Patent Publication Number: US-2006011289-A1

Title: Method of manufacturing composite structural beams for aircraft

Description:
The present invention relates to a method of manufacturing beams of composite material based on carbon fibre for the construction of aircraft.  
      In the aircraft construction field, until now, the method used for the fabrication of structural elements of the said type has comprised the lamination or deposition of carbon fibre matting pre-impregnated with resin in a mould. The mats are over size with respect to the final dimensions of the beam to be formed. After a polymerisation phase in an autoclave a beam is obtained the edges of which must subsequently be trimmed by means of a cutter. The cut edges must then be re-covered by securing a fabric or cladding of glass fibre with an adhesive for preventing the cut edges from being able to initiate corrosion phenomena, particularly because of the moisture in the presence of low temperatures.  
      In many applications in the aeronautical field, for structural reasons it is required that the web of the beam should have some locally thickened reinforcement regions. To achieve these reinforcements doublers are fabricated separately by means of lamination of carbon fibres pre-impregnated with resin. These reinforcements are polymerised separately and then cut to shape with a mill, thus obtaining a series of doublers (for example of flattened frusto-pyramid form) which are finally secured by means of adhesive onto one or both sides of the web of the beam.  
      The present invention seeks to achieve the object of providing a method of manufacturing elongate structural elements of the type specified above, mainly addressing the problem of reducing the time, cost and the number of stages in the manufacturing process. In particular, it is desired to reduce the number of polymerisations in autoclaves, eliminate the traditional operations of trimming or cutting the edges and the subsequent final phase of application of the cladding of glass fibre onto the cut edges.  
      Another object of the invention is to reduce the amount of footing necessary for the traditional cutting of the edges, as well as the cost of labour for the final application of the glass fibre cladding.  
      A further object of the invention is to produce monolithic structural elements having a greater structural strength than those obtained by means of the traditional fabrication process discussed above.  
      These and other objects and advantages which will be better understood hereinafter are achieved according to the present invention by a method as defined in the annexed claims. 
    
    
      One preferred, but non-limitative, embodiment of the invention will now be described making reference to the attached drawings, in which:  
       FIG. 1  is a transverse sectional view which schematically illustrates the main components of a beam formed according to the invention;  
       FIG. 2  is a perspective view which schematically shows a cutting stage of the method of the invention;  
       FIGS. 3, 4  and  5  schematically illustrate shaping and assembling stages of the blanks of which the beam of  FIG. 1  is to be composed;  
       FIG. 3A  is an enlarged view of a detail of  FIG. 3 ;  
       FIG. 6  illustrates a subsequent curing stage in an autoclave with a vacuum bag applied on a series of shaping tools of the type illustrated in  FIGS. 4 and 5 ; and  
       FIG. 7  is a transverse sectional view of the finished beam. 
    
