Process for manufacturing pre-cured parts of composite material with green-applied stiffeners

Abstract: A process for manufacturing pre-cured parts of composite material with green-applied stiffeners, comprising the lamination of superimposed pre-impregnated composite material sheets to obtain a base part (1) and a second part (2) intended to be bonded with it; curing the base part (1); hot forming the laminate destined to form the second part (2); removing the second part (2) from the forming tool thereof and introducing it in angle pieces, tightening the angle pieces by sections and deposit it over the base part (1), intercalating a structural adhesive sheet between both of them; closing the assembly of both parts inside a vacuum bag and loading said bag in an autoclave, carrying out a curing cycle of the second part (2) under pressure and temperature, so that it is strongly adhered to the base part (1). The invention is applicable to the field of aeronautics.

Description: FIELD OF THE INVENTION The present invention refers to a process for manufacturing pre-cured parts of composite material with green-applied stringers, in which at least two parts manufactured in composite materials are structurally bonded, of which a first part called base part is cured and a second part called stringer is in green condition and in which the bonding of both parts is carried out by means of a structural adhesive sheet in such a way that the stringer is compacted against the base part, with adequate crosslinking of the resin of its composite material and so strongly adhered to the skin of the base part that the adequate resistance of the adhesive sheet is assured. More specifically, the object of the invention is to develop the necessary theoretical concepts and their corresponding manufacturing processes to obtain a bonding system by co-gluing two or more structural parts manufactured in composite materials. The bond is carried out by means of autoclave curing of a structural adhesive sheet interposed between two surfaces of parts made of composite material. The resin is cured in one of these parts and green in the other one. The curing of the resin of the part which is in a green condition is also produced in the same autoclave cycle. One of the most productive industrial applications of the present invention is the manufacture of big size parts where the quality of the base part is assured, that is, the part arriving already cured to the process. BACKGROUND OF THE INVENTION The adhesive joints of structural parts have a reduced application in composite materials due to the fact that their use and development degree is limited as corresponds to their recent appearance in the technical background. In the case of co-gluing, the applicant's close background refers to the bonding of spar stiffeners of the horizontal stabilizer torsion box for the Airbus A330-340 aircraft (currently in production), the bonding of the longitudinal stiffeners of the coatings of the wing torsion box for the CASA 3000 aircraft (in prototype stage), the bonding of spars to the wing coating of the EFA-European Fighter Aircraft (in pre-production phase) and the bonding of the FB.5-1 test box of the LFS-Large Flying-Surfaces development programme. However, the closest prior art to the present invention is represented by a project developed by the applicant in connection with the construction of the aeroplane Airbus A340 Growth. This project is explained in internal documents belonging to the applicant, where a process for manufacturing the various sections of aeroplane spars is disclosed. Such process comprises the steps of laminating the base part of the spar and curing the same on a corresponding tool by using a pressure and temperature cycle; laminating pairs of half-stiffeners which are then transferred to forming tools, and carrying out a temperature and vacuum forming cycle; bringing the half-stiffeners of each pair together while the same are housed in moulding tools so as to obtain a complete stiffener; mounting the assemblies each comprised of a green stiffener and a moulding tool on the cured base part, and curing the stiffeners on a curing tool; and demoulding the stiffeners after the same have been cured and bonded to the spar base part. From the results of the previous experiments as well as from other studies and manufacturability tests, it is deduced that the application of the process of the present invention is both achievable and reliable for its use in parts with highly demanded resistant structures and with strict quality requirements. FIELD OF APPLICATION OF THE INVENTION This invention is applicable to the manufacture of composite material parts in which one or several of its components are in a cured condition (bade part components), and the other components are in a green condition (stringer components), provided that the bonding between both of them is made. These parts may be any of those belonging to: Aircraft structures such as flying-surface coatings, spars, ribs, fixtures. Space vehicles. Marine and land vehicles. Industrial machinery and equipment. The manufacturing processes involved are: Laminating of composite material (manual or