FR2712939A1 - Fibre reinforced composite fastener for an article - Google Patents

Abstract

Fibre reinforced composite fastener has an integral head and shank and is tapered between them. The fastener is formed by spirally winding superimposed composite sheets including reinforcing fibres, pref. parallel to the longitudinal axis of the fastener. The sheets are of thermosetting or thermoplastic resin and of different sizes positioned mostly at the head end. Also claimed is at least two members (40, 42, 44) with aligned openings (45) and joined by two such fasteners (36, 38) whose heads are on opposite sides of the openings in the article. Also claimed are methods of making the fastener and joining members using the fasteners.

Description

COMPOSITE ASSEMBLY ELEMENT, METHOD FOR
MANUFACTURE AND COMPOSITE ARTICLE USING
THIS ASSEMBLY ELEMENT
The present invention relates to composite articles comprising members at least partially assembled by mechanical means, such as aerodynamic elements of blades, fins, masts, etc. of turbomachines, and it relates more particularly to an element of fiber-reinforced composite assembly which is suitable for fixing composite materials to composite materials, or for fixing composite materials to metals.

 The increasing use of composite materials in the primary and secondary structure of aircraft has led to a greater interest in improving general quality and long-term serviceability, while retaining lightness. Composite aerodynamic elements, which are often hollow and contain various types of reinforcing structure, use a variety of composite materials including graphite / epoxy, graphite / glass / epoxy, and other similar composite materials, reinforced with fibers.

When a structural member is metallic, the risk of galvanic corrosion is extremely high; aluminum fasteners and even stainless steel fasteners can be affected. Titanium fasteners are therefore frequently specified.

 Composite fasteners for composite aerodynamic fasteners have been the subject of previous development programs. For example, fiber-reinforced, injection molded thermoplastic rivets and bolts were evaluated in a program of the US Air Force, the report of which is presented in the document Report AFML-TR 79-4044 , Project IR-453-7 (1), June 1979. Reinforcement with short fibers, however, limited the tensile strength properties of such pre-formed thermoplastic rivets. There is therefore always a need relating to the development of simple composite assembly elements, having better mechanical properties, for the assembly of a composite material on a composite material, or the assembly of a composite material on a metallic material, in the formation of aerodynamic elements and other aircraft structures. The present invention aims to satisfy this need.

 A main object of the present invention is to provide composite elements with improved fibers which can be used to define a composite assembly element reinforced with fibers.

 Another main object is to provide an improved method for joining composite structures on composite and composite structures on metal.

 An additional object is to provide an article constructed from a set of members at least part of which is formed by the fiber-reinforced composite fastener.

 Briefly, one form of the present invention relates to a composite assembly element usable in the construction of aircraft components having a composite structure, in particular aerodynamic elements. In one of its broadest aspects, the composite joining element is made from a composite sheet material reinforced by parallel or unidirectional fibers, of the thermosetting or thermoplastic type, which has a greater thickness at one end and which has been wound in a spiral, generally around the axis of the fibers, to form a component part of composite fiber assembly, having a decreasing section, with a head integrated at one end. In the method of assembling a composite material on a composite material or of a composite material on a metallic material, the materials to be assembled are traversed by a hole. The hole can be chamfered or not, depending on other construction considerations. It is preferable to pass through the hole several pieces of composite fasteners, either typically a pair, in both directions, or opposite directions, with the larger end, or head, of each piece of composite fastener disposed at opposite ends of the penetration hole, to form the composite fastener. Excess material can be removed from parts of composite fasteners.

The head of each assembly element part can advantageously fill a chamfered cavity. The fiber composite fastener having a decreasing cross section is then heated to a temperature suitable for curing the thermosetting material of the composite sheet.

 In a preferred embodiment of the method of the invention, a sheath or an elastic pocket surrounds a member such as the aerodynamic element constructed from a set of members which are mechanically fixed temporarily by at least one pair composite assembly elements with decreasing section, as described above. The air which is inside the elastic sheath is then evacuated. An air or gas pressure is then applied to the outside of the sheath, at the same time as heating, for example to harden the parts of thermosetting composite assembly elements having a decreasing section, and preferably to be flush with the surface of the composite joining element to the adjacent composite or metallic material. In one embodiment, when the hole is not chamfered, a heated die or forming tool can be placed over the head of each piece of fastener to form a fastener. bulging head.

