Method for the prevention of fibre distortions in the production of a stiffened panel by the cocure process of stringers on composite skins

Abstract

A method of producing a stiffened panel comprising the steps of : a) forming a band (1) of generally flat carbon fibre; b) providing, on a mould (M), a plurality of sheets (3) made of composite material soaked in uncured resin, and superimposed on one another to form a skin (4) of the panel; c) providing a plurality of sheets (5) made of composite material soaked in uncured resin, superimposed on one another and patterned to form at least one elongated stringer (6) provided with a central portion (6-a) and at least one flange (6-f); d) supporting the stringer (6) by a mandrel; e) applying an adhesive layer (11-A) to a first face of the band (1) and placing the first face in contact with the upper face (7) of the skin (4) still in an uncured state; f) applying a structural adhesive layer (11-B) on the flange of the stringer (6) at a foot end portion of the stringer (6) or a longitudinal perimeter portion of the stringer (6); g) arranging the foot end portion of the stringer (6) or the longitudinal perimeter portion of the stringer (6) above a second face of the band (1); h) arranging the flange of the stringer (6) in contact with the upper face (7) of the skin (4) and the band (1); i) covering the skin (4) and the stringer (6) with a vacuum bag (S); j ) implementing a curing cycle in an autoclave to obtain the hardening of the resin of the skin (4) and the stringer (6) and the adhesive layers so that the stringer (6) forms a single piece with the skin (4) and the carbon fibre band ( 1 ).

Description

[0001] "METHOD FOR THE PREVENTION OF FIBRE DISTORTIONS IN THE PRODUCTION OF A STIFFENED PANEL BY THE COCURE PROCESS OF

[0002] STRINGERS ON COMPOSITE SKINS"

[0003] Cross-Reference to Related Applications

[0004] This Patent Application claims priority from Italian Patent Application No. 102024000013684 filed on June 14, 2024, the entire disclosure of which is incorporated herein by reference.

[0005] Technical Field

[0006] The present invention relates to a method for the prevention of fibre distortions in the production of a stiffened panel by the cocure process of stringers on composite skins.

[0007] Background of the Invention

[0008] It is known to produce structural aeronautical components, for example wing boxes, tail assemblies and fuselage sections, comprising the following primary structures produced with composite material:

[0009] • two or more stiffened panels;

[0010] • two or more longitudinal spars;

[0011] • a given number of transverse ribs or ring frames between the panels and the above spars.

[0012] In particular, the stiffened panels consist of an external plating (called "skin") and a given number of longitudinal stiffening battens (called "stringers") that extend in the main direction, also called longitudinal, of the panel.

[0013] From the technological point of view, there are essentially the following four methods to permanently integrate the stringers with the composite skin: 1 . "Co-bonding" process ;

[0014] 2 . "Cocure" process ;

[0015] 3 . "Secondary bonding" process ;

[0016] 4 . "Mechanical assembly" process

[0017] In the first case ("co-bonding" process ) , the stringers are manufactured by curing composite material in a mould and subsequently the stringers thus formed are bonded to the skin which has not yet been cured ("wet" condition) by the interposition of a structural adhesive film arranged between the stringers and the skin . The structure thus formed undergoes a thermal ( and pressure by using a bag) cycle in an autoclave ( called precisely "co-bonding" ) during which the skin is cured ( from the wet state ) and simultaneously the stringers are adhered to the skin .

[0018] In the "cocure" process , all the elements constituting the structure ( therefore the skin and the stringers ) are instead in conditions that have not yet been cured (precisely, wet ) and are integrated with one another by a single thermal cycle of heating and pressure ( a vacuum bag covering the parts is used) during which the simultaneous curing and integration of all the elements of the structure occurs .

[0019] In the case of the " secondary bonding" process , all the skin and stringer elements are first produced by separate pressure and heating cycles and subsequently the rigid elements are integrated with a subsequent autoclave cycle and thanks to the interposition of a structural adhesive film layer . Finally, in the case of the "mechanical assembly" process , all the skin and stringer elements are first manufactured by separate pressure and heating cycles and subsequently the rigid elements are connected to one another with mechanical connection members .