automatic). Cutting of composite material. Hot forming of composite material. Handling and positioning of parts and tooling. Autoclave curing. The used materials may be integrated by different resins and different types of fibre, such as: Fiberglass. Carbon fibre Aramid fibre Boron fibre Bpoxy resin. Thermoplastic resin Other heat-stable resins SUMMARY OF THE INVENTION The object of the invention is a manufacturing process for composite material parts in which at least one green stringer is adhered with a cured base part by means of a structural adhesive. This bonding is obtained by the moulding and pressure application with special tools designed for such purpose, so that it is possible to control and retain the resin and adhesive flow produced during the autoclave cycle. The stringers will have an L- or T-shaped cross section. The process for the latter will be described in detail below. The base part may be a wing skin or that of a stabiliser or any other component needing to be stiffened in order to comply with its structural function. The stringer with a T-shape is confined by devices (angle pieces) which are the basis of the system and which control and retain the resin flow. Afterwards, a vacuum bag is fitted as will be explained later on, and the assembly is introduced into an autoclave to submit it to a curing cycle under a predetermined pressure and temperature depending on the material and part requirements. The process is applicable to the parts obtained by any lamination process (manual or automatic). The inventive process as just summarized starts from the above described process developed in connection with the construction of spars of the aeroplane Airbus A340 Growth and mainly distinguishes from the latter in that the stringers to be cured are, once they have been removed from the forming tools, introduced in angle pieces which are pressed together so as to retain the stringers and prevent resin of the composite material of same from flowing outwardly. Said angle pieces are mounted on a turning rack which allows the stringers to be turned and positioned over the already cured base part placed on a cradle, whereafter the turning rack is removed and the assembly comprised of the base part, the stringers and the angle pieces is put inside a vacuum bag and moved to a curing autoclave, where the stringers are cured and adhered to the base part. The essential features of the process according to the invention are described in independent claim 1, while the remaining dependent claims are directed to particular embodiments of said process. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the process of the present invention, the attached drawings will be resorted to, where: Figure 1 is a general perspective view of a base part-stringer assembly obtained with the process of the invention, Figure 2 shows angle pieces used in the present invention, Figure 3 is a perspective view of a stopper used in the present invention, Figure 4 illustrates the formation of a semi-stringer according to the present invention, and Figure 5 shows the bonding of the semi-stringers by using the process of the invention. DETAILED DESCRIPTION OF THE INVENTION The manufacturing process developed by the invention and applicable to composite materials is related to a set of different preliminary processes allowing to achieve co-gluing in a optimized way. The key point is the retention of resin flow of the composite material during the co-gluing process described below. This co-gluing process consists of the structural bonding of an already cured base part 1 with a stringer 2 in a green condition. The foot 3 of the stringer 2 is of a constant thickness in each section and is adapted to the shape of the coating of the base part 1, ascending and descending the slopes of the latter as may be observed in figure 1. The thicknesses developed until now for such a foot vary between 2 and 8 mm. The stringer 2 is confined between two angle pieces 4, 4' of steel, aluminium, pneumatic cushion, etc., preferably Invar, like those shown in figure 2 which are adapted to the shape of the stringer 2. These angle pieces have a series of channels where elastomeric pipes 5, 5' are housed, retaining the resin flow of the composite material of the stringer 2. The location, shape and exact sizing of such pipes is carried out according to the aforementioned resin flow optimisation and that of the grip-torque over the stringer 2 during the curing process. Once the stringers 2 have been fitted over the angle pieces 4, 4', the adhesive is applied over the feet 3 of the former ones. Then, the assembly is positioned over the base part 1 and the relevant vacuum bag which envelops the entire assembly is made. With respect to the tooling for the co-gluing process, a cradle over which base part 1 has been cured will be used as well as a turning rack which will allow the stringers 2 to be correctly positioned over said base part 1. The former ones are fastened to the rack by means of the angle pieces 4,4' described above. To retain the resin flow at the ends, stoppers 6 of steel, aluminium, carbon fibre, etc. are used as shown in figure 3. Now, the different steps of the manufacturing