The advantages of the present invention include obtaining a composite joining element having a simple design and structure, but which nevertheless retains excellent mechanical properties during use. Another advantage is that one can use the parts of composite joining elements having a decreasing section, to join a composite material with another composite material or a metal, in a simple operation. These advantages as well as others will readily appear to those skilled in the art on reading the description given here.

The invention will be better understood on reading the following description of embodiments, given by way of nonlimiting examples. The following description refers to the accompanying drawings in which
fig. 1 is a simplified side view of an aerodynamic element constructed with the composite assembly elements of the invention
fig. 2 is a section along line 2-2 of the aerodynamic element of FIG. 1
Figures 3 to 5 show a manufacturing sequence of the fiber composite fastener part, having a decreasing section, which is made from composite fiber sheets, parallel and thermosetting
FIGS. 6 to 8 are partial sections of an aerodynamic element and they respectively represent the parts of composite assembly element after their installation and after hardening, as well as the resulting structure
fig. 9 shows another hardening procedure which can be used in place of that of FIG. 7
fig. 10 shows yet another method in which it is desired to form a domed rivet head on the composite joining element
Figures 11 to 13 show the use of a single composite connection element in the assembly of structures
Figures 14 and 15 show the use of a molded composite plug in association with the composite assembly elements of the invention which are shown in Figures 11 to 13; and
fig. 16 is a bar chart which represents the results of shear strength measurements, for the composite connecting elements with decreasing cross-section of the invention, and for conventional metallic connecting elements.

 Referring to FIGS. 1 and 2, it is noted that an aerodynamic element 10 (for example a blade for a non-faired fan turbine engine) is a composite aerodynamic element in which there is a spar 12 which is provided for attachment to a hub turning. The spar 12 can advantageously be made of titanium or another metal. A metal strip 14 is fixed to the leading edge of the aerodynamic element 10, for protection against erosion and the impact of particles contained in the air. The outer skin 16 is a fiber-reinforced composite material, such as a graphite / epoxy material. The interior of the aerodynamic element 10 contains a light and porous material, forming a core, such as for example blocks 18 and 20 of cellular material, located respectively at the leading edge and at the trailing edge, which can be constituted by the same material or by different materials. The composite skin 16 is bonded in a conventional manner to the light cores. In this embodiment, metal bolts, for example titanium bolts, 22a-d and 24a-d, are placed adjacent to the base part of the aerodynamic element 10 and to the adjacent side member 12. Description aerodynamic element 10 is intended only to illustrate a structure from a variety of composite structures of an aerodynamic element that can be used in accordance with the principles of the invention.

 Composite connecting elements 26a-i according to the invention assemble the opposite parts of the composite skin 10 and the titanium spar 12 arranged internally, to mechanically hold the parts together, at least partially, in order to define an article composite, such as a blade or a turbine engine blade. Figures 3-5 illustrate the manufacture of a part of the composite joining elements 26 of Figures 1 and 2. Composite sheets 28, 30, 32 and 34, suitably sized, as shown, are formed by a composite material reinforced by parallel or unidirectional fibers, the fibers of which are generally oriented along an axis of the fibers. The fibers which form the composite sheets can be glass, graphite, ceramic, polymer, or other material, as necessary, desirable or convenient. When a thermosetting resin is used, the part of thermosetting resin which goes into the composition of the composite sheets can be constituted by any thermosetting material, including an epoxy resin, a polyimide resin, etc. . It is desirable that some, and preferably all, of the composite sheets contain substantially parallel or unidirectional fibers, extending in longitudinal direction along the length of each composite fiber 28, 30, 32 and 34. This will lead to a longitudinal orientation of the fibers. fibers in the fastener part and finally in the fastener itself. It should however be noted that the interposition of a sheet having fibers oriented differently, for example extending in a transverse direction, in particular in the upper head section of the shape comprising a head, can constitute another embodiment. can be used to obtain special effects, or in other applications. However, as shown in fig. 4, the set of composite sheets 28, 30, 32 and 34, having different dimensions in the direction of the length "L", are arranged with their first edges "E" aligned, and these sheets are generally wound in spiral around a fiber axis 39 to form a piece of fiber composite assembly element, 36, having a decreasing section, as shown in fig. 5. The assembly element 36 comprises a head part 35 and a rod part 37 which are defined by a set of composite sheets reinforced by fibers wound in a spiral, with the fibers oriented in the direction of the longitudinal axis. fibers, 39. Unlike prior composite fasteners, which use various structures of hardened composite fasteners, the fiber composite fastener part, 36, having a decreasing cross-section , is not hardened or subjected to the thermosetting operation, before its use in a composite joining element, to assemble the desired materials.

