FR3150777A1 - Geodesic structure with transmission of forces in two perpendicular directions and production method. - Google Patents

Abstract

The invention relates to a geodesic structure (1) for transmitting forces in two perpendicular directions in a panel of a given configuration architecture. The structure comprises stiffening elements (10) having faces (F1, F2, Fb) articulated on edges (A1, A2) extending parallel to one of the directions. Each element (10) has a triangular section perpendicular to this direction and the stiffening elements (10) are assembled together in at least one layer so that the faces (F1, F2, Fb) constitute, in section, a geodesic surface. At least one skin (P1, P2) is secured to each set of faces (Fb) and/or edges (A2) of end stiffening elements of the assembly forming consolidation surfaces of the structure (1). Abstract figure: [Fig. 1]

Description

Geodesic structure with transmission of forces in two perpendicular directions and production method.

The invention relates to a geodesic type structure allowing the transmission of forces in two perpendicular directions in a panel of a given architecture. The term "geodesic structure" is conventionally understood to mean a surface defining a volume and consisting of elementary spacers made of metal alloy or other rigid material, arranged to form a rigid triangular lattice. This braiding gives great resistance to the assembly because each spacer supports the forces of other spacers, even those located at a distance. This type of structure has been used in the past to produce aircraft fuselages or wings.

• porte d’avion constituée de poutres et cadre ou trappe de train d’atterrissage constituée de longerons et cadre, avec une direction dans le sens du fuselage et une direction dans le sens des cadres perpendiculaires à la première direction ;
• caisson de voilure constitué de longerons et nervures ou éléments mobiles de cette voilure (aileron, volet, lame, gouverne) avec une direction dans le sens de l’envergure et l’autre dans le sens du des nervures ;
• plancher pressurisé de cabine, constitué de lisses et cadre.

The present invention applies to architectural panels having structures capable of transmitting forces in two directions. This need for a structure is found more particularly in the aeronautical field where it is necessary to transport shear forces in two perpendicular directions, in particular in the following architectures:

• aircraft door made of beams and frame or landing gear door made of spars and frame, with one direction in the direction of the fuselage and one direction in the direction of the frames perpendicular to the first direction;
• wing box consisting of spars and ribs or moving elements of this wing (aileron, flap, blade, rudder) with one direction in the span direction and the other in the rib direction;
• pressurized cabin floor, made of rails and frame.

This need can also be found in other areas of construction: building, railways, navigation, etc.

Traditionally, double stiffening in two perpendicular directions is implemented. For example, in an aircraft door structure, this double stiffening is achieved on the one hand by beams arranged along the axial direction of the fuselage and on the other hand by frames arranged parallel along the direction transverse to the fuselage axis, with the use of at least three frames. This structure leads to numerous crossings where only one of the two stiffening elements, beams or spars, can remain continuous. It therefore involves the use of numerous connecting parts on the stiffening element cut into sections, which also weakens the assembly, and adds weight and production time.

STATE OF THE ART

In order to solve this fundamental problem, it is proposed to use sandwich structures comprising two sheet metal skins separated by a material or a lightweight structure forming the core allowing the shear force to pass in two perpendicular directions. In practice, two types of core have been developed: honeycomb structures and polymer foam structures (for example based on polyurethane, polypropylene or polyester).

This type of sandwich structure is easy to implement on simple geometries, particularly with constant thickness. As soon as the geometry becomes more complex, with thickness variations such as for a wing trailing edge, bonding the cores to the skins can become problematic. In addition, these structures absorb a large quantity of water and are difficult to drain. It is therefore necessary to guarantee the watertightness of the skins surrounding the core.

To overcome the drawbacks of the state of the art set out above, the main objective of the invention is to provide an architecture allowing continuous transmission of forces in two perpendicular directions and capable of adapting to variable thickness geometries.

To do this, the invention provides for forming the structure from identical stiffening elements extending in a single direction, either in the lengthwise direction or in the transverse direction of the architecture, thus forming either stringers or beams. These basic stiffening elements, of triangular section, are joined together to be assembled in layers.

