FR3012764A1 - METHOD FOR MANUFACTURING A NAIL HEAD AND COMPOSITE STRUCTURE COMPRISING AT LEAST ONE SUCH NAIL HEAD - Google Patents

Abstract

The invention relates to a method for manufacturing a filling element made of composite materials for filling an empty volume, this volume having a main axis and a cross section whose geometric shape is identical along this main axis. According to the invention, the following steps are carried out: a) identifying the main axis and establishing the geometric shape of the cross-section, this shape defining a first and a second profile (11, 12), b) decomposing each profile into minus a mathematical function whose curve describes said part or part of said corresponding profile, c) perform analysis by means of a computer to calculate the number and the arrangement of the plies (13) of at least two stacks (14) of plies (13) so that, when placing said stacks (14) in a forming tool (17) whose inner surface defines a footprint of said empty volume: - for each profile, the base of a stack wife to less partially this profile when fully described by the curve of a mathematical function, or for each portion of this profile described by the curve of a mathematical function, the base of a stack at least partially marries said portion of profile, and - at least two stacks (14) of plies (13) form wedges that couple in pairs the space between said profiles (11, 12) when the stacks (14) of said pair at least partially match a different profile one edge of each stack then being in contact with an edge of the other stack by a free end of the plies (13) of said stacks (14), d) making each of the stacks (14), e) introducing each stack into a forming tool (17) whose inner surface delimits a footprint of said void volume, f) subjecting the assembly thus obtained to a pressure and temperature cycle to form said filling member.

Description

TECHNICAL FIELD The present invention relates to a method of manufacturing a nail head made of materials and to a method for producing a nail head and a composite structure comprising at least one such nail head. BACKGROUND OF THE INVENTION composites. The present invention also relates to structures or structural elements made of composite materials such as stiffeners, comprising one or more nail heads obtained by this manufacturing method. Background Art It is known in the aeronautical field to use reinforcing elements such as frames or smooth, to give a thin envelope called skin, a good mechanical strength by distributing the forces in the working structure of the aircraft. These reinforcing elements are most often made of composite materials, which exhibit good fatigue behavior, absence of corrosion, mechanical properties of the same level and a significant weight gain compared to similar metal parts. Such reinforcing elements are typically obtained from long fibers such as carbon fibers embedded in a resin matrix. The latter may be a thermosetting or thermoplastic resin.

FIG. 1 shows an example of a panel comprising three reinforcing elements 1 placed side by side on one of the faces of a skin 2. These reinforcing elements 1, which here are profiles having a U-shaped section, are contiguous by one of their edges 3.

The practical impossibility of producing zero radii of curvature leaves a space between the skin 2 on the one hand, and the rounded portions facing two profiles 1 immediately adjacent, on the other hand.

It is then known to fill this space by a filling element 4 ("Core"), generally designated by the expression "nail head", because of the shape taken by its section, to ensure continuity of material in the corresponding junction area. Each nail head 4 is previously formed by a usual technique to obtain a "rolled nail head", in which firstly, by experience feedback or by calculation, the quantity of material required to fill the nail is evaluated. space separating the rounded portions, then drapes a single layer of fibers flat on a plane support before rolling it manually to obtain a "boudin". The thus obtained coil is then introduced into a hot forming tool having an imprint of the space to be filled, in order to conform the material to the geometry of the expected nail head. However, this manufacturing method does not allow precise and regular calibration of the amount of material required to fill the volume to be filled and does not provide a part having the theoretical geometry initially expected for the nail head. Also known is a method of manufacturing an "extruded nail head" in which a single unidirectional fold impregnated with a resin is taken and divided to obtain fold pieces of narrower widths. These pieces are then placed in an oven where the resin is heated to reduce the viscosity so as to facilitate the forming process. The pieces thus heated are then collected and stretched through a die to give the desired shape. Different cross-sections can thus be obtained using a single unidirectional fold.

However, this process imposes logistics supply and conservation of raw materials at low temperatures (-18 ° C) and the implementation of a dedicated manufacturing device cumbersome and expensive. In addition, this method of manufacture is limited because it allows to obtain only nail heads having sections and constant thicknesses, the number of different sections can be formed being, moreover, very small.