 Figures 6-8 respectively show sequential steps of a method constituting a form of the invention, which includes the use of a pair of fiber composite fastener parts, 36 and 38, having a cross-section. decreasing, in accordance with the present invention, to assemble the composite skin 40 and the composite skin 42 to the titanium spar 44, by means of an opening such as that indicated at 45 in the drawings. It will be noted that the materials to be assembled or to be assembled do not necessarily have to be in the order indicated, nor correspond to the number of materials indicated in FIGS. 6 to 8. The parts of composite assembly elements 36 and 38 can thus be assembled suitably a composite material to a composite material, or a composite material to a metallic material, etc. The specific illustrations which are presented in FIGS. 6 to 8 follow the construction of the aerodynamic element 10 which is described above. It will be noted that a pair of parts of assembly elements having a decreasing section is used to assemble the composites 40 and 42 to the titanium spar 44.

It will be noted that it would be possible to use more than two composite joining elements having a decreasing section, if this were necessary, desirable or convenient. In this embodiment, a chamfer 46 is formed in the composite skin 40 and a chamfer 48 is formed in the composite skin 42. The heads of the pieces of connecting elements 36 and 38 have a size suitable for filling these chamfers.

Of course, the use of such indentations is not necessary for the implementation of the invention, as will be examined later in connection with FIG. 10.

 After having inserted and blocked the pieces of composite assembly element 36 and 38 in the hole which crosses the skins 40 and 42 and the internal metal piece 44, the small ends, in excess, are cut off of the pieces having a cross section decreasing. As shown in fig. 7, it is desirable to place the assembly inside a sheath or an elastic pocket 50, in the company of a plastic forming tool 51, consisting for example of silicone rubber, as shown fig. 7. In this figure, a gas pressure, coming for example from an autoclave, is represented by arrows 53. The air which is contained in the bag is then evacuated.

 The curing operation of the resin part of the composite fastener parts 36 and 38, to form a composite fastener, uses heat from, for example, an oven or an autoclave, which is capable of maintaining an appropriate temperature, necessary for the setting or hardening of this resin. A suitable pressure can also be established in the autoclave, for example a gauge pressure of 350-700 kPa, while heating the assembly, to further densify the cured assembly member, and to help flush the heads of the connecting element 41 and 43 to the other surfaces of the composite skins 40 and 42, as shown in FIG. 8. Note that the resulting hardened composite assembly element 52, in accordance with the invention, is a one-piece structure which firmly holds the assembly members together, at least partially mechanically. It has been observed that if the hardened composite joining element 52 is made of the same material, it generally has a homogeneous composition and excellent tensile strength. It will obviously be noted that the two or more composite assembly element parts which are used in the present invention for assembling the composite skins 40 and 42 to the metal beam 44, may have a different composition. Although the method described in connection with Figures 6 to 8, using a sheath or an elastic pocket 50, is convenient, other methods can be used. For example, as shown in fig. 9, a pair of heated forming tools or dies can be used, 54 and 56.

 When chamfers in one or both of the outer surfaces are not desired, heated tools or dies, 54 and 56, shown in fig. 10, to form a rounded head 55 or a domed head rivet. The dies have appropriately shaped cavities, as shown, in which these heads are shaped during the application of pressure.

