COMPOSITE GRID STRUCTURE

Abstract

Grid structure, such as a lattice or grid-stiffened structure and a process of manufacturing such a grid structure. Fiber material is laid up on a base tool to form intersecting ribs defining a grid with a plurality of cavities. In the same step fiber material is laid to form one or more; local substructures. Blocks are placed, at the positions of the cavities. The fiber material of the ribs and the local substructures is impregnated with a resin. Optionally, one or more layers of fiber material are placed on the base tool and/or over the ribs and the blocks to form an outer skin. The ribs, the local substructure and optionally the outer skin jointly consolidated to form, the grid, structure.

Claims

1. A process of manufacturing a grid structure of a layered composite material comprising fibers and resin, the process comprising: laying fiber material up on a base tool to form intersecting ribs defining a grid with a plurality of cavities; laying fiber material to form one or more local substructures; placing expansion blocks at positions of the cavities; impregnating the fiber material of the ribs and the local substructures with a resin; consolidating the ribs and the local substructures jointly to form the grid structure.

2. The process according to claim 1, wherein parts of the fiber material forming the ribs are interwoven with ends of the fiber material forming the local substructures.

3. The process according to claim 1, wherein the fiber material forming the ribs comprises prepreg tows and the fiber material forming the local substructures comprises prepreg plies.

4. The process according to claim 1, wherein at least one of the one or more local substructures comprises a laminate with a section with a thickness gradually increasing towards a row of terminal ends of the intersecting ribs.

5. The process according to claim 4, wherein terminal ends of the ribs are at a distance from an outer edge of the grid structure, the outer edge being formed by extending plies of the gradually thickening laminate section.

6. The process of claim 5, wherein the intersecting ribs have ends near the laminate edge section which are leveled with the outer laminate edge.

7. The process of claim 4, wherein layers of the intersecting ribs terminate in the laminate edge section.

8. The process of claim 7, wherein layers of the ribs overlap lower tows to gradually reduce a height of the ribs.

9. The process of claim 1 wherein after curing connection structures are provided in the laminate edge.

10. The process according to claim 1, wherein a compaction pressure is exerted onto the laminate edge during consolidation.

11. The process according to claim 1, wherein the one or more local substructures include one or more patches.

12. The process of claim 11, wherein the patches comprises one or more plies.

13. The process of claim 11, wherein a tool is used with a recess tailored to accommodate material for the one or more patches.

14. The process according to claim 1, wherein tows or plies are stacked to form a local substructure, wherein ends of at least some of the tows or plies forming the local substructure are interwoven with the tows forming an adjacent rib.

15. The process of claim 14, wherein the local substructure bridges adjacent ribs.

16. A grid structure of a layered composite material comprising fibers and a resin, the grid structure comprising a grid of ribs with open spaces between the ribs, and one or more load introduction structures between at least some of the ribs, the load introduction structure having one or more layers with layer ends extending between layers of one or more adjacent ribs.

17. The grid structure according to claim 16, wherein the load introduction structures include a laminate edge section with a thickness gradually increasing towards a row of terminal ends of the intersecting ribs.

18. The grid structure according to claim 17, wherein layers building the intersecting ribs gradually built off in the direction of laminate edge section.

19. The grid structure according to claim 18, wherein terminal ends of the intersecting ribs are leveled with the laminate edge section.

20. The grid structure of claim 16, wherein at least a part of the layers of the rib overlap layer ends of the laminate edge section, wherein the overlapping parts of the layers of the rib gradually drop off, while the overlapped parts of the layers of the laminate edge section gradually build up.

21. The grid structure according to claim 20, wherein the overlapping parts form an inclined stack of interwoven layer ends with an angle of inclination relative to the surface of the laminate edge section of at most about 70 degrees.

22. The grid structure according to claim 16, wherein the load introduction structures include at least one structure in a cavity bordered by intersecting ribs, said structure interfacing at least one of the intersecting ribs.

23. The process of claim 1 and further comprising placing one or more layers of fiber material on the base tool and/or over the ribs and the expansion blocks to form an outer skin.

24. The process of claim 23 wherein consolidating includes consolidating the ribs, the local substructures and the outer skin jointly to form the grid structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Aspects of the invention are further explained with reference to the accompanying drawings, showing exemplary embodiments.