[0020] The four methods described above di f fer from one another especially from the point of view of the manufacturing flow (number of operations to be performed and the number of curing cycles ) and the technological risk ( in relation to the quality of the final manufactured article ) associated therewith .

[0021] The "cocure" process is the one which, in absolute terms , provides the shortest manufacturing flow in terms of time ( to which lower recurring costs are associated) , since only one heating and pressure cycle in an autoclave is necessary for simultaneously curing and integrating the stringers with the skin . Furthermore , in this case no assembly steps are provided . This manufacturing method of aeronautical components is very coveted for the consequent overall weight savings , but is at the same time more di f ficult to apply and at greater risk of waste and / or qualitative non-conformities in the part , with respect to the other methodologies .

[0022] In fact , the "cocure" process requires the use of very precise moulds produced with materials with a low coef ficient of thermal expansion ( Invar or composite type ) .

[0023] In more detail , it is necessary to provide a mould that comprises a lower portion that allows the skin to be received in the wet state , defining the external surface of the panel and at the same time keeping the stringers in place and supporting them, since the stringers are also in the uncured state . In particular, they must be used for placing the stringers of the mandrels that copy the internal profile of the stringers and which are constrained to the lower portion of the mould . As described in the prior art referred to in patent application no . 102020000025525, it is also necessary to cover the skin by a caul plate to define the internal surface of the panel and give the parts a good surface finish, attenuating defects that could occur at the interface between the areas of the skin covered by the mandrels and those instead left uncovered .

[0024] The presence of the mandrels and caul plates that necessarily lie on the skin o f the panel , during the manufacturing steps and subsequently during curing, cause defects such as imprints and li fting towards the fibre interstice of the composite material forming the skin on the cured parts , at the relative j oints or abutments .

[0025] These fibre undulations or deviations are generated by the fact that when curing the resin or matrix of the composite material , the component , which is being formed, is in a vacuum bag in an autoclave with high external pressure ( for example 6 bar ) and temperatures up to 180 ° C .

[0026] During a first heating step in an autoclave , and precisely when the temperature is in the range between 90 ° C and 120 ° C for epoxy-type resins , the matrix or resin softens and greatly reduces the viscosity thereof , passing from a geltype consistency (which it had at room temperature ) to that very similar to water in the liquid state .

[0027] In such condition, the pressure acting on the bag and, through any interposed equipment , also on the plies of the composite material , exerts a very high hydrostatic pressure on the resin inside the laminate (precisely close to 6 bar ) . In such context , i f under the bag, in the laminate , and / or between the equipment and the laminate , there are cavities or spaces, even if small but empty, the pressure difference generates a very strong flow in the resin liquid that moves from areas where the pressure is greater towards said cavities, where the pressure is lower.

[0028] With such movement of resin, however, the fibres or reinforcement of the composite material are also moved by the pressure of the resin in motion and bent in the direction towards the cavity or space to be filled.

[0029] When the resin at 180 °C then completes its chemical reaction and becomes solid, in the areas where the fibres were moved, a defect is created with potential impacts not only on the (surface) appearance but also of a structural type (thickness) .

[0030] In fact, it is known that, when the fibres of a composite material are not flat and / or straight but are hunched, bent and / or destabilised outside the theoretical plane, under compression loads, i.e., those oriented in the direction of the fibre, they have a strength lower than the strength expected for that type of fibre. The photographs illustrated in la, lb and lc in fact illustrate typical structural defects of the prior art (enclosed by dashed parts in Figure la) , in which the fibres that have an abnormal trend due to the movement of the resin can be seen.

[0031] In particular, a very critical area is represented by the superimposition area between the terminal portion of the stringers and skin, in this area the stringer has a flat end portion (also called "foot") that forms a 90° angle with respect to the underlying skin portion (see Figure 2 representing the prior art) with "square-edge" arrangement.

