Abstract:
A laminated composite stringer having a termination at one end in its longitudinal direction, and including a laminated stack of composite structural plies, wherein internal plies in the stack are terminated consecutively towards the stringer termination to provide a taper of reducing stack thickness. Also, a composite structure comprising a panel and the stringer; and a method of manufacturing the stringer. The composite structure may be used in aircraft.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a laminated composite stringer having a taper at its termination, and to a method of forming such a stringer. 
       BACKGROUND OF THE INVENTION 
       [0002]    A panel, such as the skin of an aircraft wing or fuselage, may be reinforced by a series of elongate stringers which run along the length of the panel. The stringer typically comprises a web extending at right angles to the panel, and a flange engaging the panel. 
         [0003]    Stiffeners have to be terminated in certain areas due to obstructions so that the panel/stringer structure assumes a flat plate geometry. In the case of an aircraft, stiffeners have to be terminated at, e.g. the wing tips, access holes, etc. At the stringer termination, stress supported by the stringer is transferred into the panel. Stringer terminations therefore cause areas of local stress concentration and the panel and stringer have a tendency to separate in these areas. 
         [0004]    To improve resistance to separation of the panel and stringer, the stringer flange may have a widened portion near the termination, a so-called “stringer foot”. This may be bolted or otherwise affixed to the panel. Alternatively, or additionally, a finger plate or cover plate may be provided over the stringer foot and bolted to the panel. These measures aid in dispersing the stringer load into the panel but high stress concentrations remain. 
         [0005]    The web typically has a taper of reducing height (perpendicular to the panel) near the termination in order to facilitate load transfer from the skin to the stringer, by providing a gradual increase in transverse bending and axial stiffness and relieving local stress concentrations. 
         [0006]    WO2008/132498A describes a stringer wherein the web has a taper of reducing height near the termination. EP1566334A describes a stringer wherein the web has two tapered regions of reducing height near the termination separated by a plateau. 
         [0007]    The fairly basic stringer termination geometries of the prior art have changed little over the years with the movement from metallic to composite stringers. However, these stringer termination geometries do not fully exploit the benefits of constructing stringers from composite materials. 
       SUMMARY OF THE INVENTION 
       [0008]    A first aspect of the invention provides a laminated composite stringer having a termination at one end in its longitudinal direction, and including a laminated stack of composite structural plies, wherein internal plies in the stack are terminated consecutively towards the stringer termination to provide a taper of reducing stack thickness. 
         [0009]    A second aspect of the invention provides a composite structure comprising a panel and the stringer of the first aspect bonded to the panel. 
         [0010]    A third aspect of the invention provides an aircraft comprising the composite structure according to the second aspect. 
         [0011]    A fourth aspect of the invention provides a method of manufacturing a laminated composite stringer having a termination at one end in its longitudinal direction, and including a laminated stack of composite structural plies, wherein internal plies in the stack are terminated consecutively towards the stringer termination to provide a taper of reducing stack thickness, the method comprising: cutting the composite structural plies to a desired termination profile; and stacking the composite structural plies. 
         [0012]    By contrast with the prior art, where the stack thickness remains the same in the web taper, the stringer of the present invention has a taper of reducing stack thickness. This greatly increases the geometric possibilities for the stringer termination and allows load to be far more effectively transferred from the stringer into the panel. The load transition is controlled and evenly distributed in a free-flow form instead of having to manage local stress concentrations. For most loading scenarios, the load transition is such that no additional finger plate or cover plies are required, and the size and number of any bolts can be kept to a minimum. The termination design inhibits crack initiation and improves fracture mechanics. 
         [0013]    The stringer may have a flange for engaging a panel, and the stack taper may be in the flange. The stack taper in the flange may be in the longitudinal and/or transverse direction. Stress concentrations are most effectively reduced where the flange is tapered in both the longitudinal and transverse directions. 
         [0014]    The stringer may have an upstanding web, and the stack taper may be in the web. The web is preferably bifurcated at the termination, and a non-structural filler element may be disposed between “branches” of the bifurcated web. 
         [0015]    The flange and web of the stringer may be formed by joining a pair of substantially L-shaped stacks of laminated composite structural plies back-to-back. Due to the limitation of the minimum radius of the composite ply at the corner, a cleft may be formed between the L-shaped stacks, which can be filled with a non-structural filler element. 
         [0016]    In addition to the taper of reducing stack thickness, the stringer may also be tapered such that the height of the web, and/or the width of the flange may be tapered at the termination. The width of the flange immediately inboard of the termination may be greater than the width of the flange further inboard of the termination. 
         [0017]    The plies may be cut by, for example, a laser or a water jet. Preferably, each ply is cut as soon as it is added to the stack of plies. An automatic tape laying machine may also be used to cut and lay each ply to form the stack. 