On the other hand, it is known that, during assembly operations of a nail head and stiffeners made of composite materials, it is necessary to ensure the creation of a connection between this head and the surfaces of the stiffeners while avoiding the appearance of porosities. Indeed, these porosities have adverse effects on the mechanical characteristics of the finished element and therefore impact the performance of the latter. In addition, it is regularly found, with the state of the art nail head manufacturing processes, imperfect filling rates and defects in shape of the nail heads. The presence of empty spaces then leads to a migration of pure resin, creep, from the surfaces of the stiffening elements adjacent to the empty volume that is to be filled.

This resin migration generates defects in these elements, or even deformations, which disturb the mechanical structure of these stiffening elements. The installation of the nail head then has a detrimental effect on the mechanical strength of these elements. On the other hand, it is well known that areas filled with pure resin have lower mechanical properties than areas of composite materials having long fibers embedded in a resin. In addition, because of the disadvantages mentioned above, and to limit their effects, these state-of-the-art manufacturing methods impose restrictions on the possibilities of geometries of the stiffeners. There is therefore a pressing need for a filler element, or nail head, allowing the implementation of stiffeners having complex shapes. The present invention aims to overcome these various disadvantages of the prior art by proposing a method of manufacturing a simple filling element in its design and in its operating mode, fast and economical, ensuring a uniform distribution of the material and of the resin in the filling member for filling a void space in a corresponding member. Another object of the present invention is such a manufacturing method which significantly limits the porosity of the assembly obtained by assembling a filling element and an object whose surface or surfaces delimit the space. empty to be filled in order to guarantee an adequate connection and not to disturb the mechanical characteristics of the object to which the filling element is assembled.

BRIEF DESCRIPTION OF THE INVENTION To this end, the invention relates to a method of manufacturing a filling element made of composite materials to fill an empty volume delimited by the surface or surfaces of an object made of composite materials, said volume to be filled having a main axis and a cross-sectional area to said main axis whose geometrical shape is identical or substantially identical along said main axis on at least a part of said volume. According to the invention, the following steps are carried out: a) identifying the main axis of said volume and establishing the geometric shape of said cross section, said geometrical shape defining at least a first profile and a second profile, b) decomposing each of said profiles one or more mathematical functions of which the curve described or substantially described, or the associated curves describe or substantially describe, said corresponding profile c) perform an analysis for calculating the number and the arrangement of the plies of at least two stacks of plies of unidirectional fibers so that, during the establishment of said stacks in a forming tool whose inner surface defines a footprint of said empty volume: - for each profile, the base of a stack at least partially marries said profile when is fully described, or substantially described, by the curve of a mathematical function, or for each portion of described profile, or substantially described by the curve of a mathematical function, the base of a stack at least partially wife said profile portion, and - at least two stacks of plies form wedges that occlude the space between said profiles when the stacks of said pair at least partially match a different profile, an edge of each stack then being in contact with an edge of the other stack of said pair by a free end of at least some of the plies of said stacks , so as to minimize the empty spaces between said stacks on the one hand and on the other hand between each of said stacks and the surface or surfaces of the object delimiting said volume, d) to make each of the stacks, e) to introduce each stack into a forming tool whose inner surface delimits an impression of said empty volume, f) subjecting the assembly thus obtained to a cycle of pressure and temperature for forming said filling member.

Of course, the term "base of a stack", the lowest elementary layer of this stack, that is to say that placed directly in contact with the support in step d) of manufacture of stacks, and on which are stacked the other unidirectional fiber plies. This elementary layer may consist of a single sheet or two sheets when an overlap thereof. Similarly, a profile or a portion of a profile can be reproduced by the curve of a mathematical function in an approximate manner without it being necessary to seek to further decompose this profile or this profile portion to establish curves exactly reproducing their shape. geometric. We will then say that the curve of the mathematical function "substantially describes" the profile or the profile portion. For purely illustrative purposes, this may be the case of a crenellated profile which would be described as a first approximation by an arc of a circle. This choice can be made by the operator or automatically by software.