[0032] In these superimposed areas, in addition to the imprints due to the j oints of the various elements forming the internalside caul plate , the " square edge" configuration above causes a defect on the underlying skin consisting of the deviation of the fibres ( called "waviness" ) , due to the di f ferentiated crushing ef fect of the fresh skin caused by the local pressure di f ference between the area of the skin without stringers and that with the stringers , as the two areas respectively have overlying tools with di f ferent configuration and weights . This phenomenon is illustrated in the photograph of Figure 3 , where the defective area is circled .

[0033] In order to mitigate the aforementioned phenomenon, the prior art provides for the arrangement of thin foils of harmonic steel ( about 0 . 2 mm thick) between the flat end portion (" foot" ) and the underlying skin; such rigid foi ls reduce the deformations of the fibres ("waviness" ) . Unfortunately, the use of such foils , while helping to reduce the "waviness" phenomenon, introduces other problems as the additional thickness of the foil makes an imprint on the external surface of the panel . Furthermore , the correct positioning of the foil must be managed .

[0034] Finally, the presence of the foil also entails the risk that , during the preparation operations of the bag and subsequently during heating, some layers that form the foot may move away from one another, causing a local delamination or debonding ( Figure 4 ) .

[0035] Therefore , there is a need to implement a method of producing a sti f fened panel by a cocure process of stringers on skins in which the prevention of fibre distortions is guaranteed and a correct arrangement of the foot end portion of the stringers on the skin is implemented without the need to use foils .

[0036] Description of the Prior Art

[0037] EP3501804A1

[0038] US2019381700A1

[0039] US2020262160A1

[0040] US 8197625B2

[0041] US2015246718A1

[0042] Solution of the Problem

[0043] The preceding obj ect is achieved by the present invention since the latter relates to a method of producing a sti f fened panel by a cocure process of stringers on skins of the type described in claim 1 . A single precured carbon fibre band integrated in the composite structure by means of a "cobonding" process is used; this band is placed, replacing the traditional metal caul plates , at the termination / interruption of the battens in the sti f fened panels .

[0044] Brief Description of the Drawings

[0045] The invention wil l now be illustrated with reference to the accompanying figures , which represent a non-limiting embodiment thereof , wherein :

[0046] • Figures la, lb, 1c, 2 , 3 and 4 illustrate the prior art ;

[0047] • Figure 5 schematises the method according to the present invention;

[0048] • Figure 6 details a step of the method of Figure 5 ;

[0049] • Figure 7 illustrates in perspective view a panel produced according to the present invention;

[0050] • Figure 8 illustrates a detail o f a product produced according to the teachings of the present invention; and

[0051] • Figures 9 , 10 , 11 and 12 illustrate variations to the method of the present invention .