         [0018]    The plies may be laid such that the first ply is the largest and further, smaller plies are laid on top such that the upper plies terminate before the lower plies to form the taper of reducing stack thickness at the termination. The edges of the internal plies will be exposed as the uppermost ply, laid last, will be the smallest. To protect the edges of the internal plies, a ply covering may be provided over the ply stack. Protecting the ply edges will improve peel resistance. 
         [0019]    Alternatively, the first ply may be the smallest and further, larger plies may be laid on top such that the lower plies terminate before the upper plies to form the taper of reducing stack thickness at the termination. The upper plies of this reverse lay-up will protect the edges of the lower plies and the uppermost ply will form a continuous surface. The lowermost ply lies closest to the panel when the stringer is bonded, or otherwise fixed, to the panel. The uppermost ply lies farthest from the panel. 
         [0020]    The plies are preferably fibre reinforced laminates pre-impregnated with resin, so called “pre-pregs”. Alternatively, the plies may be dry fibre laminates and resin would need to be infused into the laminates after they have been cut and stacked. The resin may need to be cured in either case, for example in an autoclave. The fibres may be of carbon, glass, or other suitable materials. The resin is preferably epoxy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
           [0022]      FIG. 1  illustrates a plan view of one end of a composite lay-up for a stringer in accordance with a first embodiment; 
           [0023]      FIG. 2  illustrates a side view of the lay-up of  FIG. 1 ; 
           [0024]      FIG. 3  illustrates a plan view of the lay-up of  FIG. 1  having a cover ply; 
           [0025]      FIG. 4  illustrates a projection of the lay-up and cover ply of  FIG. 3 ; 
           [0026]      FIG. 5  illustrates a close up of the stringer termination of  FIG. 4 ; 
           [0027]      FIG. 6  illustrates the cured, completed stringer of the first embodiment; 
           [0028]      FIG. 7  illustrates detail A of  FIG. 6 ; 
           [0029]      FIG. 8  illustrates a close up of the stack taper of  FIG. 7 ; 
           [0030]      FIG. 9  illustrates a side view of the stringer of  FIG. 6 ; 
           [0031]      FIG. 10  illustrates the section view on B-B of  FIG. 9 ; 
           [0032]      FIG. 11  illustrates an end view of the stringer of  FIG. 6 ; 
           [0033]      FIG. 12  illustrates a projection of the stringer of  FIG. 6 ; 
           [0034]      FIG. 13  illustrates a plan view of the stringer of  FIG. 6  showing rib feet locations; 
           [0035]      FIG. 14  illustrates a schematic view of the ply lay-up to form the stringer of the first embodiment; 
           [0036]      FIG. 15  illustrates a schematic view of the cured lay-up of  FIG. 14 ; 
           [0037]      FIG. 16  illustrates a plan view of a stringer in accordance with a second embodiment; 
           [0038]      FIG. 17  illustrates a projection of the stringer of  FIG. 16 ; 
           [0039]      FIG. 18  illustrates a projection from beneath of the stringer of  FIG. 16 ; 
           [0040]      FIG. 19  illustrates a schematic view of the ply lay-up to form the stringer of the second embodiment; 
           [0041]      FIG. 20  illustrates a schematic view of the cured lay-up of  FIG. 18 ; 
           [0042]      FIG. 21  illustrates a projection of a stringer in accordance with a third embodiment; 
           [0043]      FIGS. 22 to 24  illustrate cut away views of the stringer of  FIG. 21  installed on a panel and having a pad covering the stringer foot; and 
           [0044]      FIG. 25  illustrates a schematic view of the stringer of  FIG. 21  installed on a panel and having an alternative pad covering the stringer foot, and showing the direction of load transfer from the stringer to the panel. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0045]    A first embodiment of a stringer in accordance with the present invention will now be described. As shown in  FIG. 1 , the stringer  100  includes a flange  101  having a width W and an upstanding web  102  having a height H. Note that only one end of the stringer is shown in  FIG. 1 . The stringer  100  has a termination  103  at one end. Inboard of the termination, the stringer  100  has substantially constant section (not shown in  FIG. 1 ). Adjacent to the termination  103 , the flange  101  has a region of increased width W′ so as to form a stringer foot  104 . The stringer  100  is formed from a pair of back-to-back L-shaped stacks of composite laminate plies  105 . 
         [0046]    The plies  105  are arranged such that the lowermost ply  105   a  is the largest ply and the uppermost ply  105   b  is the smallest ply. The plies are cut such that the lowermost ply  105   a  terminates at the stringer termination  103 , and the uppermost ply  105   b  terminates a significant distance inboard of the termination  103 . The remaining plies  105  intermediate the plies  105   a  and  105   b  are cut consecutively so as to form a taper, or ramp, of decreasing ply stack thickness towards the termination  103 . 
