The invention relates to a method for repairing an external skin of a composite structure, comprising an internal cellular core between a pair of external skins, including one which is subject to repair.
Such a composite structure is encountered in a lot of devices, notably transportation, railway, seagoing, or aeronautical machines; for example on ship hulls, or on external streamline shapes, leading edges of wings, aircraft ailerons and radomes.
The skin of the structure which faces the outside is often damaged to a degree which imposes its replacement. A new skin portion is laid by drape moulding on the structure at the location to be repaired and left to harden. In order to guarantee a nice smooth repaired surface, and without any difference in level with the original skin at the edge of the new portion, a counter-form is laid and pressed onto the new skin portion before hardening, in order to bring it in extension with the original skin.
There is a risk of a bonding failure between the new skin portion and the underlying core in spite of this precaution. The object of the invention is to eliminate this risk and to provide quality of the adhesive bonding of the external skin on the core.
The bonding failure may come from the lack of setting up pressure of the core on the skins during the baking which accomplishes the hardening, itself caused by the pressure of the air which is entrapped in the cells of the core and which the pressure of the counter-form prevents from emerging from under the edges of the new skin portion. In the usual methods, the portion to be repaired of the structure is wrapped up in a bag forming a chamber in which a vacuum is applied in order to pump the excesses of resin and drain this air, but draining is not always achieved in a satisfactory way.
In a general embodiment, the invention relates to a method for repairing a composite material structure comprising an internal cellular core between two opposite external skins, consisting of replacing a portion of one of said external skins with a new portion, laying a rigid counter-form on the new portion and around it, pressing the counter-form onto the composite structure and hardening the new portion, characterized in that it comprises, before hardening, a step for piercing the other of said external skins right up to the internal core and for draining the air contained in the internal core. In a more complicated but further general embodiment, the invention relates to a method for repairing a composite material structure comprising an internal cellular core between two opposite external skins, consisting of replacing a portion of one of said external skins with a new portion, laying a rigid counter-form on the new portion and around it, pressing the counter-form on the composite structure and hardening the new portion, characterized in that it comprises, before hardening, a step for piercing the other of said external skins right up to the internal core and for draining the air contained in the internal core, followed, if necessary, by filling up the holes with resin in order to guarantee the seal of the structure.
These aspects of the invention, as well as other ones, will now be described in connection with the following figures:
and
The structure of
Traditional tooling comprises a counter-form 8 usually as a plate which is laid on the new skin portion 6. The lower face of the counter-form 8, pressed onto the new skin portion 6 and, around it, on a border of the original external skin 2, has a shape and surface quality corresponding to those which is intended to be obtained for the external skin 2 after the repair. The counter-form 8 is pressed onto the external skin 2 and compresses the new skin portion 6. This is achieved by confining the portion to be repaired of the structure in a vacuum bag 9, the vents 10 of which on each face of the structure are connected to a vacuum pump 11 or a similar apparatus. Heating carpets 12 are slipped into the vacuum bag 9 in order to bake the resin-impregnated polymer layers of the new skin portion 6. The lower face of the counter-form 8 is often coated with polytetrafluoroethylene (PTFE) in order to provide easy removal of the mold after hardening; a pealable sheet 13 may also be slipped under it, in contact with the new skin portion 6. Finally, environment fabrics 19 are placed between the internal face of the vacuum bag 9 and the heating carpets 12. Draining is carried out through them.
The vacuum pump 11 sucks the air included in the vacuum bag 9, but it is incapable of suitably doing this for the air included in the cells 5 of the internal core 1 under the new skin portion 6 since the pressure of the counter-form 8 prevents it from flowing through underneath the edge of the new skin portion 6 while producing sufficient adherence for the seal with the original portion of the external skin 2. Bonding defects between the internal core 1 and the new skin portion 6 may appear after hardening. This is why, according to the invention and as illustrated in
An alternative embodiment will now be described in
Number | Date | Country | Kind |
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05/52951 | Sep 2005 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2006/066814 | 9/27/2006 | WO | 00 | 3/13/2008 |