Method of fabricating a stringer-stiffened shell structure using fiber reinforced composites

[0002] This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 198 32 441.3, filed on Jul. 18, 1998, the entire disclosure of which is incorporated herein by reference.

[0003] The invention relates to a method for manufacturing or fabricating a shell structure that is stiffened with stringers using fiber reinforced composite materials molded on a shell mold. The method may be used to fabricate complexly shaped fiber reinforced synthetic structures that include a shell-shaped skin and profile members or stringers stiffening this skin. The method may be used for the fabrication of all stringer-stiffened shell structures with fiber reinforced composite materials, but is particularly applicable for fabricating the rudder and vertical stabilizer assembly, the elevator and horizontal stabilizer assembly, and the wings of aircraft with particular technological advantages.

[0004] It is generally known in the art to use stringer-stiffened structures made of fiber composite materials in the field of aircraft construction. For example, these may be used as primary structures of a passenger aircraft, whereby the primary application known in the art is the empennage or rudder and vertical stabilizer assembly of known Airbus aircraft, which is completely fabricated of carbon fiber reinforced composite materials. In this context, two publications each provide certain impressions or ideas for realizing the known stringer-stiffened structures using fiber composite materials. See U. Bieling, “Serieneinsatz von Faserverbundwerkstoffen im Flugzeugbau—dargestellt am Seitenleitwerk des Airbus”, “Series Application of Fiber Composite Materials in Aircraft Construction—Represented in the Rudder and Vertical Stabilizer Assembly of the Airbus”, VDI Berichte No. 965.1, pages 77 to 88, VDI publishers Düsseldorf, Germany, 1992; and also J. Rouchon, “Certification of Large Aircraft Composite Structures, Recent Progress and New Trends in Compliance Philosophy”, Proceedings of the 17th ICAS Conference Stockholm, Sweden, 1990.

[0005] In part, Bieling discusses the fabrication of stiffening internal structures of highly integrated carbon fiber composite components for a middle shell or middle box of a rudder and stabilizer assembly. Among other things, Bieling also addresses the complex fabrication that is required in the so-called modular core technology. In this context, Bieling discusses in detail the fabrication process for a middle box shell of a rudder and stabilizer assembly, in which the carbon fiber composite pre-cut blanks that are supplied in a sorted manner must first be separated into skin layers and so-called bandages. The skin layers are manually laid into laminating molds, while the bandages are wrapped or wound around the so-called modular cores in a partially automated process. Multiple layers of these bandages are wrapped around the cores in a single process operation, when rectangular modular cores are used for the shells of the rudder and stabilizer assembly. The entire wrapping or winding process involves high technical demands, especially requiring a sufficiently high and reproducible adhesive bonding characteristic of the prepregs. In this context, trapezoidal or geometrically complex modular cores must be manually wrapped layer by layer.

[0006] Thereafter, the wrapped modular cores are arranged in a row in an exactly prescribed order on a rotatable support. In this context, the prepregs arranged between the lined-up modular cores, after curing, form the stiffening internal structure of the highly integrated carbon fiber composite components. After completion of a subsequent laminating process for the skin layers and fixing of all the modular cores, the rotatable support is rotated in such a manner that all of the cores hang downward, and then the rotatable support is lowered down onto the skin layers laying in the laminating mold. After releasing the cores from the rotatable support, the cores remain on the laminating form and the component is made ready for the curing process.

[0007] After the curing has been completed, the components are removed from the forming mold. In this context, first the middle parts of the three-part modular cores and thereafter the side parts are removed or pulled out of the component. After all of the core parts have been removed, the component is lifted out of the forming mold and subjected further to a mechanical machining. The modular cores are cleaned of any residual resin and then once again provided with separating members. The laminating mold is cleaned and provided with separating members. To repeat the above described steps of the process, the individual core parts are then joined together again to form a modular core which is used for carrying out the next wrapping process. After all of the above mentioned steps have been carried out, the components are subsequently mechanically machined, for example by boring, milling, perimeter machining or flash removal or the like, and are then subjected to a non-destructive testing.