[0052] Preferred Embodiment Example

[0053] With reference to Figure 5 , the method of producing a sti f fened panel according to the present invention comprises the steps of : a ) forming a band 1 of generally flat carbon fibre by arranging several sheets made of carbon fibre , impregnated with resin, superimposed on one another and implementing the hardening of the resin by a pressure and temperature cycle in an autoclave ; b ) providing, on a mould M ( indicated schematically) , a plurality of sheets 3 made of composite material ( for example carbon fibre ) soaked in uncured resin, and superimposed on one another to form a skin 4 of the panel ; c ) providing a plurality of sheets 5 made of composite material ( for example carbon fibre ) soaked in uncured resin, superimposed on one another and patterned to form at least one elongated stringer 6 along a longitudinal direction L provided with a central portion 6-a and at least one flange 6- f ( or two flanges , see Figure 7 ) ; d) supporting the stringer 6 by a mandrel (not illustrated for the sake of simplicity, being of the prior art ) ; e ) applying a first adhesive layer 11-A to a first lower face 1-b of the band 1 and placing the first face 1 -b in contact with an upper face of the skin 4 still in an uncured state ; f ) applying a second structural adhesive layer 11-B on the flange 6- f of the stringer 6 at a foot end portion of the stringer 6 ( see Figure 5 ) or a longitudinal perimeter portion of the stringer 6 facing a second upper face 1-u of the band 1 ( see Figure 9 ) ; g) placing the stringer 6 with respect to the skin 4 so that the foot end portion of the stringer 6 ( see Figure 5 ) is arranged above a first portion Pl of the second face 1-u of the band 1 , leaving a second portion P2 of the band adj acent to the first portion Pl uncovered, or the perimeter portion of the stringer 6 is arranged above a first portion Pl of such second upper face 1 -u of the band 1 ( see Figure 9 ) , leaving a second portion P2 of the band adj acent to the first portion Pl uncovered; h) arranging the flange 6- f of the stringer 6 in contact with the upper face 7 of the skin 4 and the second face 1-u of the band 1 ; i ) covering the skin 4 and the stringer 6 with a vacuum bag S ( schematically illustrated with dashes and dots ) constrained to the mould M subsequently implementing vacuum in the bag; j ) implementing a curing cycle in an autoclave to obtain the hardening of the resin of the skin 4 and the stringer 6 and the adhesive layers while the vacuum bag S presses on the underlying structures . At the end of the curing process , the stringer 6 forms a single piece with the skin 4 and the carbon fibre band 1 and the carbon fibre band comprises the first portion Pl which is arranged below the stringer 6 and the second portion P2 which is free and is facing outside the piece .

[0054] The adhesive layers 11 -A, 11-B can have , for example , a thickness of the order of 0 . 2 mm . The stringer 6 in the example illustrated in Figure 6 has a T-shaped section and comprises a central wall 6-a and two flanges 6-f that rest on the upper face 7 of the skin 4.

[0055] The stringer 6 could have a different section, for example J-shaped comprising a central portion and a single flange. In more detail, the band 1 is formed (see Figure 6) by a layered structure comprising: a first sheet 13 made of unidirectional carbon fibre, formed by fibres which extend parallel to a first direction DI parallel to the longitudinal direction L of the stringer 6, the first sheet 13 being impregnated with resin (see Fig. 6) .

[0056] A second sheet 14 of unidirectional carbon fibre formed by fibres which extend parallel to a second direction D2 perpendicular to the longitudinal direction L of the stringer 6, the second sheet 14 being impregnated with resin (see Fig . 6 ) .

[0057] A third carbon fibre fabric sheet 15, made by weft and warp arranged parallel to the two directions DI and D2 of the sheets 13 and 14, also impregnated with resin (see Fig. 6) . The sheets 13, 14 and 15 are arranged on top of one another in a stepped manner with a typical ramp of 1:30 (see Figure 5) , along the edges of the band 1 which fall inside the skin 4 and under the stringer 6, and this in order to allow a soft adaptation of the relative external surfaces without fibre distortions.

[0058] Obviously, the configuration of the band 1 can be different from that illustrated and can be modified in terms of constituent materials, number and orientation of the sheets in order to increase the stiffness of the band. Typically, the band 1 can be formed on a non-planar type mould for more pronounced curvatures or profiles . "Peel-ply" sheets of polyester fabric P ( see Figure 6 ) are arranged on the opposite faces of the layered structure . This fabric P is necessary to prepare the surface of the band 1 for the subsequent bonding, during the cocuring of the panel , to the skin and the stringers .

[0059] The "peel-ply" sheets P can be replaced by other material adapted to prepare the surfaces for the subsequent structural bonding, or they can even be eliminated from the configuration i f a speci fic surface treatment is provided for preparing the bonding to be done after curing, for example sanding or priming .

[0060] The layered structure arranged between the peel-ply sheets is subj ected to a curing process in an autoclave by applying controlled pressure and temperature ( 180 C ° , 6 bar for example ) . A caul plate can be used that presses on the face of the band 1 that is not in contact with the plate on which the vacuum bag inside which the band itsel f is placed is sealed . This gives the same polished finish to both faces of the band 1 .