[0032] FIG. 1: shows an exemplary embodiment of a grid-stiffened composite structure;

[0033] FIG. 2: shows an exemplary embodiment of a lattice structure;

[0034] FIG. 3: schematically shows a process for manufacturing the structure of FIG. 1;

[0035] FIG. 4A: shows in detail a rib end and a laminate edge section of the structure of FIG. 1;

[0036] FIG. 4B: shows the build-up of plies at the midplane cross section of the section shown in FIG. 4A;

[0037] FIG. 4C: shows a third exemplary embodiment;

[0038] FIG. 4D: shows a fourth exemplary embodiment;

[0039] FIG. 5A: shows in front view a local substructure of a fifth embodiment;

[0040] FIG. 5B: shows the structure of FIG. 5A in perspective rear view;

[0041] FIG. 6: shows a local substructure of a sixth embodiment;

[0042] FIG. 7: shows a schematic cross-section of the structure of FIG. 6;

[0043] FIG. 8: shows a similar cross section as FIG. 7 of a different embodiment.

DETAILED DESCRIPTION

[0044] FIG. 1 shows an exemplary embodiment of a grid-stiffened composite structure 1 with a local substructure 2. The structure 1 comprises a shell or skin 3 reinforced by a grid 4 of regularly spaced intersecting ribs 6, 7, 8 running in three directions. The ribs 6, 7, 8 intersect at nodes 9 and form a repeated pattern of triangular cavities 12. Alternative embodiments may be designed with any other suitable grid patterns, includingbut not limited toanisogrid patterns.

[0045] In this exemplary embodiment of FIG. 1 the local substructure 2 is a laminate section 13 with a thickness gradually increasing towards a row of terminal ends 14 of the ribs 7, 8.

[0046] At the terminal ends 14 of the ribs 7, 8 the height of the ribs has been levelled with the laminate section 13. Here the section of gradually increasing thickness is continued as a laminate edge 16 of substantially even thickness. The local substructure 2 forms a load introduction edge provided with openings 17 for mechanical fastening means (not shown). Due to the gradual thickening of the laminate section 13 loads exerted via the fastening means are distributed over the grid structure, while stress peaks and concentrations are minimized. Alternatively, the load introduction edge 16 can be used for attachment via an adhesive bond, in which case the openings 17 would no longer be required. The edge 16 and the section 13 gradually guide any bending, tensile, shear and compression loads into the grid structure.

[0047] FIG. 2 shows an alternative embodiment of a grid stiffened structure 10, which is identical to the structure 1 of FIG. 1, except that the structure 10 is a lattice without a skin 3.

[0048] FIG. 3 shows schematically how the structure 1 of FIG. 1 can be manufactured by means of a one-shot curing process. First, a base 21 is prepared by applying a release film or release agent over its surface. In a next step tows of uncured prepreg material are laid on top of each other to form the intersecting ribs 6, 7, 8 in the desired grid pattern. Also the tows forming the local substructures 13, 16 (not shown in FIG. 3) are laid with ends being interwoven with the tows forming the intersecting ribs 6, 7, as will be explained hereafter. Due to the tackiness of the tows, the tows stay attached on top of each other.

[0049] In a next step, expansion blocks 23 are positioned in the cavities 12 between the ribs 6, 7, 8. Then a skin 25 is placed covering the grid 4 and the expansion blocks 23.

[0050] To ensure the dimensional stability of the structure 1 during curing a stiff frame 27 is built at the edges of the grid 4 to keep the ribs 6, 7, 8 straight, if the grid composite structure 1 is a panel or other non-cylindrical structure.

[0051] A caul plate 28 is then put on top of the skin 25 and a breather fabric 29 is placed over the assembly. In a next step the resulting assembly is put into a vacuum bag 31 and positioned in an autoclave or oven (not shown).

[0052] During cure the expansion blocks 23 expand and compact the ribs 6, 7, 8 to define the final shape and dimensions of the ribs 6, 7, 8. During compaction the height of the ribs increases and the ribs become tall enough to co-cure with the skin. The skin 25, the ribs 6, 7, 8 and the local substructures 13, 16 are jointly cured to form durable cohesion between the local reinforcements structure 13, 16, the skin 25 and the grid 4. After cure the expansion blocks 23 are removed from the structure 1.