[0008] It is desirable to use fiber composite materials for the primary structures such as an empennage, e.g. a rudder and stabilizer assembly, of an aircraft in view of the many advantages such as a weight reduction, very good specific strength and stiffness characteristics, a reduced total number of individual components, very good corrosion resistance, very good fatigue performance, among others. However, from the above description of the known fabrication processes, it is clear that further improvements in the fabrication of such primary structures are desirable or even necessary to achieve a rational fabrication with a low expense and effort in the way of tooling, equipment, and process steps, while reliably meeting the high technical demands that are typical in the field of aircraft construction. These considerations especially apply in the construction of high capacity civilian commercial aircraft. A further subordinate requirement is the use of rationalized production technologies for achieving stiffened primary structures using fiber composite materials, which may be applied to empennage structures as well as the wings of aircraft.

[0009] For the above reasons, it is important in the field to effectively accommodate or adapt known production techniques in order to achieve the simplest possible application of stiffening members such as stringers onto complexly curved shells or skin layers. In this context, it is generally known in the field that significant problems arise when fitting stiffening members such as stringers onto the curved shells or skin layers in the production of stringer-stiffened structures of fiber composite materials. When pre-fabricated and pre-cured stiffening members are combined with a shell or skin structure having a relatively sharp curvature, various assembly and fabrication difficulties arise in achieving a proper alignment and conformance of the parts, and ultimately a warping deformation of the finished component results due to imperfect matching or conformance of the various parts. On the other hand, any attempt to use non-cured semifinished parts for the stiffening members have previously always required the use of very complicated and costly molds or forming tools for properly molding the stiffening members, bonding or joining the stiffening members onto the shell structure, and curing the stiffening members. The above mentioned publications of Bieling and Rouchon provide no disclosure or suggestions toward how the fabrication of complexly shaped stringer-stiffened structures can be improved while simultaneously avoiding the above mentioned problems.

[0010] In view of the above, it is an object of the invention to provide a method of fabricating or manufacturing a stringer-stiffened shell structure using fiber composite materials, which combines the advantages and avoids the disadvantages of using pre-cured stiff components and un-cured deformable components in combination to form the stiffening stringers. The inventive method aims to make it possible to use pre-fabricated hard material blanks that have a curvature exactly matched to that of the shell structure, to serve both as forms or molding cores for the stringers during the fabrication process and also as strengthening elements in the finished structure. It is a further object of the invention to completely avoid the need for complicated and cost-intensive forming tools or molds for the curing process, and particularly for the stringers. The invention also aims to completely avoid an undesirable warping deformation of the finished component as a result of curvature caused by the shape of the pre-cut material blanks. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.

[0011] The above objects have been achieved according to the invention in a method for fabricating a stringer-stiffened shell structure using fiber composite materials, wherein the shell structure is molded on a shell mold. According to the invention, a hard fiber reinforced skin is laid onto the curved surface of the shell mold. The hard fiber reinforced skin has a curvature that matches and conforms to the curved surface of the shell mold. Then, at least one vertically arranged fiber reinforced stiff strip-shaped pre-cut stringer core blank is positioned to extend along the skin, whereby a narrow edge of the stringer core blank has a curvature corresponding to the curvature of the skin and lies conformingly in contact on the skin.

[0012] Next, a respective lengthwise extending, deformable, fiber-reinforced stiffening profile member is laid against and along each respective broad side of the pre-cut stringer core blank. Each one of these lengthwise stiffening profile members has a contact surface that contacts the skin and adapts conformingly to the curvature of the skin, and a protruding web that conformingly contacts the broad side face of the stringer core blank. There-after, the entire assembly or construction is packed or sealed in a vacuum-tight manner and subjected to the external application of pressure and heat, which causes the various elements of the overall structure to be adhesively bonded to each other and to be cured and hardened in this condition. Particularly, each lengthwise stiffening profile member undergoes a rigid and permanently unreleasable mechanical joining or bonding with the pre-cut stringer core blank on the one hand and with the skin on the other hand, during the curing process.

[0013] In the above process, the hard skin member and the stiff pre-cut stringer core blank have the prescribed contour or curvature and positions relative to each other and thereby serve as a mold member or forming core against which the lengthwise stiffening profile members are shaped and adapted during the assembly and curing process. Once the entire assembly is fully cured and hardened, the lengthwise stiffening profile members serve to provide a strong and permanent connection between the pre-cut stringer core blank and the skin member, and also provide stiffening and strengthening of the finished structure with the cured and hardened webs or flanges of the stiffening profile members. Namely, a respective stringer is formed by one of the stringer core blanks sandwiched between the protruding webs of the two adjoining stiffening profile members, while the stringer is integrally bonded to the shell or skin member by the base webs and contact surfaces of the stiffening profile members.

