Abstract:
A method for repairing damage to the coating of an aircraft fuselage, the coating being made of fiber-reinforced composite material. This method comprising making a cut-out in the fuselage with a substantially parallelepipedal outline around the damage and drilling a circular hole substantially centered on the intersection of the sides of the outline of the parallelepiped of the cut-out at each of the corners of the outline before the cutting out operation.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for repairing aircraft primary structures. More specifically, the invention relates to a method for repairing damage to the coating of an aircraft fuselage, which coating is made of fiber-reinforced composite material. 
     While in operation, aircraft structures are frequently subjected to impacts with birds, vehicles, tools, airport installations, etc. Such impacts create damaged areas within the structure, which have lower mechanical resilience; these areas become prime sites for the start of defects such as cracks, which are likely to propagate. 
     More specifically, in the case of a fuselage made of fiber-reinforced composite material, the actual damaged area may be significantly larger than the visible impact area. 
     When an aircraft is subjected to such damage, it must absolutely be repaired so as to recover all the structural strength and avoid the propagation of defects from the damaged area before it can be put back into service. 
     It must be possible to carry out such repairs within the shortest possible immobilization time for the plane. 
     A known repair mode involves covering the damaged area with a generally circular liner or patch, whose area is significantly larger than said damaged area, then fastening it to the portion of the structure that remains sound with any appropriate means, such as rivets, bolts, welding or bonding. 
     This solution has the advantage of being relatively simple to implement. It does, however, have the disadvantage of keeping the damaged zone, which remains under loading and therefore may initiate the propagation of defects, even though the presence of the liner limits the flow of forces to which said area is subjected. Consequently, such repairs can only be very temporary and need to be monitored very closely until a permanent repair can be effected. 
     In addition, the liner must be shaped to match the shape of the fuselage in the area under consideration. This shape may be complex and non-involute, such that it requires specific shaping of the liner, which being circular in shape must then be cut-out of a larger-size plate, itself made by any sheet metalworking means. 
     According to another embodiment described, for example, in international patent application WO2007135318 in the name of the applicant, a polygonal cut-out is made around the impact area so as to eliminate the whole of the damaged area. A liner, also polygonal in shape but with a larger surface area, is then fastened to the part that remains sound so as to close the cut-out. This method avoids the initiation of defects in the damaged area, since this has been eliminated. Nevertheless, it is then necessary to ensure that the cutting-out operation itself does not introduce any defects. In the case of patent application WO2007135318, holes centered on the intersection of the sides of the polygon are drilled at each corner of said parallelepiped before cutting-out to facilitate this and to avoid unfortunate “saw-cuts” in the delicate cut-outs at the corners. The method divulged in this patent application aims therefore to avoid the formation of cutting defects which may give rise to cracks at the corners of the polygonal cut-out. Effectively, it is in these areas that the beginnings of cracks are most likely to occur, subsequent to a lack of precision on the part of the operator assigned to cutting-out. 
     In the case of a fiber-reinforced composite material fuselage, defects such as delamination can be caused during cutting-out, irrespective of the care taken by the operator. The action of the cutting implement, of whatever kind it may be, can easily break the cohesion of plies located at the edge of the cut-out and thus initiate delamination. Where the material that is cut-out is a fiber-reinforced composite material, the risk of introducing defects at the edge of the cut-out depends essentially on the rate of advance of the tool. At a given cutting speed, too high a rate of advance favors the occurrence of delamination, whereas too slow a rate of advance causes thermal degradation of the matrix (burning or melting). This type of cutting-out operation in fiber-reinforced composite materials must therefore be performed at a controlled rate of advance, within a narrow band of admissible speeds. In the case of repair operations, however, the cutting-out advance movement is generally communicated manually to the machine by the actions of the operator who moves it along the path to be cut. Even if a judicious choice of tool geometry can increase the range of favorable cutting-out conditions, it remains difficult or even impossible to prevent delamination of the surface plies from occurring, except where sophisticated methods of automatic advance are implemented, which control the cutting parameters and, in particular, the rate of advance/cutting speed combination along complex trajectories that match the shape of the fuselage. 
     There is therefore a requirement for a repair method for an aircraft fuselage made of composite material that can be implemented with adequate safety in conditions compatible with the means of airport maintenance workshops and requiring the shortest possible immobilization time for the plane. 