[0053] FIG. 4A shows in detail a section where a terminal end of a rib 7, 8 meets the laminate section 13. FIG. 4B shows schematically a cross section along a vertical midplane of the detail shown in FIG. 4A. The rib 7 is built-up with a stack of plies 32. In a first transition zone 33, part of the plies are terminated. In this transition zone 33 lower terminated ply ends 32A are terminated at more distance from the laminate section 13 than next higher terminated ply ends 32B, gradually leveling the height of the rib 7 with the surface of the laminate edge section 13. The terminated ply ends 32A, 32B 32C, 32D are alternately laid between plies that extend into the laminate edge section 13 to form a second transition zone 34. Optionally, multiple plies can be terminated at a single point in a single layer. In the second transition zone 34, plies 32 of the rib 7 gradually drop-off with higher ply ends 32Z extending beyond lower ply ends 32Y to define a linearor optionally a non-lineardrop-off rate. Plies 35 of the laminate edge section 13 are gradually built-up, with lower ply ends 35A extending beyond higher ply ends 35B with a build-up rate corresponding to the drop-off rate of the rib plies 32 in second transition zone 34. During curing the transition zone 34 will be compressed and become leveled with the surface of the laminate edge section 13.

[0054] The ply ends 32 of the rib 7 extending into the second transition zone 34 are interwoven with the ply ends 35 of the laminate section 13. The overlapping ply ends of the rib 7 and the laminate edge section 13 in the second transition zone 34 form an inclined stack of interwoven layer ends. The angle of inclination a of the stack relative to the surface of the laminate edge section may be any suitable angle between 0 to 90 degrees, in practice it will be less than about 70 degrees, e.g., less than about 30 degrees, e.g., less than 15 degrees.

[0055] In FIGS. 4A and 4B the rib plies 32 start to drop-off prior to entering the laminate section 13. This may result in a structure shown in FIG. 4C. Alternatively, the ply ends of the rib 7 may end in a section where the laminate section is already fully builtup. An example of such a structure is shown in FIG. 4D.

[0056] FIGS. 5A and 5B show a section of a further exemplary embodiment of a grid-stiffened composite structure 40 in front view (skin-side; see FIG. 5A) and rear view (rib-side; see FIG. 5B), respectively. The grid stiffened structure 40 has a skin 41 and intersecting ribs 42, 43, 44 defining a regular grid pattern with triangular cavities 45 at a rib-side of the skin 41. The grid pattern is interrupted by a central hexagonal cut-out 49 bordered by intersecting ribs 42, 43, 44. At the side opposite to the rib-side a patch 51 is applied on the skin 41 bridging ribs at the outline of the cut-out 49 with adjacent ribs. The patch 51 is formed by a single ply of prepreg material, or by more plies if so desired. In the shown embodiment, the patch 51 completely surrounds the cut-out 49. In an alternative embodiment the patch may for example only partly border a cut-out. It has been found that in a wide variety of cases such a patch 51 suffices to maintain the buckling resistance close to the same level as the undisturbed grid-stiffened structure.

[0057] A further type of local substructure is shown in FIG. 6 with a section of a grid-stiffened structure 55. Three intersecting ribs 56, 57, 58 intersect at nodes 61 and define a triangular cavity 59. A fourth rib 63 crosses one of the nodes 61. At this node 61 the cavity 59 is provided with a local rib interfacing substructure 65 locally bridging the two intersecting ribs 56, 58. In this exemplary embodiment the local substructure 65 is a load introduction structure provided with an opening, e.g., for cooperation with mechanical fastening means.

[0058] The local rib interfacing substructure 65 is formed by stacking tows or plies 67, similar to the build-up of the ribs. The tows or plies 67 forming the local substructure 65 have ends 69 interwoven with those of the adjacent ribs 56, 58, as is shown in FIG. 7 (showing one of the two ribs only, before and after compaction and curing). In the local substructure 65 every ply 67 extending into the ribs 58 is between two pairs of plies 71 not extending into the ribs 58. The amount of plies extending into the ribs may be varied subject to engineering considerations.

[0059] In FIGS. 6 and 7 the local substructure 65 has about the same height as the ribs 56, 57, 58, 63. Alternatively, the local substructure 65 may have a smaller height, as shown for example in FIG. 8.

[0060] The invention is not restricted to the above described embodiments which can be varied in a number of ways within the scope of the claims.