[0014] In order that the invention may be clearly understood it will now be described in connection with example embodiments, with reference to the accompanying drawings, wherein:

[0015] FIG. 1 is a schematic plan view of a plate of fiber rein forced composite material from which strip-shaped stringer core blanks are cut;

[0016] FIG. 2 is a schematic perspective view of strip-shaped stringer core blanks and lengthwise stiffening profile members arranged on a curved skin shell that is supported on a shell mold cooperating with an auxiliary jig frame resting on the perimeter of the mold for fabricating a stringer-stiffened shell structure; and

[0017] FIG. 3 shows an enlarged detail end view of the detail area III in FIG. 2.

[0018] The invention provides a technology and a process for fabricating stringer-stiffened shell structures having a complex curved configuration, wherein the stringers can be formed to the curvature of a skin or shell member in a simple manner without using additional cost-intensive tools or process steps. The skin or shell member is known in the art as such due to its structure, composition, and appearance, and generally has a configuration of a curved panel with length and width dimensions that are substantial multiples of the relatively small thickness dimension. The fabrication of such stringer-stiffened shells according to the invention requires substantially less effort and expense than the conventional technologies discussed above.

[0019] In order to carry out the fabrication method according to the invention, it is first necessary to provide pre-cut strip blanks or so-called “blades” or stringer core blanks 5 of fiber reinforced material in a preliminary step. Preferably, as schematically represented in FIG. 1, the stringer core blanks 5 are cut from a previously laminated or pressed and fully cured and hardened plate 4 of fiber reinforced composite material. The blanks 5 have a strip shape, namely an elongated shape with two narrow edges, two broad side faces, and an elongation length that is substantially greater than the dimensions of the narrow edges and of the broad side faces. At least one of the narrow edges has a contour shape matching the contour shape of the skin member to which the stringer core strip will be joined.

[0020] FIG. 1 shows a plan view of the hard flat plate 4, and thereby shows the side view shape and particularly the pre-defined curvature of each pre-cut stringer core blank 5. It should be noted that the pre-cut stringer core blanks 5 have already been cut out of the plate 4 as shown in FIG. 1, so that only the remaining holes in the plate 4 are evident. Alternatively, FIG. 1 could be taken as showing a situation in which the pre-cut blanks 5 have already been cut or separated from the remaining material of the plate 4, but have not yet been removed therefrom. The cutting can be carried out by any known means, such as stamping, saw cutting, laser cutting, water jet cutting, hot wire cutting or the like. It is well understood in the art how the exactly required curved configuration of each pre-cut stringer core blank 5 can be achieved, for example using a computer numerical control for the cutting process.

[0021] The pre-cut stringer core blanks 5 that have been previously prepared in the above mentioned manner are thereafter integrated into the construction in carrying out the process as shown in FIG. 2. Thereby the stringer core blanks 5 will serve as forming cores or guides during the process, and will serve as strengthening cores of finished stringers that stiffen the shell structure in the finished product which has been fully cured and hardened.

[0022] FIG. 2 shows a stationarily arranged shell mold 1 having a mold surface with a predefined curvature, which may involve a curvature in one plane or two planes for example. A curved shell or skin member 6, which consists of one or more layers of fiber reinforced material and which is to be stiffened with stringers, is laid onto the curved surface of the shell mold 1. The curvature of the skin or shell member 6 is matched or adapted to the curved surface of the shell mold 1 in a surfacially conforming manner. The skin member 6 may be a pre-fabricated, pre-curved member that has been previously fully cured and hardened in a configuration matching that of the curved surface of the shell mold 1. Alternatively, the skin member 6 may not yet be cured and hardened or only partially cured and hardened so that its curvature remains flexible and conformable when it is placed onto the shell mold 1. Preferably, the skin member 6 has been previously cured and hardened and may be a prefabricated stock component that is stored until needed.

[0023] Once the skin member 6 has been laid onto the curved surface of the shell mold 1, a plurality of hard pre-cut stringer core blanks or blades 5 are positioned to extend along a strip length 1 along the upper surface of the skin member 6. Particularly, one of the two narrow edges 51 of each pre-cut stringer core blank 5 is in contact with the surface of the skin member 6, while the width of the respective stringer core blank along the broad side faces 52 extends substantially vertically or perpendicularly relative to the local surface of the skin member 6. The pre-cut curvature of the narrow edge 51 that lies in contact with the surface of the skin member 6 corresponds to or mates with the curvature of the surface of the skin member 6 at each respective location. This is achieved by properly cutting or shaping the respective stringer core blank 5 so that its narrow edge 51 has the proper curvature as seen in a side view and as discussed above in connection with FIG. 1.

