members have a mean thickness of at least 0.02 mm and no more than 0.30 mm.

6. The lightning strike protection material according to claim 1 , wherein the grid comprises openings having a four-sided shape.

7. The lightning strike protection material according to claim 1 , wherein a distance between centers of adjacent nodes in the grid is at least 1.0 mm and is no more than 10.0 mm.

8. The lightning strike protection material according to claim 1 , wherein the lightning strike protection material has a mass per unit area of at least 20 g/m 2 and no more than 400 g/m 2 .

9. The lightning strike protection material according to claim 1 , wherein the grid has an open area of at least 30% and no more than 95%.

10. The lightning strike protection material according to claim 1 , wherein, as calculated by number, at least 10% of the grid-forming members and/or nodes have the outward-facing surface that is at least partially concave.

11. The lightning strike protection material according to claim 1 , wherein:

substantially none of the grid forming members have the outward-facing surface that is at least partially concave; or

none of the nodes have the outward-facing surface that is at least partially concave.

12. The lightning strike protection material according to claim 1 , wherein at least 80% of the at least partially concave outward-facing surfaces, as calculated by surface area, face substantially in one direction.

13. The lightning strike protection material according to claim 1 , wherein:

the grid comprises a sheet having two faces; and

at least 80% of the at least partially concave outward-facing surfaces face at least partially away from one of the two faces of the sheet.

14. The lightning strike protection material according to claim 1 , wherein:

the grid comprises a sheet having two faces; and

at least 80% of the at least partially concave outward-facing surfaces are on one of the two faces of the sheet.

15. The lightning strike protection material according to claim 1 , wherein at least one of the at least partially concave outwardly-facing surfaces is elongate and has a same cross-sectional shape along a length thereof, which cross-sectional shape is concave.

16. The lightning strike protection material according to claim 1 , wherein at least one of the at least partially concave outwardly-facing surfaces has a shape consistent with a volume of revolution and/or has a conical shape, a bell-shape, a frusto-conical shape, or a horn shape.

17. The lightning strike protection material according to claim 1 , comprising an indicator to indicate which of two faces of the lightning strike protection material comprises the at least partially concave outward-facing surfaces.

18. A laminate for an aircraft structure, the laminate comprising:

the lightning strike protection material according to claim 1 ;

a composite material; and

at least one layer of paint overlying the lightning strike protection material.

19. An aircraft structure comprising:

the laminate according to claim 18 ; and

a fuselage or part thereof, a wing or part thereof, a tail or part thereof, a flight control surface or part thereof, a flap or part thereof, an elevator or part thereof, an aileron or part thereof, a rudder or part thereof, a radome or part thereof, or an engine cowling or part thereof.

20. A method of forming a lightning strike protection material, the method comprising:

providing a sheet of blank material; and

forming a grid in accordance with the lightning strike protection material according to claim 1 .

21. A method of forming the lightning strike protection material according to claim 1 , the method comprising:

providing a multiplicity of grid-forming members, a plurality of the grid-forming members each comprising a concave surface; and

forming a grid from the plurality of grid-forming members.

22. A method of forming the laminate according to claim 18 , the method comprising:

contacting un-cured composite material with the lightning strike protection material; and

curing the composite material.

23. A method of forming the aircraft structure according to claim 19 , the method comprising:

contacting un-cured composite material with the lightning strike protection material, in a mold or former shaped to provide the aircraft structure or part thereof; and

curing the composite material.

24. A lightning strike protection material comprising:

an electrically-conductive sheet that has a first face and a second face;

wherein the sheet is in a form of a lattice that comprises multiple lattice members and multiple nodes;

wherein the nodes are located at respective meeting points of the lattice members; and

wherein a plurality of the lattice members have a concave surface that faces away from the first face.

25. An aircraft comprising the lightning strike protection material according to claim 1 .