         [0047]    As can be seen from  FIG. 1  the taper of decreasing ply stack thickness in the flange  101  is in both the longitudinal direction X and the transverse direction Y. As can be seen from  FIG. 2 , the taper of reducing stack thickness in the flange  101  is also in the vertical direction Z. Furthermore, as can be seen from  FIGS. 1 and 2 , the taper of reducing stack thickness in the web  102  is in the transverse direction Y. Near the termination  103 , the width W of the flange  101  also tapers, as does the height H of the upstanding web. The taper  106  in the width W of the flange  101  is set at approximately 45° to the longitudinal direction X. A second tapered region set at around 5° to the longitudinal direction X helps to blend between the taper  106  and the constant width section W′ of the stringer foot  104 . The taper  107  in the height H of the web  102  is set at approximately 30° to the horizontal. The stack of plies  105  is cut such that only the lowermost ply  105   a  remains at the termination  103 . 
         [0048]    The exposed edges of the cut plies  105  in the tapers of reducing stack thickness in the flange  101  and web  102  can be susceptible to delamination. This can be overcome by providing a cover ply  105   c  over the uppermost ply  105   b.  The cover ply  105   c  is dimensioned so as to be at least as large as the lowermost ply  105   a  such that the cover ply  105   c  covers all of the other plies  105 . As shown in  FIG. 3 , the cover ply  105   c  further includes edge portions  108  which extend beyond the lowermost ply  105   a . These edge portions  108  may be used to fasten the stringer  100  to a panel. It is to be noted that any such fasteners are non-structural and are provided only to prevent peeling of the cover ply  105   c  from the panel. Fastener holes  109  in the edge portions  108  are shown in  FIG. 6 . 
         [0049]    The cover ply  105   c  when applied over the stack of un-cured plies forms a gap at the termination due to the taper of reducing stack thickness in the other plies  105 . This gap  109  is best seen in the close up view of  FIG. 5 . When the lay-up of plies has been completed, curing the stringer  100  causes the cover ply  105   c  to conform to the taper of reducing stack thickness of the other plies  105  below. 
         [0050]      FIG. 6  shows the completed stringer, after cure, where the cover ply  105   c  forms a continuous outer surface; Detail A of  FIG. 6  is shown in  FIG. 7 ; and a close up of the tapering stack of plies in the web  102  is shown in  FIG. 8 . From  FIG. 8  it can be seen that the plies  105  are each parallel in the longitudinal direction X and by successively terminating the plies the thickness of the stack in the transverse direction Y reduces to form the taper. The cover ply  105   c  is, in fact, formed of two plies  105   c . These cover plies  105   c  are not parallel to the longitudinal direction X in the taper, but instead conform to the angle of the taper. The small voids  110  between the edges of the cover plies  105   c  and the remaining plies  105  are filled with resin. 
         [0051]      FIG. 9  shows a side view of the stringer  100  and  FIG. 10  shows the section view on B-B. From  FIG. 10 , the pair of back-to-back L-shaped stacks of composite plies  105  in the region of constant cross section on the stringer foot  104  can be seen. Each ply  105  is continuous such that the thickness of the web portion  102  is double the thickness of the flange portion  101 . Due to the minimum radius r dictated by the laminate plies  105 , a cleft may be formed between the back-to-back L-shaped stacks. This cleft may be filled with a non-structural filler  111  such that the lower surface  112  of the stringer  100  is substantially planar such that the stringer  100  may be securely bonded to a panel. 
         [0052]    An end view of the stringer  100  is shown in  FIG. 11  and a projection view of the completed stringer  100  is shown in  FIG. 12 . The stringer  100  is intended to be fixed to a cover, or skin, of an aircraft wing. Aircraft wings typically include ribs having cut-outs through which the stringers pass. The ribs are typically connected to the wing cover, or skin, and to the stringers by rib feet.  FIG. 13  shows locations  113  at which rib feet may be connected to the stringer  100 . 
         [0053]      FIG. 14  shows a schematic of the lay up of the plies. The plies between the lowermost ply  105   a  and the uppermost ply  105   b  form a stepped configuration in the taper of reducing ply stack thickness. The cover plies  105   c  extend beyond the lowermost ply  105   a.  The stack of plies is cured in an autoclave whereby applied heat and pressure P cause the cover plies  105   c  to conform to the taper geometry of the other plies  105 .  FIG. 15  shows the cured plies upon removal from the autoclave. 