[0024] The vertical positioning of the stringer core blanks 5 may, for example, be carried out with the aid of an auxiliary jig frame 2 that is set onto the shell mold 1 around the perimeter thereof, so as to establish the proper positioning and spacing of the stringer core blanks 5. To achieve this, the inwardly facing sides of the wall 21 of the auxiliary frame 2 are provided with slits or slots 22 that extend substantially vertically overall, or locally perpendicular relative to the local surface of the skin member 6, and with defined spacing distances between adjacent slits 22. Specifically, the slits 22 are formed as grooves or recesses in the wall 21 of the auxiliary frame 2. The end portions of the stringer core blanks 5 are inserted into the respective slits 22 so as to hold and properly position the stringer core blanks 5. Moreover, thereby the stringer core blanks 5 are held parallel to each other and properly positioned in their vertical orientation with the respective narrow edges 51 thereof held and pressed against the surface of the skin member 6.

[0025] Next, a respective lengthwise extending stiffening profile member 7 is laid between respective adjacently paired ones of the stringer core blanks 5. Specifically, each stiffening profile member 7 is laid along and in contact with a broad side 52 of a respective stringer core blank 5 and along the adjoining surface of the skin member 6. The details regarding the configuration and arrangement of the lengthwise stiffening profile members 7 will now be described in connection with FIGS. 2 and 3.

[0026] The stiffening profile members 7 are U- or L-sectional profile members consisting of fiber reinforced composite material which may be in a raw or non-cured state or in a merely partially cured state when it is used in the process as shown in FIGS. 2 and 3. In this context, the stiffening profile members 7 may be pre-formed and resin pre-impregnated prepreg intermediate parts. As an alternative, the stiffening profile members 7 may be blanks of fiber textile materials pre-cut to the required shape and laminated with a resin film.

[0027] The profile members extend along the length of the stringer core blanks 5 in the direction of the strip length 1, and each respectively include a protruding web 72, which can represent a vertically oriented flange of a U-sectional profile member or a vertically oriented shank of an L-sectional profile member, as well as a base web or shank 71 including a contact surface 71′ extending substantially horizontally along the surface of the skin member 6. The vertically oriented protruding web 72 of each stiffening profile member 7 extends in a form-fitting manner along a respective lengthwise extending broad side 52 of the respective stringer core blank 5. On the other hand, the base web 71 of a respective stiffening profile member 7 runs with its contact surface 71′ in a form-fitting manner along the curved surface of the skin member 6 adjoining the stringer core blank 5. In a case of a U-sectional profile member, two of the vertically protruding webs 72 are interconnected by the base web 71, whereby these two protruding webs 72 respectively run along the opposite facing broad sides 52 of two adjacent stringer core blanks 5.

[0028] While laying the lengthwise stiffening profile members 7 onto the assembly (and thereafter while pressurizing and curing the assembly) during the fabrication process, the stiffening profile members 7 are flexible and deformable because they are not yet cured or only partially cured as mentioned above. Thereby, the vertically protruding webs 72 of the stiffening profile members 7 are easily conformed to the shape of the broad sides 52 of the stringer core blanks 5, while the base webs 71 are similarly conformed to the curved shape of the surface of the skin member 6. This can be achieved by a manual lay-up process or by a semiautomated or completely automated lay-up process of laying the stiffening profile members 7 into position along the stringer core blanks 5 and the adjoining surface of the skin member 6 and then smoothing or pressing the profile members 7 along and against the stringer core blanks 5 and the surface of the skin member 6.

[0029] The assembly of the stringer core blanks 5 and lengthwise stiffening profile members 7 described above may then be directly subjected to a curing process as will be described below. However, as an alternative, an additional strengthening and fixing of the above described arrangement of stringer core blanks 5 and stiffening profile members 7 on the skin member 6 may be provided before the curing process, as follows. Respective additional stiffening or strengthening profile members 3 that are made of fiber reinforced composite or metal and that have a relatively short length k may be arranged at prescribed locations between the parallel extending stringer core blanks 5, for example at the midpoint and at regular spacings along the strip length 1 of the blanks 5. Preferably, each short strengthening profile member 3 has a U-sectional profile shape, but may instead have an L-sectional profile shape.