[0061] At the end of the curing process , the band is ready for the method of steps b ) - j ) . The peel-ply sheets P are removed before the arrangement of the adhesive 11-a 11-b on the band 1 .

[0062] The method of the present invention has the following multiple advantages :

[0063] • Elimination of defects related to the deviation of the fibres of the composite material sheets at the end of the stringers , as the precured band 1 , extending partly below the flanges 6- f of the stringers (portion Pl ) and partly outside the flanges (portion P2 ) , protects the sheets 3 of the skin 4 therebelow from any deformations induced by the di f ferent pressure induced by the overlying stringer ;

[0064] • Cancellation of the risk of triggering delamination or debonding of the stringers 6 due to the absence of harmonic steel foils ;

[0065] • Elimination of imprints or marks on the surface of the component ;

[0066] • Simpli fication of the tool chain, eliminating the use of harmonic steel foils at the termination / interruption of the battens .

[0067] • Producing of a lighter final product for the improved structural performance expected as a result of the elimination of the typical defects of the cocure process ;

[0068] • Furthermore , it has been demonstrated by experimental tests carried out by the applicant that the flexibility of the precured band is suf ficient to allow the adaptation thereof to the curvature of aeronautical panels and any ramps present in the skin ( i . e . , 20 : 1 and 10 : 1 ) .

[0069] • destructive checks performed on the cocured panel at the precured band show the complete absence of defects such as the deviation of skin fibres at the stringer termination ( Figure 10 ) and the imprints caused by the mandrel j oints ( Fig . 7 ) .

[0070] • With the technical solution of the present invention, the cocure of the structural components can also be carried out by providing a tool chain of the mould and caul plate type with several coordinated elements to be flanked ( referred to in the prior art of patent application no . 102020000025525), since the cocure of the aeronautical component will not have structural defects in the relative joints. The use of a tool chain with full coverage, of both the external and internal surfaces of the aeronautical component, entails considerable performance advantages (such as compliance with the aerodynamic tolerances on the external profile OML) and industrial (cost and flow) advantages since the improved geometric precision and surface quality of the internal surface IML of said components will greatly facilitate the operations of mounting on the substructure, reducing the need for shims or fillers for any gaps below the connecting members and non-conformities to be managed with out-of-sequence operations.

[0071] The use of the band 1 described above can also be extended for stringers of the "square edge" type longitudinally, i.e., having the edge of the flanges at 90°, instead of with the classic bevel angle, for the entire length (Figure 8) , both in the case of skin / stringer cocuring and co-bonding with precured stringer and skin in "wet" conditions.

[0072] More in general, the precured band in question, in the composite parts, both on the IML and OML side, can be integrated at the joints of tools / machined plates / moulds (Figure 9) in order to avoid, on the cured parts, defects such as fibre deviations and / or imprints on the surface in these areas. In this case, in the event of the skin / stringer cocure process, the use of the precured band can also be provided, in the skin, at the joint of the stringer mandrels in the stringer termination / interruption area (Figure 10) instead of the usual caul plates / harmonic steel foils. Reference Numerals