     BRIEF SUMMARY OF THE INVENTION 
     To achieve this, the invention proposes a method for repairing a damaged area of the fiber-reinforced composite coating of an aircraft fuselage comprising the following steps:
         realize a polygonal cut-out around the damaged area;   fit a coating liner with a polygonal outline, an area greater than the cut-out and adjusted to the shape of the fuselage so as to close the cut-out;   fasten said liner to the fuselage skin that remains sound, with at least two rows of fasteners laid out parallel to the perimeter of the liner;   a circular hole centered on the intersection of the sides of the polygon having been cut-out in the fuselage at each of the corners of the polygonal outline of the cut-out before the cutting operation.       

     This method, implemented on a composite material fuselage, has two advantages:
         prior cutting-out of the circular holes is essentially an axial machining operation, which can be carried out by portable means, giving precise control over the cutting conditions;   the sides of the polygon can be cut out manually, using any appropriate means such as an angle grinder, jig-saw, portable milling machine or router, therefore with less efficient control over the cutting conditions, in particular the advance movement, because, surprisingly, according to this embodiment, these areas are not under loading in operation once the liner has been fitted.       

     Thus, the cut-outs at the corners are free of defects because of the strict control over the cutting conditions; as for cutting out the sides, even if they were to have defects, these would not be likely to propagate because of the weak stresses to which these areas are subjected. 
     Advantageously, the diameter of the holes will be between 20 and 40 mm. This diameter is adequate, firstly, to remove the loading from the sides of the cut-out over an adequate width while, secondly, still remaining compatible with conventional drilling means, such as step-drills mounted on an automatic drilling machine (ADM) or portable orbital drilling systems, that allow these holes to be cut out in a single operation, with a single tool and thus avoid the risks linked to adjustment operations. 
     According to an advantageous embodiment, the liner is not fastened to the fuselage at the corners. Typically, when N rows of fasteners are to be installed, those fasteners located on the outside of an n×n diagonal are not installed. Advantageously, the value of n is between 2 and N. 
     This embodiment allows for better distribution of the load over all the fasteners and therefore better dissipation of the forces into the structure that remains sound. 
     According to an embodiment of the method which is the subject of the invention, the liner is made of metal. Thus it will be easier to adjust to the shape of the fuselage using conventional metalworking means. More specifically, if the fuselage is made of carbon-fiber reinforced composite material, the liner will advantageously be made of titanium alloy, both for their galvanic compatibility and for their closely-matched coefficients of thermal expansion. 
     According to another embodiment, the liner is made of plies of fibers pre-impregnated with organic resin, manually laid-up, shaped directly onto the fuselage on the area of application of the repair. 
     According to this embodiment, the shape of the liner adjusts itself to the shape of the area it covers during the shaping operation. Advantageously, when the resin is a thermosetting type, curing can be entirely realized on the fuselage. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will now be described more precisely in the context of preferred non-limiting embodiments shown in  FIGS. 1 to 8  in which: 
         FIG. 1  represents a synopsis of the repair principle which is the subject of the invention, with the drilling of the end holes in  FIG. 1A , the cutting out in  FIG. 1B  and the fitting of the liner in  FIG. 1C ; 
         FIG. 2  shows a detail of  FIG. 1  C, which highlights the distribution of the fasteners at the corners of the liner; 
         FIG. 3  illustrates the distribution of the fasteners at the corners of the liner in a general case; 
         FIG. 4  represents the distribution of the service stress in the fuselage in the vicinity of a repair according to the prior state of the art; 
         FIG. 5  represents the distribution of the service stress in the fuselage in the vicinity of a repair according to an example of realization of the invention; 
         FIG. 6  is an example of repair in a general case that requires cutting out a longitudinal stiffener in front view; 
         FIG. 7  is a B-B cross-section view along the thickness of the repair shown in  FIG. 6 ; 
         FIG. 8  is a transversal A-A cross-section view of this same view in the immediate vicinity of the repair. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     All these figures represent an example of realization that implements a parallelepipedal cut-out and liner. The person skilled in the art will adapt these lessons to the more general case of a polygonal cut-out. 
       FIG. 1A : the coating of a composite fuselage ( 1 ) has been subjected to an impact ( 100 ). To perform the repair, the operator starts by drilling four holes ( 10 ) using an axial machining device, which holes delimit a rectangular area surrounding the impact. The dimensions of said rectangular area are given by the plane&#39;s maintenance manual, depending on the characteristics of the coating and on the characteristics of the impact. 
       FIG. 1B : from these holes, the operator realizes the cut-out lines ( 11 ) to extract the recommended rectangular area from the coating. This cut-out is realized with a radial advance movement tool, to which the advance movement is communicated manually by the operator. As an example, for a fiber reinforced composite coating, this tool can consist of an angle grinder, a jig-saw or a router such as a portable milling machine available under the brand name ONSRUD® manufactured by LMT Tools. 