 Figures 11, 12, 13, 14 and 15 show other embodiments of the invention. They use the single piece of spiral-wound composite fastener, 57, of FIG. 11, which has first and second ends 63A and 63B and which is placed in an opening 45. As shown, the ends 63A and 63B are accessible by the ends of the opening. The part 57 is then fixed by the application of pressure, for example using tools 59A and 59B of suitable shape, as shown in FIG. 12. The components to be assembled are preheated to reduce the viscosity of the resin matrix, or to melt the thermoplastic material. This allows the parts 36 and 38 of FIG. 6 to deform during the passage through the opening 45 and allows the resin to fully wet the hole. This promotes adhesion of the connecting element to the hole and to the chamfers, when used. Tools 59A and B can be heated, fig. 12, as well as the tools 54 and 56, FIGS. 9 and 10. According to a variant, these tools can be silicone rubber forming tools placed under a pressure pocket, as shown in FIG. 7. As indicated by the arrows 61A and 61B, these tools are moved towards the respective ends of the part 57 to separate the material from the part 57, so that it fills the chamfers 46 and 48, as shown respectively on the Figures 12, 13 and 14, or so that it extends over the surface of the parts 40 and 42, as shown in FIG. 15, if a chamfer is not used and a domed type head is desired, as described in relation to FIG. 10. This action gives the configuration which is shown in FIGS. 13 and 15, in which the edges 62 and 64 of the part 57 are formed, with the formation of cavities 66A and 66B, and a reduced thickness for the edges 63 and 64 which are formed, as in fig. 13. If it is desired to fill these empty regions, for example for aerodynamic reasons, it is possible to add composite plugs 58 and 60 of suitable shape, as shown in FIGS. 14 and 15, to fill the cavities 66A and 66B; these plugs will generally have a conical shape, due to the filling of openings having cross sections which are generally circular. In this form of the invention, the plugs 58 and 60 are arranged in the respective cavities and they are fixed to the rest of the assembly element, for example by heating and compression, using heated tools or the processes of the type autoclave described above. Alternatively, if necessary or desirable, plugs 58 and 60 can be added and joined to the rest of the assembly member during the latter's forming process, as described above. By way of example, these plugs can be pre-molded composite conical plugs, and they can be used in the form of part of the tools 59A and 59B in FIG. 12, the plugs separating from the tools when the latter are removed when the operation is complete.

Various mechanical tests have been undertaken to demonstrate the excellent tensile strength provided by the fiber composite fasteners of the invention. The specific material used to fabricate the fiber reinforced composite fasteners of the invention, for this assessment, was Scotchply 250-E36-RC (Minnesota Mining and Manufacturing Co.,
Minneapolis, Minnesota). This material is a prepreg material with parallel glass / epoxy fibers, containing 35% resin and type E glass fibers, and having a maximum hardening temperature of 1210 C. A bolt was also tested for comparison. made of titanium, as well as a treated steel bolt.

 A shear test was carried out comprising the following operations: manufacture of samples of the type shown in FIGS. 8 and 9, then joint traction of the two outer composite plates 40 and 42, in mutually parallel directions, with traction of the titanium spar intermediate 44 in the opposite direction. Fig. 16 shows the result of this shear test.

In fig. 16, bar A represents the failure load for a composite assembly assembled by means of a titanium alloy bolt; the failure mode was failure in the bolt. Bar B represents the failure load for an assembly assembled using a treated steel bolt; in this case, the test load was limited by the local crushing of the composite materials assembled by the bolt. Bars C and D represent failure loads for composite assembly elements of the invention, having respectively domed heads and submerged heads in both cases, the failure resulted from a failure of the element assembly. It is noted that the mechanical shear strength of the two types of composite fiber jointing elements of the invention has exceeded that of the titanium bolt, which could also be used to assemble composite skins with titanium. The good shear strength exhibited by the assembly elements of the invention manifests itself as well for composite assembly elements with domed head as for those with submerged head.

 Another important characteristic of the use of the invention is linked to the fact that parts of composite fiber assembly elements having a decreasing cross-section provide greater design flexibility than bolted assemblies.

For bolted connections, the surfaces to which the bolt and nut bear must be parallel, or else must be conical surfaces having the same axis. This was the case for the bolts which have been tested and whose test results are shown in fig. 15. This limitation on the axial alignment of metal bolts may require incorporating features suitable for bearing surfaces, such as chamfers, into parts such as fan blades. The composite fastener of the present invention does not have such restrictions. Indeed, the holes for the blower blade assembly elements have been chamfered on the exterior composite surfaces, which means that the axis of the chamfer was perpendicular to the surface, but not necessarily parallel to the axis of the rod. of the assembly element. This flexibility of design provided by the use of composite fiber assembly parts having a decreasing section, in accordance with the invention, further contributes to the utility of the invention, compared to rivets or conventional metal bolts, in many applications.