More generally, step c) of the method can be carried out by means of a computer. Preferably, the fibers of the plies in the stacks are parallel to each other. This makes it possible to avoid the appearance of empty volumes in the stacks when they are pressurized.

The method of the present therefore advantageously makes it possible to: quantify the quantity of material to be formed, that is to say precisely determine the number of layers of unidirectional fibers required to fill the volume of the opening or cavity to be filled. This also results in optimized mastery of the material with little or no material drop. calibrating the material during forming to obtain a nail head of precise geometry very close to the theoretical section targeted, preforming salient angles without resorting, as in the state-of-the-art methods, to the random creep of the resin to fill the bottom or edges of the nail head, - to improve the quality of the finished product, that is to say of the assembly formed by the nail head cooked and assembled to the object whose cavity or the opening is to be filled. In particular, there is a significant reduction in porosity and lack or excess of material. In addition, the integration of the nail head in the cavity or the opening of the object is performed without any disturbance and / or deformation of the folds of the object, adjacent to the nail head. - to offer the possibility of forms and / or changes of sections, - not to limit the geometry of the section of the nail head, - to limit the costs of industrialization and to facilitate the management of the materials, in particular as regards the duration of ambient life, storage ...

The method of the present invention allows the manufacture of filling elements having complex profiles either because they have a radius of curvature, or because they are the combination of different geometrical shapes (arc of circle, rectilinear, .. ..). This represents a major advantage of the present invention with regard to the prior art constituted by the so-called extruded nail heads, which can indeed have only one geometry, namely straight extruded profiles. According to an embodiment, the method of the present invention is applied to a method of manufacturing structures made of composite materials comprising several parts assembled together. The composite material filling element can then be designed to occupy an empty volume delimited by surfaces of a plurality of composite material parts in a junction zone of at least three parts, said empty volume having a main axis of the junction area and a cross section. The present manufacturing method finds applications in the fields using composite materials such as aeronautics, space, automotive, etc. In various particular embodiments of this manufacturing process, each having its particular advantages. and capable of numerous possible technical combinations: in step c), a coefficient of expansion of said stacks is taken into account so that, after polymerization of these stacks, the surface of the filling element intended to be placed towards the outside of said empty volume is flush or substantially level with the surface or surfaces of said object adjacent to said void volume. This ensures advantageously that the outer surface of said filler element is not curved or recessed but instead follows the surface or surfaces of the composite material object, adjacent to the empty volume to be filled. in step c), the stacks are chosen so as to limit the required number of these stacks to fill the empty volume. In other words, among the different possible strategies for filling the empty volume, one will choose the one that requires the realization of a minimum number of stacks to fill the empty volume so as to reduce the manufacturing time and inherent costs. Alternatively, or cumulatively, in step c), the stacks can be chosen so that the cumulative percentage of empty spaces between these stacks on the one hand and on the other hand between each of said stacks and the surface or surfaces of the object delimiting said empty volume is less than 5% of the internal volume of said empty volume. in step d), and for at least one of said stacks, the plies are deposited on a support by shifting at least some of said plies with respect to the corresponding immediately lower ply in this stack so that a lateral edge of this stack substantially reproduces the curve of the mathematical function describing or substantially describing said profile or said corresponding profile portion. For purely illustrative purposes, this lateral edge of the stack follows an arc or a portion of an ellipse.

This gives a stack with a side edge has steps. the plies of this stack are deposited by overlapping at least a portion of them. Preferably, at least the lower layers of said stack are deposited with respect to said support, by overlapping them. This arrangement makes it possible to confer on said elementary layers of the stack a "sliding effect" facilitating the arrangement of these during the forming step. Advantageously, each of said stacks is manufactured by automated deposition of resin impregnated unidirectional fiber plies, said plies being arranged in the form of a strip which is cut at the lengths calculated in step b) for the stack considered. in step d), layers are deposited on a support to form a coating stack whose outer surface is intended to be flush with or substantially level with the surface or surfaces of the object next to said volume to be filled; edges of said coating stack being straight or substantially straight, - the support on which the stacks are made is flat or non planar as rounded.