process according to the invention will be described. A - Lamination of the part or parts This consists in the superposition of composite material layers in a pre-impregnated condition, so that the orientation of the fibre is adapted to the structural requirements of the part to be manufactured. For such a purpose, the necessary reinforcements will be placed between the different layers of the so produced laminates, designated with number 7 in figure 4. The distribution of layers must be such that their lamination and forming is permitted without producing wrinkles or fibre distortion and also that the part, once cured, has no permanent deformations due to thermal stresses. On one hand, the base part 1 and on the other hand the assembly of basic stacks forming the stringers 2 are laminated. The base parts 1 and stringers 2 made with the present process may be flat or curved, with or without changes of thickness, both transverse and longitudinal. They may be subjected to a posterior cutting according to the convenience of laminating several base parts and stringers at the same time. B - Forming of the stringers and preliminary operations The different laminates 7 forming the semi-stringers 2 are assembled over a bench and introduced in the bed plate of a thermo-forming machine. Control thermocouples are fitted and the tooling parts which may damage the machine membrane are coated with airweave (aerating fabric). The hot forming cycle is obtained by applying heat and vacuum. In this way, formed laminates 7 are obtained forming semi-stringers (figure 4) over the thermo-forming tools 8; the stringer 2 is obtained by means of bonding the semi-stringers by couples (as shown in figure 5). ue to the thermo-forming tool 8 geometry, the feet 3 of the stringers 2 are adapted to the coating shape of base part 1, ascending and descending the slopes existing on it. C - Assembly on the turning rack The stringers 2 are removed from the forming tools 8 and then introduced in the angle pieces 4, 4' previously fitted in the turning rack. The angle pieces 4, 4' are tightened by sections and a cradle with the already cured base part 1 on which the reinforcements have been fitted is placed below the rack. The stringers 2 are turned over base part 1, for which the rack is provided with movement and mechanical and optical positioning means permitting to obtain the desired accuracy. Then, the rack is removed and the tightness is assured to prevent resin of the composite material from flowing outwardly and formation of the vacuum bag is started. The positioning tolerances of the parts between each other and of the latter with the tools are related to the thickness and geometrical characteristics thereof. D - Autoclave process preparation Once the green stringers 2 have been positioned over the cured base part 1, the vacuum bag is prepared, for which three layers of different materials will be used. In the last layer, a series of perforations will be made to locate valves which will extract air from the bag, hence obtaining the required vacuum. The bag will remain sealed to the tool, preventing loses thanks to a strip of mastic fitted around the perimeter. To assure tightness, the bag will be tested once cold. Re-usable bags of silicone, etc. may also be used. E - Autoclave curing After preparing the vacuum bag, the assembly is loaded over the autoclave support tool. This is hermetically closed and the corresponding curing cycle is carried out rating the value of the following essential parameters, depending on the characteristics of the material and the part: Pressure: 5.95 - 10.5 kg/cm 2 . Temperature: up to 190ºC according to the material. Rate of heating: 0.5 - 2ºC/min. Once the autoclave chamber reaches the environmental conditions of pressure and temperature, the already cured and perfectly consolidated material is removed. The stringers 2 initially in a pre-impregnation condition are now perfectly adhered to the cured base part 1 and in a condition to support the structural stresses for which they have been designed. As already described in its different stages, in the process of the present invention, the key factor lies in retaining the resin flow of the composite material of the stringer 2 during the curing process, for which the previously described gluing process is applied, in which the stringer 2 is confined between two steel, aluminium, pneumatic cushion angle pieces 4, 4' etc., like those shown in figure 2, the latter having a series of elastomeric pipes 5, 5' which are those retaining the resin flow. The process of the invention is applicable for different thickness (3.5 - 16 mm for the core and 2 to 8 mm for the foot of the stringer) and for different laminates 7. The dimensions are very variable, both in height and length. Until now, parts reaching a 17 m length with suitable tooling have been developed. The basic features of the invention have been remarked above, although, as will be understood, it will be possible to make amendments of certain details of the manufacturing process developed by the applicant. For this reason, it is intended that the scope of the invention is limited only by the contents of the attached claims.

