Patent Abstract:
A composite panel with protected parallel stiffeners has a multiply front face coming to an edge and extending between two lateral flanks of outside plies, symmetrically relative to a median plane (Pm). A device for protecting each stiffener includes a structural section that caps the stiffener symmetrically relative to this median plane (Pm). This structural section includes a so-called upper section formed above the edge and configured to envelop a space above the edge, a so-called lower section framing the lateral flanks by means of longitudinal walls (Pi) fastened to these flanks, and an intermediate section bearing at least partially on the edge via transverse walls (Pb) so as to minimize the propagation of impact forces to the lower section.

Full Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a method for protecting composite panel stiffeners and to a device adapted to implement that method. It also concerns a composite material panel including stiffeners protected by said device against impacts on their edges. 
         [0002]    The field of the invention is the protection of the edges of composite material structures composed of superposed plies of epoxy resin reinforced by fibers with different orientations, in particular panels self-stiffened by extensions forming stiffeners. 
       BACKGROUND OF THE INVENTION 
       [0003]    Aeronautical structures utilize this type of panel in a wing or ventral beam box section, for example. These panels, highly loaded in compression, are generally sized for stability in terms of compression if holed and damage tolerance. 
         [0004]    Damage tolerance is a particular penalty for this type of panel with low damage tolerance in the event of impacts on their edge. Indeed, their superposed carbon fiber ply structure open on the edge is greatly weakened by an impact on that edge. The shock of the impact in fact induces “decohesion” (i.e. separation) of the plies through the exercise of out-of-plane forces. 
         [0005]    The precise field of the invention is therefore protecting the edges of self-stiffened panels. 
         [0006]    In order to alleviate this fragility and this lack of damage tolerance, the panels are protected by rigidly fastened edge-cover structures. One edge-cover of this type is known from the patent document FR 2 902 689 or FR 2 869 871, for example. 
         [0007]    Such edge-covers have a limited efficacy because they have a U-shaped overall structure that does not allow good absorption of impact energy. This energy is in fact transmitted to the structure or to the panel, the shape having no really damping character. This transmission may result in damage to the panel and its connections to other parts. 
         [0008]    Moreover, these edge-covers, entirely stuck to the stiffener, are not demountable. To remove them, the assembly must be destroyed, which leads to the destruction of the protection itself. The structure to be protected is therefore not easy to inspect after an impact. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    An aspect of the invention aims to reduce the influence of the damage tolerance criterion by protecting the stiffener by means of a structural section that has a configuration absorbing edge impacts without propagation of the impact energy to the protected structure. 
         [0010]    An aspect of the invention consists in a method of protecting composite panel stiffeners. This method comprises coupling a structural section to each stiffener to be protected, the stiffeners each including an edge and lateral flanks. The structural section has a so-called upper section extending at a distance from the edge and configured in the form of an envelope adapted to absorb impacts on the edge by plastic deformation, a so-called lower section framing the lateral flanks and an intermediate section bearing at least partially on the edge so as to minimize the propagation of impact forces to the lower section. The method thereafter consists in coupling the lower section framing the lateral flanks to be demountable by detachment of fixing elements without deformation of the structural section. 
         [0011]    In particular embodiments:
       the upper, intermediate and lower sections are interconnected to form a continuous structural section of substantially constant thickness;   the upper section has a small overall size laterally, preferably less than or equal to the distance between the lateral flanks.       
 
         [0014]    Another aspect of the present invention also consists in a device for protecting stiffeners of composite panels with parallel stiffeners each including a multiply front face coming to an edge and extending between two lateral flanks of outer plies, symmetrically relative to a median plane. The protection device includes a structural section symmetrical with respect to this median plane that caps the stiffener. This structural section consists of a so-called upper section formed above the edge and configured to envelop a space above the edge, a so-called lower section framing the lateral flanks with longitudinal walls rigidly fastened to these flanks, and an intermediate section bearing at least partially on the edge via transverse walls, so as to minimize the propagation of impact forces to the lower section. 
         [0015]    According to preferred features:
       the upper section has in section a configuration chosen from a rectangle, an oblong shape, a double-lobe shape, and a T-shape with single or double stem;   the double-lobe geometrical configuration produces, symmetrically relative to the median plane, a transition with the supports of the intermediate section;   the framing lower walls of the lower section of each structural section are rigidly fastened to the lateral flanks of each stiffener by clamping means, in particular:
           a circular plate including an interface for sticking it to said flanks; the fixing interface is formed by stud/nut pairs, each stud coming from the plate and passing through the lower section by means of notches produced at regular intervals in the border of this wall;   a clip that clips onto the flank of the structural section, notably by way of a trough formed by a shoulder and a projection;   elastic clamping on the flanks of the stiffener, the material and conformation of the structural section advantageously enabling retention by spring loading of the lateral walls of the structural section;   
           the framing lower walls of the lower section of each structural section feature lateral projections, notably trough-shaped extending longitudinally and serving to support and attach elements glued to the flanks of each stiffener to fasten together said lower walls;   the framing lower walls of the lower section of a structural section feature at least one notch that is filled with a mastic;   the material of the structural section is chosen from alloys based on aluminum, alloys based on titanium or an epoxy aramide composite.       