This support may, in fact, have a shape conferring on the overlay stack the desired rounded shape allowing for example a conservation of the curvature of the outer surface of a structure in which the filling element is integrated. in step e), additional plies or portions of plies 20 are added to fill the empty space (s) of said volume to be filled with fiber plies and / or resin, said plies being impregnated with resin or dry. The manufacturing method of the invention advantageously makes it possible to use webs in an uncooked state, which are still called raw. The present invention also relates to a composite material structure comprising one or more fillers obtained by the method as described above. Preferably, this structure comprises at least one piece of bent structure defining an inner and outer surface, a filling member extending to the top of the inner surface of the bent portion of said piece on one side of the latter . In a particular embodiment, this part having an L-shaped or U-shaped section, two contiguous branches of said L or U form an angle of a value less than or equal to 90 °. Alteratively, this angle may be greater than 90 °.

According to one embodiment, this structure comprises two pieces of U-shaped or L-shaped sectional structure contiguous or whose edge of a first piece is placed opposite an edge of a second piece so that this structure has a H, W, T, Z or Y-shaped section. The nail head may have a cross-section selected from the group consisting of a T, L, Z, U, triangle shape, and combinations of these forms. BRIEF DESCRIPTION OF THE DRAWINGS Other advantages, aims and particular features of the present invention will emerge from the description which follows, for an explanatory and in no way limiting purpose, with reference to the appended drawings, in which: FIG. 1 shows schematically a skin having adjoining stiffening elements on one of its faces, the empty space between two consecutive elements being filled by a nail head; FIG. 2 shows different strategies for filling an empty volume having a section of triangular shape, as determined in step c) of the method according to a particular embodiment of the present invention, - Figure 3 schematically shows one of the stacks of unidirectional fiber layers produced in step d) of the method of manufacture, after selecting the filling strategy of the empty volume shown in FIG. 2c); FIG. 4 shows the organizational diagram of the stacks in a forming tool whose inner surface delimits a footprint of the volume to be filled, as well as the steps of introduction and positioning of these various stacks, FIG. Fig. 6 is a sectional view of the filling member of Fig. 6; 5 and stiffeners defining the empty volume filled by this filler element; FIG. 7 is a partial and enlarged view of the assembly of FIG. 6 showing the arrangement of unidirectional fiber webs in the filler element. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Firstly, it is noted that the figures are not to scale.

Figures 2 to 4 describe the main steps leading to the manufacture of a filling element of an empty volume delimited, on the one hand, by the rounded edges of two U-shaped sections section contiguous and, on the other hand on the other hand, by a flat wall such as a skin covering the bases of the U-shaped profiles to define a structural element such as a self-stiffened panel according to an embodiment of the present invention. This empty volume to be filled comprises a longitudinal axis which is here the axis of junction of the profiles, along which the cross section of this volume is identical. In addition, this empty volume has a median plane, the geometry of the cross section being symmetrical with respect to this plane by defining a first profile 11 and a second profile 12. The geometric shape of this cross section has thus been determined, identifies that each profile of the cross-section can be reproduced by an arc 18. A calculation unit then makes it possible to determine the parameters of the mathematical function whose curve, an arc of circle, makes it possible to reproduce each profile. For this purpose, the computing unit (not shown), such as a computer, comprises a software-controlled processor, at least one memory system connected to the processor and adapted to record at least data to be used such as functions. mathematics, at least one display system adapted to provide information in a visible manner such as the curves reproducing the profiles of the cross section, and at least one input system designed at least to exert an action on the functions of operation of the computing unit and / or to enter data.

