MECHANICAL FIXING AT THE END OF A COMPOSITE TUBE

Abstract

The invention relates to a structural element (100) with a composite tube (1) for a three-dimensional structure, configured to be attached to an anchor point comprising a dowel (9) having a radially expandable outer ring (91) for static friction fixing of the dowel (9) in the composite tube (1), said composite tube (1) comprising: - longitudinal fibers (3), - an embedding resin in which the longitudinal fibers (3) are embedded, characterized in that it further comprises at least one reinforcement (5) of fibers (51) over a predetermined length at at least one end, the fiber(s) (51) forming the reinforcement (5) being wound around the composite tube (1) and embedded in the embedding resin, said fibers (51) of the reinforcement (5) forming an angle (α) of at least sixty degrees with the longitudinal axis (L) of the tube (1). Figure for the abstract: [Fig. 1]

Description

Title of the invention: MECHANICAL FIXING AT THE END OF A COMPOSITE TUBE technical field

[0001] The present invention relates to the general field of composite material reinforcements, particularly for use in aeronautics, for example as a stiffening structure for balloons, aircraft fuselages, etc.

[0002] The invention relates to the field of tubes and three-dimensional composite structures, with particular mechanical properties arising from their internal structure, and in particular to the fixing of tubes to an extreme fitting in a solid manner by means of a mechanical fixing. Previous techniques

[0003] Composite materials, and in particular carbon fiber tubes coated with resin, make it possible, with equal or even greater strength, to produce lightweight and high-strength reinforcements or three-dimensional structures.

[0004] This lightness is particularly suitable and sought after in the aeronautical field, both for aircraft, gliders and airplanes, and for aerostats, balloons and airships.

[0005] The structures are notably composed of cylindrical tubes, having longitudinal fibers giving them great strength. These cylindrical tubes are fixed to anchor points with, generally, metal parts such as sleeves or external end caps bonded to dowels and screw-nut systems.

[0006] For example, document WO2022 / 123124 describes a fastening device using a dowel with a truncated conical insert, inserted into the end of the tube. Securing and tightening the end fitting inserts the truncated conical insert into the dowel to exert an outward radial force that secures the dowel to the tube.

[0007] The screw then serves to connect to the anchor point, with a strength allowing a significant tensile resistance, in particular greater than one tonne or even ten tonnes.

[0008] The outward radial force exerted by the peg allows for the joining, but is likely to cause a bursting of the bundle of fibers forming the core of the tube.

[0009] To prevent this bursting, document WO2022 / 123124 and prior art devices use an outer metallic ring or tubular sleeve, which covers the end of the tube along the length of the pin, in order to counter the radial force and prevent bursting.

[0010] This results in a still significant weight, even when using titanium for the metal parts, whereas lightness is central to performance in aeronautics. Description of the invention

[0011] To meet this need, the invention proposes a composite tube structural element for three-dimensional structures, configured to be attached to an anchor point, comprising a dowel having a radially expandable outer ring for fixing the dowel in the composite tube by static friction, said composite tube comprising: - longitudinal fibers, - a coating resin, in which the longitudinal fibers are coated, characterized in that it further comprises at least one fiber reinforcement over a predetermined length at at least one end, the fiber or fibers forming the reinforcement being wound around the composite tube and coated in the coating resin, said reinforcement fibers forming an angle of at least sixty degrees with the longitudinal axis of the tube.

[0012] This additional reinforcement can be easily added around a pre-existing, pre-cut and packaged tube, and advantageously replaces the metal sleeve of the prior art, being lighter with comparable strength.

[0013] The composite tube structural element may in particular have one or more of the following characteristics, taken alone or in combination.

[0014] The fiber or fibers forming the reinforcement form an angle of at least eighty degrees with the longitudinal axis of the composite tube.

[0015] The reinforcing fibers can be pre-impregnated resin fibers.

[0016] The reinforcing fibers may, as an alternative or in addition, comprise fibers dry resin-coated materials.