[0030] These short strengthening profile members 3 are pre-fabricated, stiff, hard and rigid auxiliary insert members that are fitted and pressed into the spaces between the protruding webs 72 of the lengthwise extending stiffening profile members 7 lying along the broad side faces 52 of the stringer core blanks 5, in order to exactly position the stiffening profile members 7 and stringer core blanks 5 relative to each other and fix the horizontal spacing and positioning of these members relative to each other. In this context, the protruding webs 31 of the short strengthening profile members 3 are pressed to extend in contact along the protruding webs 72 of the lengthwise extending profile members 7. This achieves a wedging or jamming action in the manner of a type-setting box, which practically prevents a sliding or shifting displacement of the non-cured stiffening profile members 7 in a horizontal direction.

[0031] The above described assembled or laid-up structure is then packed in a vacuum-tight manner, in order to carry out a curing process is in an autoclave. For example, the assembled structure can be “bagged” in a conventional manner for bag molding, is then optionally subjected to vacuum for gas extraction, and is then subjected to pressure and heat in the autoclave to press the profile members 3 and 7 against the stringer core blanks 5 and the surface of the skin member 6. Thereby the components of the assembled structure are adhesively bonded intimately and securely to each other and then completely cured in this condition.

[0032] During the curing process, the protruding webs 72 of the stiffening profile members 7 are securely adhesively bonded to the stringer core blanks 5, by an intermingling or interpenetration of the resin of the non-cured protruding webs 72 into the pre-cured or partially pre-cured stringer core blanks 5. Similarly, the base webs 71 of the stiffening profile members 7 are securely adhesively bonded to the contacting surface area of the skin member 6 by the resin of the members 7. It is simply necessary that compatible resins and reinforcing fiber materials are used for the various components, but it is especially preferred that the same fiber composite material is used for each of the components. Thereby, during the curing process, a rigid, secure and permanent mechanical connection among the components is achieved.

[0033] Thereafter the hardened structure is removed from the autoclave and from the vacuum-tight packing such as a curing and molding bag. In the finished, hardened, and completely cured integral composite structure, respective stiffening stringers are each formed by one of the stringer core blanks 5 sandwiched between two protruding webs 72 of the two adjoining stiffening profile members 7, for example as shown especially in the detail of FIG. 3. It should be noted that the detailed view of FIG. 3 ignores or omits the curvature of the skin member 6. The molded form or configuration as well as the position and longitudinal extension of each stringer is determined by the molding and fixing of the adjoining stiffening profile members 7 onto the previously hardened and properly shaped stringer core blank 5. The position of the stringers relative to each other is fixed by the strengthening members 3. There is no need of providing a complicated molding tool for molding the shape of each stringer on the skin member.

[0034] The additional short strengthening profile members 3 that wedged the stringer bodies into position may remain rigidly connected to these stringers in the finished product, for example, as a securing point for ribs or the like that are to be added later, and as additional local strengthening and/or mounting points in the finished composite structure. Alternatively, the auxiliary strengthening profile members 3 can be removed from the resulting structure after the curing step, whereby these members 3 are seen merely as molding auxiliary inserts that can be reused for subsequent molding operations.

[0035] The described method for carrying out a one step fabrication of complexly shaped stringer-stiffened shell structures using fiber composite materials makes it possible to easily adapt the shape of the stringers to any desired curved or complexly configured skin member 6. It is simply necessary to pre-cut the stiff or hard blades or stringer core blanks 5 to have the proper curvature along the narrow edge 51 that will be in contact with the skin member 6, and then to form the deformable stiffening profile members 7 therealong during the fabrication process. With this method, it is also possible to use stringer core blanks or blades 5 that are not straight or linear, so as to form finished stringers that are not linear. The various devices such as jigs and molding tools that have previously been necessary for establishing the form and for supporting the non-cured structure are no longer necessary, or at least are not necessary to the extent previously required. As a result, the expense and effort necessary for manufacturing the molding tools, releasing the finished components from the molding tools, cleaning the molding tools and the like, is substantially reduced. Additional or subsequent changes of the wall thickness of the form parts require substantially lower efforts in view of the few forming tools that are still required.

[0036] Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims. It should also be understood that the present disclosure includes all possible combinations of any individual features recited in any of the appended claims. The terms soft, flexible, deformable and uncured herein refer to the characteristics of a fiber composite component that is not yet fully cured, while the terms hard, stiff and rigid (or substantially so) refer to the characteristics of a fiber composite component that has been fully cured and do not necessarily imply complete rigidity and stiffness.