Using this calculation unit, an analysis is carried out to define a strategy for filling the empty volume by means of unidirectional fiber plies 13 arranged in the form of stacks 14. The plies 13 consist of unidirectional fibers. extending longitudinally without discontinuity being parallel to each other. The nature of the webs 13 of unidirectional fibers and the nature of the resin in which they are impregnated will be chosen according to the application envisaged, from the fibers and resins usually used in the field of composite materials. Thus the plies 13 of unidirectional fibers may consist of carbon fibers and the resin may be a thermosetting resin such as a phenolic or epoxy resin, or a thermoplastic resin such as a PEEK resin (polyetheretherketone). A "canvas" 15 representative of the optimal arrangement of the fibers, in the cross section of the geometry of the empty volume, makes it possible to obtain, by calculation, different arrangements of the plies 13 of fibers in the empty volume. FIG. 2 diagrammatically represents three examples of strategies for arranging webs 13 that are favorable to the technique of implementation by hot forming.

The choice of the strategy is then specific to the geometry of the cross-section of the expected filler element to fill the void volume. Here, it is chosen to limit to the minimum the required number of stacks 14 to fill the void volume, leads to retaining the strategy shown in Figure 2c). According to this strategy, two stacks 14 of unidirectional fiber plies will be arranged symmetrically with respect to the median plane. The largest volume remaining to be filled will then be filled by a supply of a complementary stack 16 of unidirectional fiber plies 25. Figure 4 shows how these three stacks 14, 16 will be introduced and arranged in a forming tool 17 whose inner surface is an imprint of the empty volume to be filled to obtain the filling element.

The number of webs 13 of unidirectional fibers deposited in a section (N) of the filling element is an integer number of webs 13 derived from the ratio of the surface area of the section of the filling element (A) to the surface area. of the section of a fiber (a), from which is subtracted the sum of the areas of the empty zones, ie uncompleted areas taking into account the thickness of the fibers, ie the calculation: N = A - 2. s - Ep a where: A = Area of the section of the filling element, which (Ir is given by the formula = 2xR2 x 1-- 4, R = Radius on external faces of the adjoining parts delimiting the empty volume or else the geometry of the filling element intended to fill this empty volume, Ep is the thickness of a ply in the fired state and x1 is the offset value of the first ply with respect to the elementary layer determining the base of the stack, determined from the equation of the circle (FIG 3) As shown in FIG. 4, two stacks 14 of plies form wedges that fill the space between the profiles in the shape of an arc of a circle when the base of each stack at least partially marries a different profile, one edge of each stack comes into contact in the median plane with an edge of the other stacking through one end of their plies 13 so that the empty spaces between these stacks 14 on the one hand and on the other hand between each of these stacks 14 and the profiles are limited. In order to have this contact between the plies of the stacks 14 and even the interlocking plies 13 for some of them, at the level of the median plane, the free end of each ply of a stack is offset with respect to the ply. placed immediately below the sheet considered in the stack so that each stack has a side edge reproducing the curve describing the profile considered, here an arc of a circle. After having thus determined the number, the orientation, the order of placement and / or spacing of each of the plies 13 of each stack, these stacks 14 are manufactured on a plane support. 3 represents a stack whose base is designed to fit at least partially the profile of the cross-section, the first two plies 13 ', 13 "have a small overlap which is necessary for the arrangement of the plies 13 of fibers according to the strategy determined advantageously, the arrangement of these two first plies 13 ', 13 "of unidirectional fibers at the interface between the forming tool and the stack 14 of unidirectional fibers plies pre-stacked loose. of folds gives a "sliding carpet" effect to facilitate the arrangement of the plies 13 ', 13 "during the forming step, then each stack 14 is introduced into a forming tool whose inner surface delimits an imprint of the volume empty to be filled and subjecting the assembly thus obtained to a cycle of pressure and temperature to form the filling element in an embodiment of the present invention. 5 to 7 show on the one hand a nail head obtained by the manufacturing method of the present invention and a structure comprising two U-shaped section stiffeners contiguous whose space between the rounded edges is filled with this nail head. It emerges from the study of a micro section on a section of this structure that the filling rate in the nail head is optimal and homogeneous. In addition, a control of this piece by ultrasound reveals a very good health.