More specifically, the present invention relates to a geodesic structure of a panel of a given configuration architecture, in particular of an aircraft door, this structure comprising stiffening elements having a triangular section perpendicular to one of the directions, each stiffening element having two lateral faces articulated on an inner edge extending parallel to this direction, and a base face opposite the inner edge. This base face forms lateral edges with the lateral faces. each element having a triangular section perpendicular to this direction. The base faces of the stiffening elements are assembled together by their lateral edges in at least one layer having a surface continuity so as to constitute, in section, a geodesic surface. At least one skin is secured to each set of assembled faces and/or inner edges of end stiffening elements of the assembly forming a surface continuity for consolidating the structure. When it is unique, the skin is an inner skin joined on the inner edges.

Thus, the transmission of forces takes place along the faces of the components to be exerted in two directions capable of carrying shear forces: the one in which the stiffening elements extend and the one perpendicular to this direction of extension of these elements, as in a geodesic structure. In addition, the stiffening elements have lengths and a triangular section of dimensions adapted to the configuration of the architecture.

In addition, the structure thus produced has high torsional resistance with the advantages of sandwich structures without their drawbacks, in particular without the risk of the skins detaching.

• la couche d’éléments raidisseurs est compacte par agencement d’une configuration tête-bêche d’éléments raidisseurs adjacents ;
• une tête de renfort (« noddle » en terminologie anglaise) est agencée entre deux arêtes appartenant à deux éléments raidisseurs adjacents ;
• les éléments raidisseurs sont, pour au moins certains d’entre eux, des tubes vides de matière, ce qui permet de faire circuler des câbles ou tout type de matériel de longueur et de souplesse suffisante pour les traverser ;
• les éléments raidisseurs sont adaptés en longueur et en découpe à une configuration d’architecture donnée ;
• les éléments raidisseurs et les peaux sont constitués en un matériau choisi entre un matériau composite, un alliage métallique et un matériau plastique.

According to preferred embodiments:

• the layer of stiffening elements is compacted by arranging a head-to-tail configuration of adjacent stiffening elements;
• a reinforcement head (“noddle” in English terminology) is arranged between two edges belonging to two adjacent stiffening elements;
• the stiffening elements are, for at least some of them, tubes empty of material, which allows the circulation of cables or any type of material of sufficient length and flexibility to pass through them;
• the stiffening elements are adapted in length and cut to a given architectural configuration;
• the stiffening elements and the skins are made of a material chosen from a composite material, a metal alloy and a plastic material.

Advantageously, the composite material is made up of layers of carbon fibers (or glass, aramid such as Kevlar®, or equivalent) reinforced with an epoxy resin, the metal alloy is an aluminum alloy and the plastic material is a high-performance thermoplastic, in particular PEEK (polyetheretherketone), PEI (polyetherimide), PPS (polyphenylene sulfide) or equivalent.

• mise en rotation de mandrins de sections triangulaires prédéterminées;
• drapage de chaque mandrin par enroulement d’une fibre préimprégnés de résine thermodurcissable sur une longueur prédéterminée de chaque mandrin;
• assemblage des éléments raidisseurs en couche de structure géodésique;
• cocuisson de l’assemblage avec les peaux d’extrémité accolées aux surfaces d’extrémité dudit assemblage.
• démoulage des mandrins.

The invention also relates to a method for producing such a stiffened structure. The method comprises the following steps for preparing a composite material structure:

• rotating mandrels of predetermined triangular sections;
• draping each mandrel by winding a fiber pre-impregnated with thermosetting resin over a predetermined length of each mandrel;
• assembly of stiffening elements in geodesic structure layer;
• co-firing of the assembly with the end skins attached to the end surfaces of said assembly.
• demolding the mandrels.

