Abstract:
Methods for manufacturing elongated structural elements are provided. Such methods provide composite material having optimal properties such as weight and strength which can be produced at much lower costs compared to conventional methods. Composite materials including such elements are also provided. In addition, commercial products incorporating such structural elements are provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and benefit of Italian Patent Application No. TO2014A000739 (which corresponds to Italian Patent Application No. 102014902294265 under the new numbering system which was introduced in 2015 with the implementation of the IPTO online filing platform) filed Sep. 18, 2014, the contents of which are incorporated by reference in their entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of aircraft construction in general and refers more specifically to methods for manufacturing elongated structural elements of composite material, such as wing and empennage spars, fuselage stringers, and beams in general. Methods according to the present invention may be applied to structural elements having at least one node between a flange and a web. More particularly, but not exclusively, the present method may be applied to the manufacture of beams or spars having a cross section in the form of a double T (or H), T, or J, and to co-cured monolithic components having stiffenings with the aforesaid cross sections, such as multi-spar boxes or stiffened panels. 
       BACKGROUND OF THE INVENTION 
       [0003]      FIG. 11  shows a step in the manufacture of a spar or beam of composite material with a double T cross section, according to a conventional method. The spar is initially assembled in an uncured state, by bringing together the webs of two profiled components  15 ,  16  with cross sections in the form of opposed C-shapes. Fillers F of carbon fibre and/or structural adhesive, with a pseudo-triangular cross section, are then applied along the longitudinal recesses R which are present along the junction areas between the webs and flanges of the two profiled sections. 
         [0004]    Problems have arisen concerning the quality of the junction areas (or “radial” areas) of the spars, owing to the geometrical imprecision of the fresh (uncured) fillers. The imprecision may be due to the filler production process, which is usually carried out by vacuum forming or extrusion. Conventional fillers cannot repeatedly provide the geometrical precision (in terms of radii and thicknesses) specified on the drawings of components which are to be cured on the filler. In other cases, the imprecision may be seen after the step of co-curing with the other components of the spar, with the appearance of wrinkles, accumulations of resin (called “resin pockets”) and fibre distortion. This has led to an increase in rejects, repairs and structural operations in support of the evaluation of the acceptability of the various types of defects encountered. 
         [0005]    Moreover, the presence of fillers in the nodes of beams makes it difficult to inspect the radial areas of the spars with ultrasonic methods, since it disperses the signal. 
         [0006]    The filler is an integral part of the beam or spar. Its use is dictated by the need to fill the cavity in the junction area to enable the radii of the spars to be compacted correctly during the autoclave curing, which takes place under pressure in vacuum bags. The filler provides a “support” or “reaction” function without which the beam would collapse in the radial area under the action of the pressure of the autoclave and the vacuum bag. The reactive force exerted by the filler also allows correct curing of the C-section profiled components in the radial area, thereby ensuring the absence of porosity. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides methods of manufacturing beams of composite material having an optimal weight, while also avoiding the drawbacks discussed above. More particularly, methods disclosed and claimed herein provide lighter beams for a given structural loading, reducing the direct costs of production (the cost of materials and labour for producing a conventional filler), improving the geometrical quality of the beam in the junction (radial) or node area, simplifying the steps of non-destructive testing, and reducing the non-recurring costs of managing typical defects in the region of the filler. 
         [0008]    The present invention is based on the fact that, in structural terms, the filler usually only serves a marginal purpose: the two C-shaped sub-elements are designed to bear the whole of the load specified for the beam, and the filler is therefore an element which essentially provides increased weight. 
         [0009]    The aforesaid objects and advantages as well as others, which will be more fully evident from the following text, are achieved according to the present invention described and claimed herein. 