 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0025]    Other data, features of and advantages of the present invention will become apparent on reading the following description of one embodiment given with reference to the appended figures, in which: 
           [0026]      FIGS. 1   a  and  1   b  are partial perspective views of one example of a titanium-based alloy structural section with a double-lobe upper section, respectively a rectangular upper section, mounted on the flanks of each stiffener by gluing plates; 
           [0027]      FIGS. 2   a  and  2   b  are partial perspective views in section of one example of an aramide-epoxy structural section mounted on a panel stiffener and featuring shoulders for attaching clips (also known as patches); 
           [0028]      FIGS. 3   a  and  3   b  are partial perspective views in section of one example of a T-shaped aluminum-based alloy structural section mounted on a stiffener and featuring projections for attaching mounting clips on the flanks of the stiffener; and 
           [0029]      FIGS. 4   a  and  4   b  are partial perspective views in section of one example of an aramide-epoxy structural section mounted on a panel stiffener and featuring trough-shaped shoulders for attaching clips. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    In the following detailed description, the qualifiers “upper” and “lower” respectively relate to the space situated above the edge of each stiffener and to the spaces situated at the level of the lateral flanks of each stiffener. Furthermore, the term “lateral” qualifies any extension in a plane parallel to the plies of the stiffeners and the term “transversal” qualifies any extension in a plane perpendicular to those plies. Moreover, the term “external” is in opposition to the term “internal” by virtue of its greater distance from the median plane Pm. Moreover, identical reference signs in the figures refer to the same elements defined with reference to the passages that describe them. 
         [0031]    The detailed description relates more specifically to composite panel stiffener protection embodiments including a multitude of stiffeners. Such a panel is formed in a manner known in itself from U-section preforms produced by stacking carbon-fiber-reinforced epoxy resin plies in order to form the stiffeners. 
         [0032]    Referring to  FIG. 1   a , the perspective view shows a first example of a structural section  1  in titanium-based alloy, shaped in a hydraulic press. The upper section  11  of this structural section features two upper lateral walls F 1  with “reentrant” double lobes La and Lb, formed symmetrically with respect to the median plane Pm of the stiffener  2 , and a substantially plane upper wall F 2  parallel to the edge  20  of this stiffener  2 . The summits S 1  and S 2  of the reentrant lobes come as close as possible to the median plane Pm. 
         [0033]    The lobes Lb have transverse walls Pb that come to bear on the edge  20  to form the intermediate section  12  of the structural section  1 . These walls Pb are extended by framing lower walls Pi extending along the respective lateral flanks  21  of the outside plies of the stiffener  2 . The lower walls Pi form the lower section  13  of the structural section  1 . These walls Pi are rigidly fastened to the flanks  21  by circular plates  3  introduced between the lower walls Pi and the flanks  21 . To be more precise, the plates  3  are equipped with studs  30  mounted centrally and perpendicularly to the plane of the plates  3 . The studs  30  pass through the walls Pi via notches  31  formed in the extreme longitudinal border B of these walls. The internal faces  3   a  of the plates  3  are then stuck by means of epoxy resin to the flanks  21  and the studs  30  are fixed against the walls Pi by nuts  32  tightened onto the external face of the lateral flanks  21 . 
         [0034]    The notches  31  are produced at regular intervals along the borders B of the walls Pi, for example every 100 mm. The titanium alloy structural section has a thickness substantially equal to 0.5 mm and the upper section  11  has a height H substantially equal to 20 mm. Under these conditions, deformation of the upper section  11  created by an impact on the upper face F 2  comes to bear on the edge  20  via the intermediate section  12 , which makes it possible to limit the loading in shear of the lower walls Pi fixed against the flanks  21 . The impact force is then distributed over the edge: the stress and the forces out of the stacking planes are thus limited and damage to the structure is prevented. 
         [0035]    Moreover, the structural section  1  is easily demountable by loosening the nuts  32 . Alternatively, “anchor” type spring fasteners may be used to produce a reversible fixing of the structural section to the flanks  21  of the stiffener  2 . 