• afin de faciliter leur démontage, les mandrins sont des mandrins dits à clé ayant des portions de parois bloquées par un noyau central, le retrait du noyau central entraînant la désolidarisation des portions de parois et donc le déverrouillage du mandrin ;
• les mandrins sont en silicone thermo-expansible avec un noyau métallique, ces mandrins se contractant en refroidissant, ce qui facilite la récupération des éléments raidisseurs ;
• alternativement, les mandrins étant formés d’un sac rempli de microbilles sous vide et agencé autour d’un noyau métallique, l’élimination du vide permet d’évacuer les billes et de libérer ainsi les éléments raidisseurs de leurs mandrins;
• alternativement les mandrins étant constitués par un noyau métallique moulé dans un matériau soluble dans un solvant dans une plage de températures donnée, après drapage l’ensemble est plongé dans ce solvant à une température de ladite plage, ce qui permet de dissoudre le matériau et de retirer les éléments raidisseurs du noyau;
• des têtes de renfort sont insérées au niveau des arêtes de deux éléments raidisseurs adjacents lors de l’assemblage des éléments raidisseurs en couche de structure géodésique.

According to advantageous forms of implementation:

• in order to facilitate their disassembly, the chucks are so-called key chucks having portions of walls blocked by a central core, the removal of the central core causing the separation of the portions of walls and therefore the unlocking of the chuck;
• the mandrels are made of thermo-expandable silicone with a metal core, these mandrels contracting when cooling, which facilitates the recovery of the stiffening elements;
• alternatively, the mandrels being formed from a bag filled with microbeads under vacuum and arranged around a metal core, the elimination of the vacuum makes it possible to evacuate the beads and thus release the stiffening elements from their mandrels;
• alternatively the mandrels being constituted by a metal core molded in a material soluble in a solvent in a given temperature range, after draping the assembly is immersed in this solvent at a temperature of said range, which makes it possible to dissolve the material and to remove the stiffening elements from the core;
• Reinforcement heads are inserted at the edges of two adjacent stiffening elements when assembling the stiffening elements into a geodesic structure layer.

PRESENTATION OF FIGURES

Other characteristics and advantages of the present invention will emerge from the following reading of a detailed exemplary embodiment without limiting its scope, with reference to the appended figures which represent, respectively:

- there , a partial perspective view of an example of an elementary geodesic structure according to the invention;

- there , a sectional view of an example of a compact layer geodesic structure according to the invention;

- there , a perspective view of a rotating mandrel on which a carbon fiber is wound at the start of draping in the method of manufacturing a stiffening element according to the invention;

- there , there and the , cross-sectional views of examples of mandrels that can be used when implementing said method;

- there and the , a schematic front and perspective view of an aircraft door equipped with a geodesic structure of stiffening element panels extending in a beam according to the invention;

- there , a schematic perspective view of an aircraft landing gear door equipped with a geodesic structure panel according to the invention, and

- there , a schematic perspective view of an aircraft wing trailing edge equipped with a geodesic structure panel according to the invention.

DETAILED DESCRIPTION

In the figures, identical reference signs refer to the same element and to the corresponding passages of the description. “Stiffener”, “stiffening element” and “element” designate the same object.

There illustrates a partial perspective view of a first example of a geodesic structure 1 according to the invention. This structure is called “elementary” insofar as it is made up of identical stiffening elements 10 successively secured along the lateral edges A1 to form a layer of stiffeners C1.

Each stiffening element 10 has a prismatic geometric shape of triangular section S1 which extends parallel to a direction D1. The stiffening elements 10 are secured along the lateral edges A1 so that the base faces Fb between the lateral edges A1 form a base surface Sb that is flat along the direction D2 perpendicular to D1, or generally slightly curved as in the example illustrated. To do this, two base faces Fb of two consecutive stiffeners form an angle of small amplitude, a few degrees, along their common lateral edge A1. The curvature thus obtained makes it possible to globally adapt the structure to a targeted curved architecture.

Alternatively, the overall curvature of the structure can also be obtained from triangular sections with a curvilinear base, the stiffening elements being formed from flat side faces and curved base faces compatible with a continuous overall curvature.

The stiffening elements 10 are tubular (empty of material) and made, in the example, of composite material based on carbon fibers and epoxy resin. A consolidation skin called inner skin P1 is secured to the inner edges A2 formed by the lateral faces F1, F2 of the stiffeners 10. The inner skin P1 ensures the continuity of the structure on the inner side thereof while closing the intermediate spaces between two stiffening elements 10.