    
      In the example illustrated and described herein refers to the manufacturing of a beam as schematically illustrated in section in  FIG. 1 , having a substantially I or H or “double T”) section with local reinforcements or thickenings on one or both faces of the web, intended to support the so-called “upper deck” of an aircraft. Clearly, the reference to this possible field of application must not in any way be interpreted as limiting the scope of the patent.  
      With reference to  FIG. 1 , the reference numeral  10  generally indicates a double T beam with local reinforcements  11  on one of the faces of the web  12 . These reinforcements (only one of which is visible in section in  FIG. 1 ) are spaced longitudinally along the web as is known to those skilled in the art. The beam  10  is obtained from the union of various blank elements which are then cured in a single curing phase in an autoclave as described above. These blank elements comprise: two C-shape elements  13 ,  14  counterposed with respect to one another which together constitute the main part of the web and part of the flanges, two flat elements  15 ,  16  which complete the top and bottom parts of the flanges, and a series of reinforcements  11  (so-called “doublers”) or local thickenings on one of the two faces of the web.  
      With reference to  FIG. 2 , each of the partly worked elements  11 - 16  is prepared by making a flat lamination of unidirectional carbon fibre mats  20  pre-impregnated with epoxy resin (also called “carbo-resin matting”). The carbo-resin mats  20  are superimposed on a support surface B thus obtaining partly worked products defined here as “flat laminates”, each constituted by a stratified succession of mats  20 . The flat laminates are then cut along their edges by means of a cutting machine, preferably a numerically controlled machine, which controls the movements of a cutting tool CT which is suitably inclinable to cut the edges of the flat laminates along a predetermined cut angle with respect to the plane in which the mats  20  lie.  
      An important characteristic of the method of the invention is that some of the edges of the flat laminates are cut at a cut angle different from 90° with respect to the plane in which the mats  20  lie. In particular the oblique edges  11   a  of the reinforcement  11  and some edges  13   a ,  14   a  of the flat laminates intended to constitute the “C” shape elements  13 ,  14  are cut obliquely. Thanks to this arrangement, at the end of the subsequent hot shaping phase ( FIG. 3 ) in which the terminal parts  13   b ,  14   b  of these elements are bent at a right angle, the edges of these stratifications  20  together define a flat surface  13   a ,  14   a  orientated perpendicularly of the plane of the bent parts  13   b ,  14   b . These edge surfaces  13   a ,  14   a  do not need any further trimming or cutting operations.  
      As illustrated in  FIG. 3 , the reinforcements  11  and the flat blank  13  are placed in succession on a shaping tool F 1  the shape of which they will copy during the subsequent hot shaping and curing stages. The reinforcements  11  are received in a recess R of the tool F 1 . By means of a hot shaping operation (known per se and therefore not described in detail here) the flat blank  13  is folded as indicated by the arrows A and constrained to copy the profile of the tool F 1 .  
      With reference to  FIG. 4 , similar steps to those described above (flat lamination, cutting of inclined edges and hot shaping on a second tool F 2 ) are performed on a second blank  14  constituting the second “C” section element intended to be positioned face to face with and joined to the first blank  13 . Then the two flat blank elements  15 ,  16  are applied for completion of the flanges, inserting two resin strips  17  into the connector zones.  
      The shaping tool is then closed by lateral counterplates S 1 , S 2 , placed in a vacuum bag V ( FIG. 6 ) and subjected to a curing cycle in an autoclave by applying temperature and pressure in a manner known per se.  
      It is to be noted that between the shaping tools F 1 , F 2 , S 1 , S 2  and the blanks to be cured there is preliminarily interposed a sheet of glass fibre P ( FIG. 3A ) which, at the end of the polymerisation phase ( FIG. 7 ) constitutes an outer cladding layer which satisfies the so-called FST (Flammability-Smoke-Toxicity) requirements prescribed in the aeronautical environment.  
      The final result of the process, as schematically illustrated in  FIG. 7 , is a composite beam  10  of carbon fibre with an external cladding layer of glass fibre matting P which continuously clads all the external surfaces of the beam, including its edges.  
      As will be appreciated, the method according to the invention envisages a single curing cycle (rather than two) and produces a monolithic structure with a more intimate and stronger binding of the reinforcements formed integrally with the web. The traditional phases of application of adhesive to join the reinforcements to the web are eliminated as are the operations of trimming the edges and the associated tools, and the final operations for applying the glass fibre cladding to the cut edges is no longer required. It will be appreciated, moreover, that the outer glass fibre cladding layer P is a continuous layer and intimately bound to the surfaces of the beam with consequent reduction in the risks of triggering corrosion.  
      It is intended that the invention shall not be limited to the embodiment described and illustrated here, which is to be considered as an example of performance of the process; the invention is on the other hand capable of associated modifications in shape, dimensions and constructional details of the beams. For example, the invention can equally be used to produce structural elements with sections of the most varied forms (“C”, “L”, “T”, “J” etc) with or without lateral reinforcements on the web.