         [0010]    To summarize briefly, instead of applying conventional fillers, one or more resilient inserts are inserted into the nodal junction areas between the web and the flanges of an elongated structural element. At the end of the curing (polymerization) step, the inserts are extracted from the cured structural element, leaving cavities extending longitudinally through the respective nodal areas. The structural and functional characteristics of embodiments of methods according to the invention will now be described by reference to the following figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]      FIGS. 1 to 5  are cross-sectional views which schematically illustrate steps in the assembly of a structural element according to an embodiment of the invention; 
           [0012]      FIG. 6  is a cross-sectional view of a structural element according to an embodiment of the invention; 
           [0013]      FIGS. 7 and 8  are perspective views which show resilient inserts that can be used to implement manufacturing methods according to certain embodiments of the invention; 
           [0014]      FIGS. 9 and 10  are partial schematic views of nodal areas of structural elements manufactured according to an embodiment of the invention; and 
           [0015]      FIG. 11  is a schematic cross-sectional view of a step in the assembly of a structural element according to the prior art. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Before the detailed explanation of a plurality of embodiments of the invention is given, it must be made clear that the invention is not limited in its application to the details of construction and the configuration of the components presented in the following description or illustrated in the drawings. The invention can be applied in other embodiments and can be used or implemented in different ways in practice. 
         [0017]    The methods described below relate to the manufacture of an elongated structural element  10  ( FIG. 6 ), which in this example is a beam or spar. The spar may have a “double T” (or H) cross section, having two parallel flanges  11 ,  12  and a central web  13  extending at a right angle with respect to the flanges. The double T (or H) shape is not to be interpreted as limiting. More generally, the present methods may be used for manufacturing structural elements elongated in a longitudinal direction, such as beams, spars, stringers, multi-spar boxes, or stiffened panels with cross sections of any shape, provided that they have at least one node or nodal area  14  extending longitudinally along a junction area between a flange and a web. 
         [0018]    The web extends in a direction which can form a right angle, or two angles other than 90°, with the flange (or flanges). In the example of  FIG. 9 , the web  13  extends in a direction which forms two supplementary angles other than 90° with the flange  12 . As understood in this text, terms and expressions indicating orientations such as “longitudinal” and “transverse” relate to the longitudinal direction of extension of the elongated structural element (beam, or spar, or stringer, or multi-spar box, or stiffened panel). 
         [0019]    In a known preliminary process, two profiled sections of composite material are initially preformed, these sections extending in a longitudinal direction and having C- or U-shaped cross sections. A preliminary step ( FIG. 1 ) may involve the lamination of fabrics T preimpregnated with resin, which are thermoformed on two respective mandrels  17 ,  18  ( FIG. 2 ), to produce two profiled components  15 ,  16  having cross sections in the form of opposed C- or U-shapes. 
         [0020]    In this embodiment, each profiled section  15 ,  16  has two parallel flange portions  151 ,  152  and  161 ,  162 , joined, respectively, to two web portions  153 ,  163  by means of a respective radiused portion  155 ,  156  and  165 ,  166 . In each profiled section  15 ,  16 , the parallel flange portions  151 ,  152  and  161 ,  162  have respective outer surfaces  154 ,  157  and  164 ,  167  facing in transversely opposed directions. 
         [0021]    In one embodiment (not shown) of the method, for the manufacture of a structural element with a T-shaped cross section, the profiled sections  15  and  16  may have L-shaped cross sections, including a single flange portion  151 ,  152  joined, respectively, to a web portion  153 ,  163  by means of a respective radiused portion  155 ,  165 . The angle formed between the web and the flange may be either 90° or another angle. 
         [0022]    The two profiled sections  15 ,  16  are assembled by bringing their web portions  153 ,  163  together. The two junctions extending longitudinally to the junctions between the web of the spar and the two flanges are defined as nodal areas or nodes. 
         [0023]    For each nodal area, a respective removable or extractable insert  20  is prepared, having a substantially pseudo-triangular cross section in which an essentially flat base  21  and two other sides  22 ,  23  can be distinguished, the two other sides converging to form a vertex or apex  24  ( FIGS. 3 ,  7  and  8 ). 
         [0024]    In certain embodiments, the two sides  22 ,  23  are curved and concave, with curvatures mating with, or corresponding to, the radiused portions of the first  15  and second  16  profiled section. Consequently, the vertex or apex formed at the junction of the curved sides  22 ,  23  is substantially cusp-shaped ( FIGS. 3 ,  7  and  8 ). 
         [0025]    Two inserts  20  may be prepared for the manufacture of a double T beam which has two nodal areas. The curved sides  22 ,  23  may have curvatures mating with the respective convex (or radiused) junction surfaces between the web portions and the flange portions of the profiled sections  15 ,  16 . 