 It goes without saying that numerous modifications can be made to the device and to the method described and shown, without going beyond the ambit of the invention.

Claims (32)

1. Part of composite assembly element reinforced by fibers (36), characterized in that it comprises: a head part (35) and a rod part (37) formed in one piece, which define together a shape decreasing from the head part to the rod part and oriented generally along a longitudinal axis (39); the connecting element part (36) being defined by a set of superimposed composite sheets (28, 30, 32, 34) comprising reinforcing fibers, and the sheets being wound in a spiral around the axis (39) . 2. Part according to claim 1, characterized in that the reinforcing fibers are oriented generally in a direction substantially parallel to the longitudinal axis (39). 3. Part according to claim 1, characterized in that the superimposed sheets (28, 30, 32, 34) comprise sheets of different sizes, and the sheets are positioned predominantly in the head portion (35). 4. Part according to claim 1, characterized in that the composite sheets (28, 30, 32, 34) comprise a thermosetting resin in which the reinforcing fibers are arranged. 5. Part according to claim 1, characterized in that the composite sheets (28, 30, 32, 34) comprise a thermoplastic resin in which the reinforcing fibers are arranged. 6. fiber-reinforced composite assembly element (26a-i), characterized in that it comprises: at least one pair of assembly element parts (36, 38) of claim 1, with the parts of head and rod (35, 37) of a pair of these parts disposed oppositely through an opening (45) in an article (40, 42, 44); the connecting element parts (36, 38) being in intimate mutual contact to define together the composite connecting element with opposite head parts. 7. An assembly element according to claim 6, characterized in that the reinforcing fibers are oriented generally in the longitudinal direction of the assembly element. 8. Joining element according to claim 6, characterized in that the composite sheets (28, 30, 32, 34) comprise a thermosetting resin in which the reinforcing fibers are arranged. 9. An assembly element according to claim 6, characterized in that the composite sheets (28, 30, 32, 34) comprise a thermoplastic resin in which the reinforcing fibers are arranged. 10. Composite article comprising a first part and a second part (40, 42, 44), this article comprising an opening (45) which passes through the first and second parts, and an assembly element which passes through the opening to assemble the first and second parts, characterized in that the connecting element is the fiber-reinforced connecting element of claim 6. 11. Composite article (10) comprising a first fiber-reinforced composite material (16), and at least one second material (12) selected from the group which comprises composite materials and metallic materials, the article comprising a through opening the first and second materials (16, 12) and an assembly element (26a-i) which passes through the opening to assemble the first and second materials (16, 12), characterized in that the assembly element is the fiber-reinforced composite fastener of claim 6. 12. Article according to claim 11, characterized in that the assembly element is the assembly element of claim 7. 13. Composite article comprising a first part and a second part (40, 42, 44), this article comprising an opening (45, 46, 48) which passes through the first and second parts and an assembly element (57) which passes through the opening for assembling the first and second parts, characterized in that the connecting element comprises a single piece of spiral wound composite connecting element (57) having first and second ends (63A, 638), the first and second ends (63A, 638) being flared to fix the connecting element in the opening and to fix the first and second parts together. 14. Article according to claim 13, characterized in that at least one of the first and second flared ends (63A, 63B) has a cavity inside (66A, 66B); and in that a plug (58) is fixed in the cavity. 15. Composite aerodynamic element (10) comprising a composite material (16) and a material (12) selected from the group which comprises a composite material and a metal, this aerodynamic composite element being traversed by a hole which contains an assembly element (26a-i), characterized in that the joining element is a fiber-reinforced composite joining element containing a thermosetting resin, which is formed from a composite sheet (28) in which the fibers are oriented along a fiber axis, and which is thicker at one end, the sheet being arranged in a spiral around the fiber axis to form a fiber composite fastener having a decreasing section (36). 16. composite aerodynamic element according to claim 15, characterized in that the composite fiber assembly element having a decreasing section (36) is formed from a set of sheets of parallel fibers of different sizes (28 , 30, 32, 34) so that the thick part is formed by more than one sheet. 