         [0054]    In the first embodiment described above, the stringer  100  has a stack of plies having a taper of reducing stack thickness in various directions. A common feature of each of the tapers is that the internal plies are terminated consecutively towards the stringer termination to form the taper of reducing stack thickness. The exposed edges of the upper plies may be covered by the optional cover plies  105   c  in the completed stringer  100 . 
         [0055]    In a second embodiment, the stringer  200  has a similar overall geometry to that of the stringer  100 . The primary difference between the stringer  200  of the second embodiment and the stringer  100  of the first embodiment is that in the stringer  200  the internal plies are terminated such that the edges of the plies are not exposed, even when the optional cover ply or plies are not used. The stringer  200  includes a flange  201 , a web  202 , a termination  203 , a stringer foot  204 , a taper  206  of reducing flange width towards the termination  203 , a taper of reducing web height  207  towards the termination  203 , and a non-structural tapered noodle  211 . The outer geometry of the stringer  200  is similar to that of the stringer  100 , with the exception that the web  202  has constant width up to the termination  203 . This is achieved, despite a taper of reducing stack thickness in the longitudinal direction X of the web  202 , by the tapered noodle  211 . 
         [0056]    The flange  201  and the web  202  are formed by joining two back-to-back L-shaped stacks of laminated composite structural plies, in a similar manner to the construction of the stringer  100 . 
         [0057]    The key difference between the stringer  100  and the stringer  200  is best described with reference to  FIGS. 18 to 20 . For the stringer  200 , the ply lay up is the reverse of that of the stringer  100 . As can be seen in  FIG. 18 , the plies  205  are laid up such that the smallest ply  205   a  is laid first and the largest ply  205   b  is laid last. Optional cover plies  205   c  may subsequently be laid over the ply  205   b.  With reference to  FIG. 19 , the first ply  205   a  is laid up first. The uppermost ply  205   b  (or cover ply  205   c ) is laid last so as to form an overhang. The plies of the stringer  200  may be of the same material as those of the stringer  100 . Once the stack of plies  205  has been assembled it is cured in an autoclave, or the like.  FIG. 20  shows a schematic partial section view of the plies  205  after cure. The uppermost ply  205   b  forms a generally smooth transition over the taper of reducing stack thickness towards the termination  203 , as do the optional cover plies  205   c.  The internal plies naturally deform and their edges tend towards the normal to the inner mould line (the surface upon which the ply stack is laid). 
         [0058]    In the schematic shown in  FIG. 20 , the flange  201  includes an extended portion  213  beyond the extent of the web  202 . This may be optionally provided for the stringer  200  where there is a risk of delamination of the flange  201  from the panel to which it is to be connected. The extended region  213  provides an increased bonding surface and may further be secured to the panel by a fastener, generally indicated by line  214 . 
         [0059]      FIGS. 21 to 24  illustrate a third embodiment of a stringer  300 . The stringer  300  includes a stack of plies  305  arranged in a similar manner to those of the stringer  100 . That is to say, the lowermost ply  305   a  is the largest ply and the uppermost ply  305   b  is the smallest ply. The stringer foot  304  has a free-form construction of substantially continuous curved edges, in contrast with the angular cut edges of the plies  105  of the stringer  100 . The shape of the stringer foot  304  has been optimised for load transfer into a panel to which the stringer  300  is to be attached. However, the curved edges of the plies  305  are more difficult to cut than the straight edges of the plies  105  and  205  of the stringers  100  and  200 , respectively. 
         [0060]    To improve bonding of the stringer foot  304  to a panel  350  a pad  370  is laid over the stringer foot  304 . The pad  370  effectively acts as a region of increased panel thickness for the panel  350 . The pad  370  has a generally semi-circular or D-shaped construction of substantially uniform thickness. The pad may be made of unidirectional or woven fibre composite material. The pad  370  may be pre-assembled and co-cured with the stringer  300  and the panel  350  to bond the stringer  300 , the pad  370  and the panel  350  together. The prefabricated pad  370  may be laid over the stringer foot  304  in an automated process utilising a vacuum pad on a robotic arm for positioning the pad  370  with respect to the stringer  300 . 
         [0061]    In a second example of the third embodiment, the pad  370  is replaced by a pad  380 , as shown in  FIG. 25 . The outline of the absent pad  370  is generally indicated by ( 370 ) in  FIG. 25 . The shape of the pad  380  of the second example has been optimised to follow the load-line  381  of the load as it is transferred from the stringer  300  to the panel  350 . The pad  380  is of substantially uniform thickness and has a divergent proximal end  382 , a generally parallel sided mid-section  383 , and a forked distal end  384 . The shape of the pad  380  is more difficult to cut than the simple “D-shaped” pad  370 , but provides improved load transfer between the stringer  300  and the panel  350 . 
         [0062]    Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.