         [0026]    According to one embodiment of the method, for the manufacture of a beam or spar having its web perpendicular to the flange, the two concave curved sides  22 ,  23  may be symmetrical about a theoretical plane of symmetry which runs through a mid-line of the base  21  and is perpendicular to the base ( FIG. 7 ). 
         [0027]    According to one embodiment, for the manufacture of a beam or spar where the web forms angles other than 90° with the flange, the two concave curved sides  22 ,  23  are asymmetrical ( FIG. 8 ). 
         [0028]    The inserts  20  may be made of resilient material such as pre-cured silicone rubber. 
         [0029]    Suitable materials which may be selected for the extractable inserts may include, for example, polysiloxanes (polymerized siloxanes), such as the type known as HTV (High Temperature Vulcanizing), which are vulcanized at high temperatures that cannot be reached during the process of curing the composite materials (carbon resin). Experimental tests conducted by the applicant have demonstrated that excellent results can be achieved by using inserts made of silicone rubber of the vinyl-methyl-polysiloxane (VMQ) type belonging to siloxane group Q according to the ISO 1629 standard (derived from ASTM 1418-79). In this specific case, the elastomer used has the following chemical, physical and mechanical properties which are considered to be optimal: specific weight, 1.200+0.020 g/cm 3 , Shore A hardness 70+5, minimum breaking strength 8 MPa, minimum breaking strain 250%, minimum tear strength 15 kN/m, compression set sch. 25% at 175° C., 40% maximum. However, these values are not to be interpreted as definitive or limiting and are simply provided as evidence of the excellent properties obtained by methods according to the present invention. 
         [0030]    In one embodiment, the extractable insert  20  is produced by extrusion. 
         [0031]    In other embodiments, the extractable insert  20  is coated with an outer layer  25  ( FIGS. 7 and 8 ) having properties that facilitate its separation from the cured composite material. The outer layer  25  may be conveniently made of adhesive tape based on PTFE (Teflon) or other release materials acceptable in the specific conditions of the relevant process. 
         [0032]    As shown in  FIG. 3 , the first profiled section  15  and the second profiled section  16  are preassembled together, the web portion  153  of the first profiled section  15  being brought against the web portion  163  of the second profiled section  16 . In this condition, the flange portions  151 ,  152  of the first profiled section  15  extend in opposite directions from the directions in which the flange portions  161 ,  162  of the second profiled section extend, and lie in pairs in two parallel planes. The radiused portions  155 ,  156  of the first profiled section  15  may be each adjacent to a respective radiused portion  165 ,  166  of the second profiled section  16 , so as to form, on each of the two opposed longitudinal sides of the profiled sections, a respective recess  26  extending longitudinally along the nodal areas. 
         [0033]    The inserts  20  are applied ( FIG. 4 ) along the longitudinal recesses  26 . The ridges or apexes  24  are inserted into the innermost areas of the recesses  26 , while the flat bases  21  remain facing in opposed outward directions. 
         [0034]    According to one embodiment, the inserts  20  may be designed so that their flat bases  21  lie substantially flush with the outer surfaces  154 ,  164  and  157 ,  167  of the flange portions  151 ,  152 ,  161 ,  162  of the profiled sections  15  and  16 . 
         [0035]    If necessary, flat laminated layers of composite material then may be applied ( FIG. 5 ) so as to form two respective bases  27 ,  28 , flat in this example, of the structural element. The layers of the bases  27 ,  28  may extend to cover the outer surfaces  154 ,  164  and  157 ,  167  of the flange portions and the flat bases  21  of the inserts. 
         [0036]    In a different embodiment (not shown), for the manufacture of a structural element with a T-shaped cross section, only one base  27  may be provided. 
         [0037]    The assembly shown in  FIG. 5  then may be enclosed between two flat forming or stamping tools (not shown) for imparting a flat shape to the layers of the bases  27 ,  28 , and can be enclosed in a vacuum bag according to known procedures which will not be described here. The assembly is then transferred to an autoclave and subjected to specified temperature and pressure cycles to cause the curing of the resin present in the composite material. 