[0017] The ankle may in particular be a conical ankle having: - an outer ring, forming an outer skirt with a diameter substantially equal to the inner diameter of the composite tube, with a conical inner surface and a shoulder for the force-holding of a screw or nut, which can be radially moved apart, - at least one frustoconical insert, with a cone angle equal to or greater than that of the inner surface of the outer ring, and whose insertion into the outer ring radially separates the outer ring, - a shaft passing through the outer ring and the frustoconical insert, and which exerts, when fixing the structural element, a force which inserts the frustoconical insert into the outer ring.

[0018] The external surface of the outer ring can then advantageously be textured for better grip.

[0019] The fibre reinforcement may in particular have a length corresponding at least to the inserted length of the outer ring of the conical dowel in the composite tube.

[0020] The conical pin may in particular comprise two parallel frustoconical inserts, and two inner rings having two consecutive conical surfaces, the force exerted by the shaft radially separating the outer rings when fixing the structural element.

[0021] The outer ring of the ankle may include a shoulder which, in the assembled state, bears against the extreme radial surface of the composite tube.

[0022] The outer surface of the outer ring of the ankle can be textured to increase friction with the inner surface of the composite tube.

[0023] The invention also relates to the method of assembling a structural element of a composite tube as previously described, characterized in that it comprises the following steps: - preparation of a composite tube end piece, - winding of a reinforcing fiber at the end of the composite tube, the reinforcing fiber forming an angle of at least sixty degrees with the longitudinal axis of the composite tube to form a reinforcement, - hardening of a resin impregnating the reinforcing fiber, - insertion of a dowel into the end of the composite tube equipped with the reinforcing fiber winding forming the reinforcement, - tightening the ankle in the end of the composite tube.

[0024] In particular, the winding of the reinforcing fiber can be done by exerting a constrictive stress by means of the reinforcing fiber. Brief description of the figures

[0025] Other advantages and features of the invention will become apparent from the following description of the accompanying figures, among which:

[0026] [Fig. 1] is a schematic perspective representation of the end of a composite tube for a structural element according to the invention in a first embodiment,

[0027] [Fig.2] is a schematic perspective representation of an alternative embodiment of a composite tube for a structural element according to the invention,

[0028] [Fig.3] is a longitudinal cross-sectional view of a conical dowel for forming a fitting at the end of the composite tube of the preceding figures,

[0029] [Fig.4] is an exploded side view of the conical pin of [Fig.3],

[0030] [Fig.5] is a cross-sectional view of a structural element comprising the composite tube and the ankle of the preceding figures according to a first embodiment,

[0031] [Fig.6] is a cross-sectional view of a structural element comprising the composite tube and the ankle of the preceding figures according to another embodiment,

[0032] [Fig.7] is a cross-sectional representation of a dowel with two truncated conical inserts,

[0033] [Fig.8] is a linear flowchart showing the main assembly steps of the structural element in Figures 6 and 7,

[0034] [Fig.9],

[0035] [Fig. 10],

[0036] [Fig. 11] and

[0037] [Fig. 12] illustrate the steps of the process of [Fig.8].

[0038] The examples are given by way of illustration and are not intended to be limiting. Other embodiments of the invention can easily be obtained by variations and combinations of the embodiments shown. The dimensions and proportions illustrated in the figures may differ from reality and may have been adapted to facilitate understanding of the invention. Detailed description

[0039] Figure 1 is a schematic perspective view of a composite tube 1, intended in particular to be attached at one end to an anchor point. The composite tube 1 is composed of longitudinal fibers 3, coated with resin or pre-impregnated reinforcing fiber. The longitudinal fibers 3 form, in particular, a small angle, less than thirty degrees (30°), with the longitudinal axis L of the composite tube 1. The longitudinal fibers 3 may, in particular, be long carbon fibers. The figures represent longitudinal fibers 3, therefore parallel to the longitudinal axis L (angle of 0°).

[0040] In addition, the embodiment of [Fig.1] further presents fibers 31, 33 inclined with respect to the longitudinal axis L, said inclined longitudinal fibers 31, 33 are twisted and composed of two superimposed twisted bundles at opposite and equal angles with respect to the longitudinal axis, for example of a bundle 31 at +30° and a bundle 33 at -30°.

[0041] Other angle values ​​for inclined beams 31, 33 are in particular possible, typically going up to 45°.

