Many contemporary aircraft have honeycomb sandwich structures formed in the wing leading edges. In the course of use the leading edges may sustain damage, e.g. from bird strikes and accidents during maintenance.
In some cases a honeycomb core may be crushed proximate the point of impact, but the outer skin may not show any visual damage at that location. The resulting core shear failure may extend over a larger area than the original damaged area. Such unseen core damage may result in the outer skin buckling and delaminating under stress.
Repair of the damage may require skin and core repair operations, either in the field or in a repair depot. A replacement core specific to that wing must be matched to correctly restore the aircraft's integrity. However, maintaining a stock of cores specific to each aircraft is logistically challenging, especially during field operations.
In accordance with contemporary repair techniques damaged skin and the adjacent honeycomb core are completely removed to the opposing skin. A replacement core may be shaped and sanded to fit snugly into the area being repaired. Replacement material may be selected to substantially match the original case material. However, conformity of the materials does not insure conformity to the structural and mechanical properties of the honeycomb shaped core. One type of replacement material is syntactic foam, an epoxy resin material that contains glass micro-balloons. This material can be tailored to different densities and properties. However the material may be too heavy for certain applications. Moreover, the material is isotropic in nature, i.e. the strength is equal in all directions, which may be undesirable or at least differ from the honeycomb properties of the adjacent structure.
For reference, several prior art methods to repair aircraft laminates are described below. U.S. Pat. No. 6,149,749 discloses a patch used to cover the damaged area, without any replacement of the original structure, including the lightweight structure between the two outer skins. The patch has apertures that allow air to be removed from below the patch thereby improving the adhesion.
U.S. Pat. No. 4,961,799 discloses a method for repairing damaged areas by bolting or bolt bonding a repair patch on the outer surfaces of the damaged material. This method does not provide a matching replacement honeycomb structure.
U.S. Pat. No. 5,741,574 discloses a truss reinforced sandwich structure that uses fiber bundles or tows cured in very small diameters; the preferred method of the attachment being stitching the foam core within the face sheets or laminates. U.S. Pat. No. 3,328,218 discloses using rigid filaments to manufacture the core structure. In both references the filaments must be fastened to the outer plies. Moreover, the references require the foam to be under compressive pressures of 50 to 90 psi.
U.S. Pat. No. 5,612,117 discloses an anchoring means comprising an insert introduced into a hole in the skins and held in place by the core to provide an anchoring of the entire structure.
U.S. Pat. No. 5,773,121 describes a syntactic form core structure that is produced using a powdered resin instead of a liquid resin. In order to improve or modify characteristics of the resultant structure, chopped fibers or pre-built honeycomb structure is added to the powered resin, before curing.
U.S. Pat. No. 5,547,629 discloses a method using rubber mandrels to fabricate hollow structures such as a wing. After the two skins are compressed together, with the mandrels in the hollow cavities providing support, the mandrels are pulled from one end. As they stretch, they contract and can be removed from the molded part. No foam honeycomb structure is introduced during the molding process.
U.S. Pat. No. 5,868,886 discloses the use of Z-pins to provide a mechanical link between the patch/parent structures. It suggests removal of the pins, leaving holes that are filled when the patch material is introduced into the repair area. Small pin sizes are used in order to reduce associated structural degradation caused by fiber breakage due to concentrated stress.