         [0036]    Moreover, in the event of an impact on the upper wall F 2 , the enveloping shape of the upper section  11  makes it possible to dissipate the impact energy by plastic deformation. For example, the double-lobe shape in  FIG. 1   a  enables absorption of 50 Joules produced by the impact of a hemispherical metal object without damaging the stiffener. 
         [0037]    Alternatively,  FIG. 1   b  shows a structural section  10  of a stiffener  2  that has an upper section  111  of rectangular cross-section, with no lobes. The upper lateral walls F 3  of this section  111  are therefore plane but are extended transversely by reentrant bends Lc to form an intermediate section  121  bearing on the edge  20 . The lower section  13  repeats that of the structural section from  FIG. 1   a.    
         [0038]      FIGS. 2   a  and  2   b  are perspective views in section of a second example of a structural section  100  in aramide and epoxy resin, with a thickness substantially equal to 1.6 mm. It is produced by molding in a metal mold and counter-mold with a metal core present. 
         [0039]    The upper section  112  of the structural section  100  may have a circular or deformed circular type cross section. It advantageously has an elliptical type oblong cross section as shown, with a greater extent along the median plane Pm than transversely, so as to favor impact resistance. For the intermediate section  122 , the structural section  100  is composed of lobe-shaped transverse bearing walls Pc of the same type as the walls Pb described above with a wider opening because of the oblong shape of the upper section  112 . 
         [0040]    To meet the requirements for reversible attachment to the flanks  21  of the stiffener  2 , the framing lower lateral walls Pj of the lower section  132  include shoulders  40  adapted to serve as supports for clips  4 . These clips  4  have rims  41  that bear on the shoulders  40  and longitudinal walls  42 . These walls  42  terminate in forming, by virtue of a step  43 , a face  42   c  in contact with the flanks  21  of the stiffener  2  of the panel P A . These contact faces  42   c  are then stuck to the flanks  21 , also by means of an epoxy resin. The shoulders  40  have a projecting profile in order to facilitate unfastening and therefore demounting of the structural section. 
         [0041]    In another embodiment, shown in  FIGS. 3   a  and  3   b  in section and in perspective, a structural section  200  is produced in extruded aluminum-based alloy. The structural section  200 , with a thickness substantially equal to 1.5 mm, is T-shaped in the upper section  113 . This shape is extended by a transverse wall Pd bearing on the whole of the edge  20  of the stiffener  2  to form an intermediate section  123 . This transverse bearing wall Pd is extended perpendicularly by framing lower lateral walls Pk that come against the lateral flanks  21  of the stiffener  2 . 
         [0042]    These lower walls Pk each have a longitudinal projection  50  that serves as an attachment area for clips  5  disposed along the walls Pk. These patches have a wall  51  stuck to the flank  21  and a hook  52  that clips onto each projection  50 . The patches are produced by injection molding plastic. 
         [0043]      FIGS. 4   a  and  4   b  are perspective views in section of an example of an aramide and epoxy resin structural section  300  example representing a variant of the structural section  100  from  FIGS. 2   a  and  2   b , the upper section  112  and the intermediate section  122  of which correspond to those of  FIG. 2   a . In this example, for the requirements of reversible attachment to the flanks  21  of the stiffener  2 , the lower lateral walls P 1  of the lower section  133  of this structural section  300  include trough-shaped projections  60  that serve for fastening to clips  6  disposed along the walls P 1 . These clips  6  have a wall  61  stuck to the flank  21  and a hook  62  with a shape appropriate to the trough-shaped projection  60 . These features make it possible to improve the durability and the strength of the assembly comprising the stiffener  2 , the structural section  300  and the clip  6 . 
         [0044]    Moreover, the clips  6  comprise between the wall  61  and the hook  62  an intermediate part  63  that diverges from the corresponding flank  21  so as to form a space  64  between the clip  6  and the flank  21 . This space  64  allows movement of the lower section  133  of the structural section  300  toward the lower part without acting on the clip  6 , which enables detachment of the clip  6  to be avoided in the case of impact on the upper face of the structural section  300 . 
         [0045]    Moreover, notches  65  are produced on the lower lateral walls P 1  of the structural section  300 . These notches  65 , which may notably be produced at the longitudinal ends  66 ,  67  of the lateral walls P 1 , are filled with mastic (not shown), which makes it possible to strengthen the adhesion of the structural section  300  to the flanks  21  of the stiffener  2 . 
         [0046]    The invention is not limited to the examples described and represented. For example, the material used for the structural section may consist of some other metal alloy (for example based on nickel) or some other composite material (based on glass, glass-aramide hybrid or carbon-aramide hybrid fibers). The fastening means on the lateral walls of the structural sections for the patches or clips may consist of a plurality of projections on each wall, and may be formed by molding them substantially in the middle of the wall or at its edge.

Technology Classification (CPC): 1