[0042] The composite tube 1 shown has at its end a reinforcement 5 of added fibers, composed of transverse reinforcing fibers 51, forming with the longitudinal axis L a significant angle, typically of sixty to ninety degrees (60-90°), and coated with resin.

[0043] Figure 2 shows an alternative embodiment of composite tube 1, in of which the fibers 51 of the reinforcement 5 are long fibers, which are wound around the end of the composite tube 1 at an angle of inclination α with the longitudinal axis The important L, especially greater than sixty degrees of angle, and more particularly greater than eighty degrees of angle.

[0044] The angle a can in particular be as close to ninety degrees of angle as possible, by forming a tight reinforcement of wound reinforcing fibers 51, loop against loop.

[0045] The reinforcement 5 may in particular include several superimposed reinforcing windings of fibers 51, typically from two to ten depending on the required strength.

[0046] The reinforcement 5 thus makes it possible to increase the burst resistance of the composite tube 1 at its end, in order to allow the use of a fastener exerting a radial force for its securing by static friction with the composite tube 1.

[0047] The fiber or fibers 51 of the reinforcement 5 may in particular be pre-impregnated fibers, wound by exerting a significant compressive tension, on the order of several hundred or thousands of newtons.

[0048] This results in a prestress, allowing in particular to exert a radial stress outwards without deformation up to values ​​exceeding said prestress.

[0049] As an alternative or in addition, so-called dry fibers 51, initially without resin, can be wound and then coated with resin.

[0050] The invention consists of replacing the metal sleeve with the winding forming reinforcement 5 which will take over the expansion forces of the ankle on the outside of the composite tube 1. The number of layers of the winding forming reinforcement 5 and the length of the winding forming reinforcement 5 make it possible to generate a resistance adapted to a large range of forces according to their expected value, which can in particular exceed one tonne or even ten tonnes of tensile force.

[0051] In particular, the reinforcement 5 is only present at the end of the composite tube 5, and therefore represents only a modest additional weight compared to the composite tube 1 without reinforcement 5, and above all an additional weight less than that of a metal sleeve.

[0052] In particular, [Fig. 3] shows a cross-section of a so-called conical dowel 9 that can be used in combination with the composite tube 1 of Figures 1 to 3. [Fig. 4] shows said conical dowel 9 in exploded view.

[0053] The conical pin 9 has an outer ring 91, provided with staggered longitudinal slots 97, giving a radially extensible character to the outer ring 91.

[0054] The outer ring 91 has a conical inner surface and an end 92 forming a force-bearing contact, for example, against a fuselage surface, a screw head, or a nut of the anchor point. A central bore passes through the outer ring 91.

[0055] The conical pin 9 also has a frustoconical insert 93, insertable into the outer ring 91, with an external conical surface of cone angle equal to or slightly greater than that of the internal surface of the outer ring 91.

[0056] The truncated conical insert 93 has a central bore provided with a thread, and has means for locking against rotation relative to the outer ring 91.

[0057] Inserting the frustoconical insert 93 into the outer ring 91 then moves the outer ring 91 radially apart.

[0058] A shaft 90 passes through the outer ring 91 and the frustoconical insert 93, and engages with the frustoconical insert 93. For example, the shaft 90 can be a threaded rod, cooperating with a thread in a central bore of the frustoconical insert 93, or a straight rod, provided with a shoulder engaging with the frustoconical insert 93.

[0059] When an axial force is applied to the shaft 90, the radial insert 93 is forced into the outer ring 91, which moves it radially apart.

[0060] Figure 6 shows a cross-section of a structural element 100 forming an attachment point obtained by a conical dowel 9 inserted into the composite tube 1.

[0061] The conical dowel 9 has a diameter substantially equal to the inner diameter of the composite tube 1, and has a shoulder 95 that acts as a stop during insertion. The reinforcement 5 has, in particular, a length at least equal to the length l inserted by the conical dowel 9 in the composite tube 1.

[0062] The end 92 of the pin 9 is in contact with a radial support S, for example an aircraft fuselage, or a nut or a screw head.