Advantageously, for architectures which require superior reinforcement, a second skin P2, called the outer skin, is secured to the base faces Fb, which provides structural continuity which is thus generally reinforced.

The triangular section S1 is an isosceles triangle in the example illustrated in order to distribute the forces equally on the two lateral faces F1, F2 of each stiffener 10. More generally, the stiffening elements can have a section formed of any triangle as long as this triangle is compatible with the mechanical constraints of the structure.

Under these conditions, the forces are evacuated along the stiffeners 10, parallel to the direction D1, or parallel to the direction D2 by using a geodesic structure formed perpendicular to the direction D1.

Referring to the sectional view of the , a variant of the previous single-layer structure is shown using the same stiffeners: in this structure 2, the stiffeners 10 are mounted head to tail to form a compact layer C2 of stiffening elements 10.

In addition, reinforcement heads 30 are inserted between two edges A1, A2 belonging to two adjacent stiffening elements 10. Reinforcement heads can also be inserted between two adjacent stiffening elements of the elementary structure 1 described with reference to the . The structure 2 is completed by two end skins P1, P2, respectively inner and outer. The reinforcement heads are made of material of the same type as the stiffeners 10, namely metal alloy, in particular aluminum, composite material or plastic material, in particular thermoplastic.

In order to obtain particularly strong structures, it is possible to produce a multilayer structure by stacking compact layers C2 of stiffening elements 10 intercalated between skins of the same type as the skins P1 or P2 at each layer or on the end surfaces of the stack of layers. In order to remain reasonable in terms of structure thickness while maintaining high strength, it may be advantageous to seek a compromise between the dimension of the stiffeners and the number of layers.

The perspective view of the illustrates the first steps of an example of a method for producing a structural stiffener according to the invention. In this method, a mandrel 4 is rotated (arrow R1) via a shaft 40 mounted on a drive motor (not shown). A carbon fiber 3, pre-impregnated with epoxy resin and stored on a roller 8, is wound onto the mandrel 4 of prismatic shape with triangular section corresponding to the geometry of the stiffening elements.

The carbon fiber 3 then winds along the faces 41, 42, 43 of the mandrel 4 to form the faces of a stiffening element, such as the faces F1, F2, Fb of the stiffening element 10 (see Figures 1 or 2). The draping of the mandrel 4 continues over a predetermined length beyond the desired stiffener length.

In addition, the angle of the fiber 3 on the mandrel 4 sets the desired stiffener strength. This angle is obtained by adjusting the ratio between the rotation speed of the mandrel 4 and the fiber feed 3.

Once the rotation has stopped, the stiffening element is then assembled with the other stiffening elements to form one or more layers according to the desired geodesic configuration.

Reinforcing heads 30 are advantageously arranged between the stiffening elements 10 and the assembly is arranged on the external skin P2 before putting the internal skin P1 in place (or conversely after putting the internal skin P1 in place), the end skins P1 and P2 being attached to the end surfaces in accordance with the configuration of the .

A co-cooking of the whole assembly thus arranged is then carried out in an autoclave.

As an alternative to using a resin prepreg, it is possible to use a dry fibre and subsequently impregnate the drape. This impregnation can be carried out using various known methods.

The mandrel 4 thus draped is pre-baked in an oven (not shown), then the stiffening element 10 is demolded and recovered to be cut to the desired length and end shape to form a panel according to the intended architecture (see figures 5 to 7).

It is also possible to impregnate the fiber while it is being wound onto the mandrel by passing it through a resin bath.

In order to recover the stiffening element 10 (see figures 1 or 2) after co-firing of the mandrel 4, it is advantageous to use mandrels 4a to 4c respectively conforming to the sectional views of figures 4a to 4c.

The 4a chuck of the said key is the chuck illustrated in . It comprises locking keys constituting wall portions 41a, 42b, 42c and 43d forming the faces 41, 42 and 43 and nested inside each other outside the draping zone. Each wall portion 41a and 43d, as well as the portions 42b and 42c taken together, extend in this example symmetrically with respect to the plane of symmetry Ps. This symmetry makes it possible to make the portions 42b and 42c interchangeable and not to distinguish sides or directions for the positioning of the portions 42a and 42d.