17. composite aerodynamic structure according to claim 15, characterized in that the composite sheet with parallel fibers (28) consists of a sheet of composite material with parallel fibers of the glass fiber / resin / epoxy type. 18. composite aerodynamic element according to claim 15, characterized in that it comprises a composite material assembled with a composite material. 19. composite aerodynamic element according to claim 15, characterized in that it comprises a composite material (16) assembled with a metal (12). 20. composite aerodynamic element according to claim 15, characterized in that a set of composite fiber assembly elements, having a decreasing cross section, (36, 38) form the assembly element which is placed in the hole. 21. Composite aerodynamic element according to claim 15, characterized in that the assembly element is formed by a pair of composite fiber assembly elements, having a decreasing section, (36, 38), with the thickest part of each of them arranged on opposite sides of the hole. 22. Method for manufacturing a part of a composite assembly element reinforced by fibers (36), comprising a head part (35) and a rod part (37) formed in one piece, characterized in that 'It comprises the following operations: a set of fiber-reinforced composite sheets (28, 30, 32, 34) is provided, at least some of which have a length shorter than that of the other sheets of the set; the sheet assembly (28, 30, 32, 34) is stacked on top of each other, so that the first edges of the sheets are practically superimposed and aligned; and the stacked sheets (28, 30, 32, 34) are then spirally wound, generally along a longitudinal axis oriented in the direction of the length, whereby the head part (35) which is at one end is formed in one piece with the rod portion (37). 23. The method of claim 22, characterized in that the fibers which reinforce the composite sheets (28, 30, 32, 34) are generally oriented in a direction substantially parallel to the longitudinal axis. 24. The method of claim 22, characterized in that the composite sheets (28, 30, 32, 34) contain a thermosetting resin. 25. The method of claim 22, characterized in that the composite sheets (28, 30, 32, 34) contain a thermoplastic resin. 26. Method for manufacturing a fiber-reinforced composite assembly element in place inside an opening (45) passing through an article (40, 42, 44), the opening comprising first and second opposite ends, characterized by the following operations: a pair of connecting element parts (36, 38) according to claim 1 is provided; passing through the first end of the opening, the rod portion (37) of a first of the pieces of the pair; passing through the second end of the opening the stem portion (37) of a second of the pieces of the pair; and pressing the head parts (35) of the pair of parts (36, 38) towards each other and towards the hole, to press the pair of parts (36, 38) in order to bring the parts into mutual contact. 27. The method of claim 26, characterized in that the rod portion (37) which projects from the opening is removed and heat is applied to agglomerate the pair of parts (36, 38) to form the connecting element. 28. A method of manufacturing a composite assembly element intended for use in the construction of a composite aerodynamic element (10), characterized in that: a set of composite sheets reinforced with fibers (28, 30, 32, 34), which have different sizes and in which the fibers are bonded by a resin, with the fibers of all the sheets oriented parallel along a fiber axis, and in such a way that the resulting stacking is more thick at one end than at the other end, and this stack is wound in a spiral around the fiber axis (39), to form a composite fiber assembly member having a decreasing section (36). 29. Method according to claim 28, characterized in that the sheets (28, 30; 32, 34) are composite sheets which comprise parallel glass fibers and an epoxy resin. 30. The method of claim 28, characterized in that the fibers are selected from the group comprising glass, graphite, ceramic or polymer fibers; and the resin of the composite sheet (28, 30, 32, 34) is selected from the group comprising an epoxy resin, a thermoplastic resin, a polyimide resin, a matrix material of ceramic type, and mixtures thereof . 31. Method for manufacturing a fiber-reinforced composite assembly element (57), in place inside an opening (45, 46, 48) passing through an article (40, 42, 44), l opening having opposite first and second opening ends, characterized by the following: providing a single piece of spiral wound composite fastener (57) having first and second piece ends (63A, 638) ; the assembly element is placed inside the opening, so that the first and second ends of the part (63A, 63B) are accessible at the first and second ends of the opening, and then the first and second part ends (63A, 63B) for fixing the assembly element in the opening. 32. Method according to claim 31, characterized in that the flaring operation forms a cavity (66A, 66B) in at least one of the ends of the part; and then a plug (58) is fixed in the cavity.