     A liner ( 20 ) is then applied so as to close the cut-out; it is fastened to the portion of the fuselage that remains sound using N rows of fasteners ( 30 ), 2 in this case. 
       FIG. 2 : N=2 and so as to distribute the dissipation of forces into the sound portion, the fastener provided for at position ( 31 ) located on the outside of the 2×2 diagonal ( 302 ) is not fitted at the corner of the liner.  FIG. 3 : generally, those fasteners located on the outer part of the corner of the liner are not fitted in one or several rows. In the example of  FIG. 3 , N=4, the fasteners are not fitted at the locations beyond the 4×4 diagonal ( 304 ) i.e. 6 fasteners not fitted including the 2×2 diagonal ( 302 ) and the 3×3 diagonal ( 303 ). The number of fasteners not fitted at the corner depends on the number of rows and on the intensity of the loading, i.e. of the surface area of the damaged area and its location on the fuselage. This configuration is determined by calculation and consigned in the maintenance and repair manual for the most frequent cases. 
       FIGS. 4 and 5 : comparing the services loading at the edge of the cut-out can be performed using a finite element simulation. Both figures correspond to the same macroscopic loading conditions and give the Von Mises yield criterion in the repaired fuselage in the vicinity of the cut-out. The reference loading, which corresponds to areas distant from the cut-out ( 110 ), is the same in both cases. The loading level for the other areas is defined relative to this reference loading. 
       FIG. 4 : by using the cutting out principle of the prior state of the art with skipped connections at the edges of the cut-out, the loading of the straight parts of the cut-out is between 60% and 100% of the reference loading. 
       FIG. 5 : by using the cutting out principle according to the invention, the service loading at the straight edges of the cut-out is reduced by 80%, compared with the reference loading in the immediate vicinity of this edge and still remains 40% lower when moving substantially away from the edge; the isodynamic line ( 111 ,  112 ) delimits this 40% reduction in the reference loading which extend even beyond the first row of fasteners. 
       FIG. 6  represents the general case of a fuselage repair, in particular where the area to be cut-out goes through a longitudinal stiffener or stringer ( 40 ). In the case of a composite fuselage, such stiffeners are linked to the skin by bonding, cocuring or other techniques of assembly without fasteners. In this case, the stringer is cut out over a width L, greater than the width of the cut-out. The width L is equal to the width of the liner ( 20 ) and will therefore vary as a function of the number of rows of fasteners required to fasten it to the sound part of the fuselage and to transmit the flow of forces. 
       FIG. 7 : a shim ( 120 ) whose thickness is equal to the thickness of the skin is placed in the cut-out. An inner liner ( 121 ) whose thickness is designed to compensate the thickness of the stringer&#39;s flange ( 41 ) and whose dimensions are substantially equal to those of the outer liner ( 20 ) is placed inside the fuselage. A splice plate ( 42 ) is fitted to ensure the mechanical continuity of the stiffener ( 40 ). 
     Fasteners ( 30 ) are positioned along an appropriate number of rows, which go through and assemble the outer liner ( 20 ), the fuselage skin that remains sound ( 1 ) and the inner liner ( 121 ). In the cut-out area, said fasteners go through and assemble the outer liner, the shim ( 120 ) and the inner liner. 
     Where the stringer ( 40 ) goes through in the vicinity of the cut-out, fasteners ( 32 ) go through and assemble the outer liner, the shim ( 120 ), the inner liner ( 121 ) and the flange of the stringer splice plate ( 42 ). 
     Outside the cut-out area, the same type of fastener ( 32 ) assembles and goes through the outer liner, the fuselage skin, the inner liner and the base of the stringer splice plate. 
     Lastly,  FIG. 8 : beyond the outer liner, a third type of fastener ( 33 ) assembles and goes through the outer skin, the flange ( 41 ) of the original stringer and the flange of the stringer splice plate. 
     Advantageously, the stringer splice plate ( 41 ) is made of metal, preferably titanium alloy approximately 1 mm thick. 
     The fasteners can be of rivet type but are preferably of the bolted type, such as Hi-Lite® fasteners, supplied by the company with the same name, which provide an assembly that is easily mounted and removed to perform a final repair. 
     The above description clearly illustrates that through its various features and their advantages the present invention realizes the objectives it set itself. In particular, since the section of the cut-out is designed to significantly reduce the stresses at the edges of the cut-out, the composite aircraft fuselage repair method according to the invention can be implemented with adequate safety in conditions compatible with the means of maintenance workshops of airports and requiring the shortest possible immobilization time for the plane.