A central metal core 4N keeps the wall portions 41a, 42b, 42c and 43d locked. Removing this central core 4N unlocks and separates the wall portions 41a, 42b, 42c and 43d. To facilitate the detachment of the composite material from the stiffening element 10, the wall portions 41a, 42b, 42c and 43d have inclined draft angles. These portions are then removed and the stiffening element 10 recovered or demolded if this element is already included in the panel.

Alternatively, the locking means are constituted by ratchets between the walls. Once the ratchets are unlocked, the walls are released, which allows the stiffener 10 to be released.

The 4b chuck illustrated in has a layer of thermo-expandable silicone 44 arranged around a metal core 45. The layer of silicone contracts when immersed in a low temperature environment, which allows the stiffening element 10 to be released and recovered.

In reference to the , the mandrel 4c has, in place of the silicone layer 44 of the mandrel 4b, a bag 46 arranged around the metal core 45 and filled with the microbeads 47 under vacuum. Once the vacuum has been eliminated, the microbeads are evacuated and the stiffening element 10 is released from its mandrel 4b

Alternatively, the mandrel is a metal core cast in a material soluble in a solvent over a range of temperatures, a water-soluble material in this example. After draping, the assembly is immersed in water at room temperature, which dissolves the material and removes the stiffening elements from the core.

The stiffening elements thus assembled are intended to constitute force passage panels in two perpendicular directions for given architectures in which these panels are integrated. The stiffeners extend along side members or beams depending on the architecture.

For example, for the aircraft door 5 as illustrated in front view and in perspective according to FIGS. 5a and 5b, the stiffening elements 10 of the stiffening structure of the type referenced 2 (cf. ), are assembled into a panel 50 in the form of longerons 11 extending in the fuselage direction D.

Door 5 being equipped with a central support 51 comprising a control screen and other equipment (porthole, handles, controls, etc.), the side members 11a, 11b and 11c, 11d are previously cut out to free up a space 5E ( ) which follows the contour of the support 51. Other panel assemblies are possible, for example two panels joined along the median MM'. By having an autoclave of suitable dimensions, a single panel provides greater overall rigidity than an assembly of several panels.

Landing gear door 6, perspective view according to the , takes up structure 1 of the according to a generally rectangular panel configuration in accordance with the configuration of the trapdoor architecture. Indeed, the constraints being of limited intensity, the structure 1, integrating a lower density of stiffening elements 10, can be advantageously used for this type of architecture.

Furthermore, a wing trailing edge 7 as shown in by a perspective view, uses an assembly of stiffening elements 12 of prismatic geometry with adapted dimensions. Indeed, such a trailing edge architecture 7 here uses a stiffening structure panel according to the invention of type 1 (cf. ) or 2 (cf. ), with stiffening elements 12 according to the invention which regularly decrease in dimensions as one moves towards the end 7E of the trailing edge 7. Furthermore, the triangular section of these stiffeners 12 is not inscribed in an isosceles triangle but in a triangle adapted to the reduction in distance between the walls 7a, 7b of the trailing edge 7: the section decreasing linearly as a function of the span, the triangular sections are deduced by homothety.

The invention is not limited to the examples described and shown. Thus, the mandrels may be made of a sacrificial material (wax or equivalent) that is easily destructible in order to facilitate the recovery of the stiffening elements. The core mandrels may alternatively be made of a layer of foam, in particular a polymer foam, which remains chemically bonded to the stiffening element once the core has been separated or removed in order to reinforce the stiffener.

Furthermore, the base fiber may be a glass fiber, linen, aramid fiber, in particular Kevlar®, a fiber made of thermoplastic material, in particular polyethylene, or an equivalent material.

In addition, the tooling used is preferably thermo-deformable in order to facilitate the implementation of the manufacturing steps.

The invention also applies to wing boxes or any other type of beam stiffened in two directions.