Claims: A process for manufacturing pre-cured composite material parts with green-applied stringers, wherein at least two parts manufactured in composite materials are structurally bonded, of which a first part, called base part (1), is in a cured condition, and a second part, called stringer (2), is in a green condition, and the bonding of the base part (1) and stringer (2) is carried out by means of a structural adhesive sheet, so that the stringer (2) is compacted against the base part (1), with adequate crosslinking of the resin of its composite material and so strongly adhered to the base part (1) that the adequate resistance of the adhesive sheet is assured, and wherein the following steps are carried out: a) laminating superimposed layers of composite material in a pre-impregnated condition to form laminates (7), so that the fibre orientation adapts to the structural requirements of the part to be obtained, producing from the resulting laminates (7) the base part (1) on the one hand, and on the other hand, an assembly of basic stacks destined to form the stringer (2); b) curing the base part (1) obtained in step a); c) assembling the laminate (7) destined to form the stringer (2) over a bench and introducing the assembly in a thermo-forming machine with a thermo-forming tool (8) and carrying out a hot forming cycle by applying heat and vacuum, hence obtaining halves of the stringer (2) which are then bonded to each other by couples to form the complete stringer (2) ; d) removing the stringer (2) from the forming tool (8) and introducing it in angle pieces (4,4') previously fitted over a turning rack, applying a structural adhesive sheet to the stringer (2) in the area where this should remain bonded to the base part (1), tightening the angle pieces (4, 4') by sections and placing a cradle with the already cured base part (1) beneath the turning rack, turning the stringer (2) until leaving it over the base part (1), removing the turning rack and assuring tightness to prevent resin of the composite material of the stringer (2) from flowing outwardly; e) once the stringer (2) has been positioned over the already cured base part (1), closing the assembly inside a vacuum bag and sealing the latter by means of a mastic strip fitted around its perimeter; and f) loading the closed assembly inside the vacuum bag over an autoclave support tool, closing the latter hermetically, carrying out a curing cycle of the stringer (2) under pressure and temperature and then allowing the autoclave chamber to reach pressure and room temperature conditions again, after which the assembly formed by the previously cured base part (1) and a now cured stringer (2) which is strongly adhered to said base part (1), is removed from the autoclave. A process according to claim 1, characterised in that a base part (1) and two or more stringers (2) are bonded to obtain a finished pre-cured part. A process according to claims 1 or 2, characterised in that if it is desired to laminate several base parts (1) and stringers (2) at the same time, the laminates (7) obtained in step a) are subjected to later cutting into the several separate base parts and stringers required. A process according to any of the previous claims, characterised in that the laminates (7) are obtained in a flat or curved shape, with or without changes of thickness, both transverse and longitudinal. A process according to any of the previous claims, characterised in that the curing of step f) is carried out at a pressure of 5.95 - 10.5 kg/cm 2 and at a temperature of up to 190ºC, depending on the used composite material, with a heating rate of 0.5ºC/min - 2ºC/min. A process according to any of the previous claims, characterised in that parts applicable in aerospace, marine and land vehicle structures are obtained, as well as in industrial machinery and equipment. A process according to any of the previous claims, characterised in that the base part (1) consists of the skin of an aircraft wing, a stabiliser or any other component needing to be stiffened to comply with its structural function. A process according to any of the previous claims, characterised in that the stringer (2) has an L- or T-shaped transverse section. A process according to any of the previous claims, characterised in that the stringer (2) has a thickness from 2 to 8 mm in the foot and from 3.5 to 16 mm in the core and the final pre-cured part consisting of the base part (1) and the stringer (2) has a length of up to 17 m. A process according to any of the previous claims, characterised in that a re-usable bag of silicone or similar material is used as a vacuum bag for step e). A process according to any of the claims 1-9, characterised in that the vacuum bag is formed in step e) by means of three layers of different materials, in the last of which a series of perforations are made to receive valves to extract air from the bag and hence obtaining the required vacuum. A process according to any of the previous claims, characterised in that the composite material consists of fibres and resins selected from fiberglass, carbon fibre, aramid fibre, boron fibre, epoxy resin, thermoplastic resin and other heat-stable resins.

Inventors: CEREZO PANCORBO CARLOS, DE CASTRO NODAL MANUEL, DOMINGUEZ CASADO RAFAEL, GARCIA GARCIA AQUILINO, HUERTAS GARCIA MANUEL, PEREZ PASTOR AUGUSTO, CEREZO PANCORBO, CARLOS, DE CASTRO NODAL, MANUEL, DOMINGUEZ CASADO, RAFAEL, GARCIA GARCIA, AQUILINO, HUERTAS GARCIA, MANUEL, PEREZ PASTOR, AUGUSTO
Applicants: AIRBUS ESPANA SL, AIRBUS ESPANA, S.L.
Application year: 2000
Main classification: B29C70
IPC codes: B64C3, B29C70