[0073] 1 band 1-b first lower face

[0074] I-u second face

[0075] 3 composite material sheets

[0076] 4 skin

[0077] 5 composite material sheets

[0078] 6 stringer

[0079] 6-a central portion 6- f flange

[0080] L longitudinal direction

[0081] I I-A first adhesive layer 11-B second adhesive layer

[0082] 7 upper face

[0083] 13 first carbon fibre sheet DI first direction

[0084] 14 first carbon fibre sheet D2 second direction

[0085] 15 third carbon fibre fabric sheet M mould

[0086] S bag

[0087] P peel-ply sheets

Claims

CLAIMS1.- A method of producing a stiffened panel by a cocure process of stringers on skin for the producing of a stiffened panel, characterized in that it comprises the steps of: a) forming a band (1) of generally flat carbon fibre by arranging several sheets (13,14,15) made of carbon fibre, impregnated with resin, superimposed on one another and implementing the hardening of the resin by a pressure and temperature cycle in an autoclave; b) providing, on a mould (M) , a plurality of sheets (3) made of composite material, in particular carbon fibre, soaked in uncured resin, and superimposed on one another to form a skin (4) of the panel; c) providing a plurality of sheets (5) made of composite material, in particular carbon fibre, soaked in uncured resin, superimposed on one another and patterned to form at least one elongated stringer (6) along a longitudinal direction (L) provided with a central portion (6-a) and at least one flange (6-f) ; d) supporting the stringer (6) by a mandrel; e) applying an adhesive layer (11-A) to a first face (1- b) of the band (1) and placing the first face (1-b) in contact with an upper face (7) of the skin (4) still in an uncured state; f) applying a structural adhesive layer (11-B) on the flange of the stringer (6) at a foot end portion of the stringer (6) or a longitudinal perimeter portion of the stringer (6) facing a second upper face (1-u) of the bandg) placing the stringer (6) with respect to the skin (4) so that the foot end portion of the stringer (6) is arranged above a first portion (Pl) of the second upper face (1-u) of the band (1) , leaving a second portion (P2) of the band adjacent to the first portion (Pl) uncovered, or the perimeter portion of the stringer (6) is arranged above a first portion (Pl) of such second upper face (1-u) of the band (1) , leaving a second portion (P2) of the band adjacent to the first portion (Pl) uncovered; h) arranging the flange of the stringer (6) in contact with the upper face (7) of the skin (4) and the band (1) ; i) covering the skin (4) and the stringer (6) with a vacuum bag (S) constrained to the mould (M) subsequently implementing vacuum in the bag (S) ; j ) implementing a curing cycle in an autoclave to obtain the hardening of the resin of the skin (4) and the stringer (6) and the adhesive layers while the vacuum bag (S) presses on the underlying structures; at the end of the curing process the stringer (6) forms a single piece with the skin (4) and the carbon fibre band (1) and the carbon fibre band (1) comprises the first portion (Pl) which is arranged below the stringer (6) and the second portion (P2) which is free and is facing outside the piece.2.- The method according to claim 1, wherein the adhesive layers (11-A, 11-B) have a thickness of the order of 0.2 mm.3.- The method according to one of claims 1 or 2, wherein the stringer (6) has a T-shaped section and comprises a central wall (6-a) and two flanges (6-f) .4.- The method according to one of claims 1 or 2, wherein the stringer (6) has a J-shaped section.5.- The method according to one of the preceding claims wherein the band (1) is formed providing a layered structure comprising : a first sheet (13) of unidirectional carbon fibre formed by fibres which extend parallel to a first direction (DI) parallel to the longitudinal direction (L) of the stringer (6) , the first sheet (13) is impregnated with resin; a second sheet (14) of unidirectional carbon fibre formed by fibres which extend parallel to a second direction (D2) perpendicular to the longitudinal direction (L) of the stringer (6) , the second sheet (14) is impregnated with resin; and a third carbon fibre fabric sheet (15) , made by weft and warp arranged parallel to the two directions (DI) and (D2) , also impregnated with resin.6.- The method according to claim 5, wherein the step of arranging peel-ply sheets (P) on the opposite faces of the layered structure is envisaged; said peel-ply sheets (P) are removed before carrying out step e) of the method.7.- The method according to claim 5 or 6, wherein the sheets (13) , (14) and (15) are arranged on top of one another in a stepped manner with a ramp of 1:30, along the edges of the band (1) which fall inside the skin (4) and under the stringer (6) , and this in order to allow a soft adaptation of the relative external surfaces without fibre distortions.8.- The method according to one of the preceding claims wherein the band (1) is formed on a non-flat-type shape mould .

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