Claims (15)

REVENDICATIONS1. A method of manufacturing a filling element made of composite materials to fill an empty space delimited by the surface or surfaces of an object made of composite materials, said volume to be filled having a main axis and a cross section to said main axis whose geometrical shape is identical or substantially identical along said main axis on at least a part of said volume, characterized in that the following steps are carried out: a) identify the main axis of said volume and establish the geometric shape of said cross section, said shape geometrically defining at least a first profile (11) and a second profile (12), b) decomposing each of said profiles (11, 12) into one or more mathematical functions whose described or substantially described curve, or the associated curves describe or substantially describe, said corresponding profile, c) perform an analysis to calculate the number and arrangement of the webs (13) at least two stacks (14) of unidirectional fiber plies so that, when placing said stacks (14) in a forming tool (17) whose inner surface defines a footprint of said empty volume: - for each profile (11, 12), the base of a stack (14) at least partially matches said profile when fully described, or substantially described, by the curve of a mathematical function, or for each portion of this profile described or substantially described by the curve of a mathematical function, the base of a stack (14) at least partially matches said profile portion, and - at least two stacks (14) of plies (13) form corners 30 by pairing the space between said profiles (11, 12) when the stacks (14) of said pair at least partially match a different profile, one edge of each stack then being in contact with an edge of the other stack of said pair by u non-free end of at least some of the plies (13) of said stacks (14), so as to minimize the empty spaces between said stacks (14) on the one hand and on the other hand between each of said stacks (14) and the or the surfaces of the object defining said volume, d) manufacturing each of the stacks (14), e) introducing each stack into a forming tool (17) whose inner surface defines a footprint of said empty volume, f) subjecting together obtained a pressure and temperature cycle to form said filling member. 2. Method according to claim 1, characterized in that in step c), account is taken of a coefficient of expansion of said stacks (14) so that after polymerization of these stacks (14), the surface of the filling element intended to be placed towards the outside of said empty volume is flush with or substantially level with the surface or surfaces of said object next to said empty volume. 3. Method according to claim 1 or 2, characterized in that in step c), said stacks (14) are chosen so as to limit the necessary number of stacks (14) to fill the empty volume. 4. Method according to any one of claims 1 to 3, characterized in that in step d), and for at least one of said stacks (14), is deposited the webs (13) on a support by shifting to least some of said plies (13) with respect to the corresponding immediately lower ply in this stack so that a lateral edge of this stack substantially reproduces the curve of the mathematical function describing, or substantially describing, said profile or said corresponding profile portion . 5. Method according to claim 4, characterized in that said lateral edge of said stack follows a circular arc (18) or an ellipse portion. 6. Method according to claim 4 or 5, characterized in that said layers (13) of said stack are deposited by overlapping at least a portion (13 ', 13 ") thereof. 7. Method according to one of claims 4 to 6, characterized in that each of said stacks (14) is manufactured by automatically depositing webs (13) of unidirectional fibers impregnated with resin, said webs (13) being arranged in form. a band which is cut to the lengths calculated in step b) for the stack considered. 8. Method according to any one of claims 1 to 7, characterized in that in step d), is deposited webs (13) on a support to form a coating stack whose outer surface is intended to be flush or substantially level with the surface or surfaces of the object next to said volume to be filled, the edges of said coating stack being straight or substantially straight. 9. Method according to one of claims 4 to 8, characterized in that said support is non-planar. 10. Method according to any one of claims 1 to 9, characterized in that in step e), additional webs (13) or portions of webs (13) are added to fill the non-occupied space (s) of said volume to be filled by plies (13) of fibers and / or resin. 11. Method according to any one of claims 1 to 10, characterized in that said plies (13) are impregnated with a resin. 12. Method according to any one of claims 1 to 10, characterized in that said plies (13) are dry. 13. Composite material structure comprising one or more filling elements obtained by the process according to any one of claims 1 to 12. 14. Structure according to claim 13, characterized in that it comprises at least one piece of bent structure defining an inner and outer surface, a filler element extending to the top of the inner surface of the bent portion of said part of one side 25 of the latter. 15. Structure according to claim 13 or 14, characterized in that it comprises two pieces of U-shaped section structure or L contiguous or an edge of a first piece is placed opposite an edge of a second piece so that said structure has a section shaped like H, W, T, Z or Y.