[0063] When the structural element 100 is attached to the rest of the structure, a force tending to insert the frustoconical insert 93 into the outer ring 91 is generated. This force, due to the conical shape of the frustoconical insert 91 and the inner surface of the outer ring, is transformed into a radial force tending to push the outer ring 91 apart.

[0064] The radial force exerted by the radial separation of the peg 9 is notably compensated by the reinforcement 5, and induces static friction enabling the peg 9 to resist in tension forces of several tons or even tens of tons without tearing.

[0065] Arrows illustrate these forces in [Fig.5].

[0066] In the case of a shaft 90 in the form of a threaded rod, said threaded rod terminates for example with a bolt head, and is inserted into a hole against which the bolt head and the end 92 of the pin 9 bear on one side, thus forming a screw-nut system.

[0067] Alternatively, a direct axial force can be exerted on the shaft 90, for example by means of wedges or hydraulic traction systems.

[0068] Figure 6 illustrates a variant of the pin 9 in which the outer ring 91 is textured on its outer radial surface. In particular, the outer ring 91 is provided here with a surface of annular grooves 99, giving it a sawtooth cross-section.

[0069] These annular grooves 99 are in particular forcibly inserted into the material of the composite tube 1 during the tightening of the pin 9 and the resulting radial expansion of the outer ring 91.

[0070] This texturing of the outer ring 91 makes it possible to improve static friction, and therefore the force required for the removal of the composite tube from the peg 9 and therefore of the structure thus obtained.

[0071] The external surface of the outer ring 91 can in particular be textured with protrusions of different shapes, for example with diagonal milling forming pyramidal protrusions, or with studs.

[0072] Figure 7 illustrates another embodiment of the conical pin 9 which comprises two parallel frustoconical inserts 93 in two external rings 91 mounted in series. The inner ring 91 then has two conical surfaces, each with a frustoconical insert 93, and the force applied to the shaft inserts both frustoconical inserts 93 into the external ring 91 that contains them, when the pin 9 is tightened, in order to radially separate the external rings 91 and thus generate a greater length of bearing surface in static friction.

[0073] Other models of radially spaced dowel 9 are of course usable in the context of making an attachment point with the composite tube 1 according to the invention.

[0074] Fig. 8 is a flowchart showing the main steps of the assembly process 200 of the structural element using the composite tube 1 and the dowel 9. These steps are illustrated in Figures 9 to 12.

[0075] The first step 201 is the preparation of the end of the composite tube 1, for example by cleaning, treating or applying a resin adhesion layer. The composite tube 1 alone is shown in [Fig.9].

[0076] The second step 203 is the winding of one or more reinforcing fibers 51 at the end of the composite tube 1, with an angle with the longitudinal axis L of the composite tube 1 of at least sixty degrees.

[0077] In particular, the fiber 51 used can be pre-impregnated and wound under constrictive stress, as previously detailed.

[0078] Fig. 10 shows the composite tube 1 provided with this winding forming reinforcement 5.

[0079] The third step 205 is possibly the coating and especially the hardening of resin of the winding forming reinforcement 5 of reinforcing fiber 51, particularly in the case of a pre-impregnated fiber 51.

[0080] In particular, the end of the composite tube 1 can be inserted into a heating sleeve to trigger said hardening.

[0081] Fig. 11 shows the composite tube 1 provided with the winding forming reinforcement 5 coated with hardened resin.

[0082] The fourth step 207 is the insertion of the peg 9 into the end of the composite tube 1 provided with the reinforcing winding 5 of reinforcing fiber 51. In particular, the length of the reinforcing winding 5 may be greater than the length inserted of the peg 9 into the composite tube 1.

[0083] The fifth step is then the tightening 209 of the peg 9 in the end of the composite tube 1 provided with the winding forming reinforcement 5, in order to fix it by static friction due to the radial force generated by the separation of its outer ring 91.

[0084] As previously described, the tightening of the pin 9 can in particular be confused with the step of fixing the structural element 100 to the rest of a composite structure, by tightening or tensioning the shaft 90.