Claims (15)

Geodesic structure for transmitting forces in two perpendicular directions (D1, D2) in a panel of a given configuration architecture (6, 7), in particular of an aircraft door, characterized in that this structure (1, 2) comprises stiffening elements (10, 11, 12) having a triangular section perpendicular to one of the directions (D1), each stiffening element having two lateral faces (F1, F2) articulated on an inner edge (A2) extending parallel to this direction (D1), and a base face (Fb) opposite the inner edge (A2) which forms lateral edges (A1) with the lateral faces (F1, F2), in that the base faces (Fb) of the stiffening elements (10, 11, 12) are assembled together by their lateral edges (A1) in at least one layer having a surface continuity such that the faces (F1, F2, Fb) constitute, in section, a geodesic surface, and in that at least one skin (P1, P2) is secured to each set of assembled faces (Fb) and/or internal edges (A2) of stiffening elements forming a continuity of consolidation surface of the structure (1, 2), the skin being an internal skin (P1) secured to the internal edges (A2) when it is unique. Geodesic structure according to claim 1, in which the layer of stiffening elements (10, 11, 12) is compact by arranging a head-to-tail configuration of adjacent stiffening elements (10, 11, 12). Geodesic structure according to any one of claims 1 to 2, in which a reinforcing head (30) is arranged at the edges (A1; A1, A2) belonging to two adjacent stiffening elements (10, 11, 12). A geodesic structure according to any one of claims 1 to 3, wherein the stiffening elements (10, 11, 12) are, for at least some of them, tubes empty of material. A geodesic structure according to any one of claims 1 to 4, wherein the stiffening elements (10, 11, 12) are adapted in length and cutting to a given architectural configuration (6, 7, 8). A geodesic structure according to any one of claims 1 to 5, wherein the stiffening elements (10, 11, 12) and the skins (P1, P2) are made of a material chosen from a composite material, a metal alloy and a plastic material. Method for producing a geodesic structure according to any one of claims 1 to 6, characterized in that it comprises the following steps:

• rotating mandrels (4) of predetermined triangular sections;
• draping each mandrel (4) by winding a fiber (3) pre-impregnated with thermosetting resin over a predetermined length of each mandrel (4);
• assembly of the stiffening elements (10, 11, 12) in a geodesic structure layer (1, 2);
• co-firing of the assembly with the end skins (P1, P2) attached to the end surfaces of said assembly;
• demolding the mandrels (4).

Method for producing a geodesic structure according to claim 7, in which the mandrels (4) are so-called key mandrels, each key mandrel (4a) having wall portions (41a, 42b, 42c, 43d) blocked by a central core (4N), the removal of the central core causing the separation of the wall portions (41a, 42b, 42c, 43d) and the unlocking of the mandrel (4a). A method of producing a geodesic structure according to claim 7, in which the mandrels (4b) are made of thermo-expandable silicone with a metal core, these mandrels (4b) contracting on cooling to facilitate the extraction of the mandrels (4b). A method of producing a geodesic structure according to claim 7, in which the mandrels (4c) are formed from a bag filled with microbeads under vacuum and arranged around a metal core, the elimination of the vacuum evacuating the microbeads and releasing the stiffening elements from their mandrels (4c). Method for producing a geodesic structure according to claim 7, in which the mandrels being constituted by a metal core molded in a material soluble in a solvent in a given temperature range, after draping the assembly is immersed in this solvent at a temperature in said range, causing the dissolution of the material and the extraction of the stiffening elements (10, 11, 12) from the core. A method of producing a geodesic structure according to claim 7, in which the mandrels are formed from a layer of foam arranged around a metal core, the layer of foam remaining chemically bonded to the stiffening element (10, 11, 12). Method of producing a geodesic structure according to claim 7, in which the mandrels are made of sacrificial material. Method of producing a geodesic structure according to claim 7 when this claim is attached to any one of claim 3 to claim 6, in which the reinforcing heads (30) are inserted at the edges (A1; A1, A2) of two adjacent stiffening elements during the assembly of the stiffening elements in a geodesic structure layer (1, 2). Method for producing a geodesic structure according to any one of claims 7 to 14, in which the method uses thermo-deformable tooling.