         [0038]    After the autoclave curing, the cured structural element may be allowed to cool to ambient temperature. 
         [0039]    Each insert  20  is then extracted from the spar by simply pulling it from one end; this leaves a longitudinally extending cavity  29  in each nodal area, having the shape and dimensions of the respective insert. 
         [0040]    During the curing process, the extractable insert  20  undergoes transverse thermal expansion as a result of the temperature rise. This reversible thermal expansion is stronger than the external compressive forces to which the beam is subjected, represented by the vacuum applied by the vacuum bag and the pressure within the autoclave. 
         [0041]    On completion of curing, the structural element is allowed to cool, in order to enable the insert or inserts  20  to contract thermally. Since the contraction of the resilient inserts is greater than the contraction of the cured composite material, it causes the cross section of the cavities  29  formed in the structural element to be slightly wider transversely (by a few tenths of a millimetre) than the transverse dimensions of the respective inserts at ambient temperature; this facilitates the extraction of the rubber insert after curing, avoiding the risk that the insert might remain trapped in the beam. To allow for the aforementioned thermal expansion undergone by the silicone insert during the thermal cycle of curing the composite, at the design stage its cross section is appropriately reduced by a percentage of 4% to 6% of the theoretical cross section which the composite beam is intended to have. 
         [0042]    Another effect of the compressive strength of the material of the insert  20 , combined with its transverse thermal expansion, is its capacity to consolidate in an optimal way the composite material located between the insert and the external vacuum bag (not shown). This prevents the appearance of defects in the laminated solid (porosity and fibre deviation) and in the geometry of the component (distortion, thickness variation, and resin pockets). 
         [0043]    Embodiments of the invention provide monolithic elongated structural elements which are free of filler (“filler-less”) and which have at least one web and at least one flange lying on a given geometric plane, and a nodal area extending along a junction between the web and the flange. The structural element forms a longitudinal cavity which extends longitudinally through the nodal area and has a substantially triangular cross section having a substantially flat first side  291  parallel to the geometric plane on which the flange lies and two convex curved sides  292 ,  293  with their convexities facing the centre of the cavity  29 . The curved sides converge in a vertex  294  which extends in an end area of the web. In certain embodiments, the vertices of the triangular cross section may be rounded. 
         [0044]    Exemplary embodiment described up to this point relate to methods of assembly by co-curing, in which a composite laminate is polymerized (“cured”) and simultaneously joined (“bonded”) to one or more components of uncured composite material. All the resins of the composite material and the structural adhesives applied to the interface of the components to be joined together may be polymerized in the same curing step. 
         [0045]    Alternative embodiments provide for the form of assembly known by the term “co-bonding”, in which two or more components are joined together, at least one of the components being fully cured, while at least one other component is uncured. Therefore, at least one of the components of the structural element, that is to say the profiled sections  15 ,  16  and the bases  27 ,  28 , may have been cured previously, while at least one of the other components is uncured. 
         [0046]    Both the aforesaid co-curing and co-bonding applications also can be used for incorporating a plurality of stiffeners into composite structural panels. 
         [0047]    These methods enable appreciable cost reductions to be achieved, including a reduction in the cost of the material for producing the conventional composite filler (depending on the length of the structural element and the cross section of the filler). A further reduction relates to the cost of the time required to manufacture conventional fillers and of the automatic machinery for their preparation. These savings are due in part to the recyclability of the extractable inserts  20 . 
         [0048]    These methods provide a considerable reduction in the weight of the finished structural element. Furthermore, the methods improve the geometric tolerances in the radial area in comparison to conventional processes: the radii have a regular curvature and the values of the radius lie within narrow tolerance ranges, with an absence of fibre distortions (wrinkles) in the radial area and accumulations of resin, thereby also simplifying the steps of non-destructive (ultrasonic) inspection (NDI) in the radial areas. 
         [0049]    Various aspects and embodiments of methods according to the invention have been described. With the exception of the choice between co-curing and co-bonding, each embodiment may be combined with any other embodiment. Furthermore, the invention is not limited to the embodiments described, but may be modified by a person skilled in the art based on the teachings herein while still falling within the scope of the claims which follow.