[0085] By increasing radial strength, the reinforcement 5 eliminates the need for a metallic end sleeve on the structural element 100 or attachment point. The reinforcement 5 thus reduces the weight of said attachment point. These structural elements 100 are particularly numerous in aircraft and aerostat structures, and reducing their weight significantly decreases the overall weight of the structure.

Claims

Demands

1. A structural element (100) with a composite tube (1) for a three-dimensional structure, configured to be attached to an anchor point comprising a dowel (9) having a radially extensible outer ring (91) for static friction fixing of the dowel (9) in the composite tube (1), said composite tube (1) comprising: - longitudinal fibers (3, 31, 33), - an encapsulating resin, in which the longitudinal fibers (3, 31, 33) are embedded, characterized in that it further comprises at least one reinforcement (5) of fibers (51) over a predetermined length at at least one end, the reinforcing fiber(s) (51) forming the reinforcement (5) being wound around the composite tube (1) and embedded in the encapsulating resin, said fibers (51) of the reinforcement (5) forming with the longitudinal axis (L) of the tube (1) an angle (a) of at least sixty degrees.

2. Structural element (100) of composite tube (1) according to claim 1, characterized in that the reinforcing fiber(s) (51) forming the reinforcement (5) form, with the longitudinal axis (L), an angle (a) of at least eighty degrees.

3. Structural element (100) of composite tube (1) according to claim 1 or 2, characterized in that the fibers (51) of the reinforcement (5) comprise resin-prepreg fibers.

4. Structural element (100) with composite tube (1) according to claim 1, 2 or 3, characterized in that the reinforcement (5) comprises at least one dry fiber (51) wound around the end of the composite tube (1) and coated with resin.

5. A structural element (100) with a composite tube (1) according to any one of the preceding claims, characterized in that the dowel (9) is a conical dowel (9) having: - an outer ring (91), forming an outer skirt with a diameter substantially equal to the inner diameter of the composite tube (1), with a conical inner surface and a shoulder for the force-taking of a screw or nut (92), which can be radially spread apart, - at least one frustoconical insert (93), with a cone angle equal to or greater than that of the inner surface of the outer ring (91), and whose insertion into the outer ring (91) radially separates the outer ring (91), - a shaft (90) passing through the outer ring (91) and the frustoconical insert (93), engaging with the frustoconical insert (93) and exerting, when fixing the structural element (100) a force which inserts the frustoconical insert (93) into the outer ring (91).

6. A structural element (100) with a composite tube (1) according to the preceding claim, characterized in that the reinforcement (5) of fibers (51) has a length corresponding at least to the inserted length (Z) of the outer ring (91) of the conical peg (9) in the composite tube (1).

7. Structural element (100) with composite tube (1) according to claim 5 or 6, characterized in that the conical pin (9) comprises two parallel frustoconical inserts (93), and two internal rings (91) mounted in series, having two conical surfaces each with a frustoconical insert (93), the force exerted by the shaft (90) radially separating the external rings (91) when fixing the structural element (100).

8. Structural element (100) of composite tube (1) according to any one of claims 5 to 7, characterized in that the outer ring (91) has a shoulder (95) bearing, in the assembled state, against the extremal radial surface of the composite tube (1).

9. Structural element (100) of composite tube (1) according to any one of claims 5 to 8, characterized in that the outer surface of the outer ring (91) of the conical pin (9) is textured to increase friction with the inner surface of the composite tube (1).

10. Method for assembling (200) a structural element (100) with a composite tube (1), characterized in that it comprises the steps: - preparation (201) of a composite tube end (1), - winding (203) of a reinforcing fiber (51) at the level of the composite tube end (1), the reinforcing fiber (51) forming with the longitudinal axis (L) of the composite tube (1) an angle of at least sixty degrees, - curing (205) of resin impregnating the reinforcing fiber (51) of the winding, - insertion (207) of a dowel (9) into the end of the composite tube (1) provided with the winding of reinforcing fiber (51) forming the reinforcement (5),

11. - tightening (209) of the peg (9) in the end of the composite tube (1). Method according to the preceding claim, characterized in that during the winding step (203) of the reinforcing fiber (51), said winding is carried out by exerting a constrictive stress by means of the reinforcing fiber (51).

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