Method for forming convex non-extractable formed pieces made of a composite material

In order to form shaped convex non-extractable pieces made of a composite material, a partially polymerized (stage B) preform strip (10) is placed in a forming tool (16) along a cylindrical surface having one rectilinear generator and one convex directrix curve whose shape coincides with the shape of the piece to be embodied. The particular section of the piece is obtained with the aid of an inflatable bladder (22) pressing the preform strip against a forming die (24). In this way, the area contractions are kept to a minimum, which avoids may undulations of the fibers and any falling off of the mechanical characteristics of the pieces which may accordingly arise.

FIELD OF THE INVENTION 
The invention concerns a method to form non-extractable convex formed 
pieces made of a composite material. 
BACKGROUND OF THE INVENTION 
Composite materials are more and more frequently used in many industrial 
applications, such as in aeronautical, automobile and rail transport 
applications. The particular structure of these materials enables them to 
embody pieces with extremely varied shapes and provide these pieces with 
the desired characteristics by means of a suitable orientation of the 
fibers they contain. 
The method of the invention is in particular applicable to the production 
of aeronautical structures made of a composite material, these structures 
being extremely delicate to embody. These structures are pieces with 
strong curves having non-extractable shapes, such as aircraft fuselage 
frames, door framing stiffeners, support reinforcements disposed at the 
edges of holes, etc. 
In order to embody such pieces, one first solution consists of producing a 
rectangular preform plate by draping edge to edge and in successive sheets 
strips of unidirectional fibers or fibrous fabrics impregnated with resin. 
Then a preform is cut from this plate, this preform having the shape of 
the piece to be embodied. So as to give this preform the section of the 
piece, it is subjected to one or several forming operations during which 
the uneven sections of the piece are shaped with a aid of a suitable tool. 
This known method has a large number of drawbacks. Accordingly, the 
operation consists of cutting a preform having the shape of the piece to 
be embodied from a large preform plate resulting in a high discard rate, 
which increases the dimensions of the piece. 
In addition, this method involves embodying a piece in which the 
distribution of the fibers is totally anisotropic. This in particular 
complicates the task of the engineering office which needs to take into 
account when designing the piece the evolution of the mechanical 
characteristics arising from this distribution of the fibers. Moreover, 
the unhomogeneous distribution of the fibers in the preform strip, as well 
as the carrying out of forming operations on a plain strip, result in area 
contractions and elongations unable to be accepted by the fibrous products 
in certain zones, which results in the formation of folds, etc. Finally, 
this method is a long method and cumbersome to implement. 
In one variant of this known production method, instead of embodying by 
draping a rectangular preform plate and then of cutting a preform from 
this plate, a preform is produced directly by draping whose shape 
approximates the contour of the piece to be embodied. 
If this technique makes it possible to considerably reduce the discard 
rate, it significantly increases the draping time and does not resolve any 
of the aforementioned drawbacks. 
In the document FR-A-88 10984, another method is described making it 
possible to embody a flat preform having the shape of the piece desired to 
be obtained from a rectilinear preform strip cut from a larger flat 
preform plate. This method consists of curving inwards the preform strip 
inside its plane, for example with the aid of a conical roller machine. 
Thus, a piece is obtained in which the distribution of the fibers is 
homogeneous over its entire length with a discard rate of virtually nil. 
However, so as to provide this piece with its definitive section, the 
latter needs to be subsequently subjected to one or more forming 
operations. During these operations, the sections internal to the radius 
of curvature of the piece are shaped by the apparent elongation of the 
evolute of the fibers which are then stretched. On the other hand, the 
sections outside the radius of curvature and previously deformed with the 
aid of the conical roller machine are shaped by the apparent area 
contraction of the evolute of the fibers outside the radius of curvature, 
which are then compressed. 
In pieces produced in this way, these compressed fibers singe owing to the 
friction stresses existing between them and the reduction of the evolute 
is absorbed by the undulation of the fibers. The undulated fibers then 
lose all their stiffness and resistance characteristics in the stratified 
piece, which then more strongly stresses the binding resin. This results 
in a falling off of the characteristics, this proving to considerably 
damage structure pieces. 
In addition, this method requires, like the preceding methods, a relatively 
large number of stages, thus increasing the duration and cost of the 
method. 
SUMMARY OF THE INVENTION 
The precise object of the invention is to provide a new method for forming 
formed pieces made of a composite material making it possible, like the 
method described in the document FR-A-88 10984, to virtually reduce to 
zero the discard rate and to embody a piece in which the fibers are 
distributed homogeneously and controlled, whilst limiting to an absolute 
minimum the elongations so as to avoid any area contractions at the time 
of forming, this method further making it possible to reduce as far as 
possible the stages, as well as the production time of such a piece. 
To this effect, the present invention proposes a method for the forming of 
non-extractable convex pieces made of a composite material from a plain 
preform strip including fibers preimpregnated with resin and orientated 
along at least two different non-longitudinal directions, wherein said 
method consists of: 
placing the preform strip in a forming tool along a cylindrical surface 
having one rectilinear generator and one convex directrix curve which has 
the shape of the piece to be embodied, this tool including a forming die 
having a formed surface which has a complementary section of the piece to 
be embodied, and 
applying by pressure the preform strip against the formed surface of the 
forming die. 
Preferably, at least the preform strip is partly applied against the formed 
surface with the aid of an inflatable bladder placed on the other side of 
the preform strip with respect to the formed surface opposite at least one 
section of this strip to be deformed. 
In this case, it is possible to apply the preform strip against the formed 
surface, either by inflating the inflatable bladder, or by deforming the 
bladder, after having firstly inflated it, with the aid of a mobile member 
of the forming tool. 
So as to enable the preform strip to undergo deformations without it 
breaking up during forming, this strip is preferably compacted by placing 
it in a partial vacuum or by rolling it between press rollers before being 
placed in the forming tool. This mechanical action, associated with the 
coherence characteristic of the preform strip, creates cohesive links 
which ensure the proper behaviour of this strip during forming. 
In the particular case where the piece to be embodied is a closed frame, 
the two extremities of the preform strip are interconnected with the aid 
of at least one seam, preferably zigzag, before placing this strip in the 
forming tool; 
The preform strip generally includes at least one portion not to be 
deformed, which corresponds in most cases to the internal portion of the 
piece to be embodied. So as to isolate this portion with respect to the 
rest of the piece, it is possible to maintain it by mechanical clamping in 
the forming tool when applying the preform strip against the formed 
surface, or to embody at least one longitudinal seam at the limit of this 
portion not to be deformed, or finally to add to this portion at least one 
additional lap of fibers orientated longitudinally with respect to this 
preform strip. The seam or additional lap are then placed before the 
preform strip is placed in the forming tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As diagrammatically shown on FIG. 1, the embodiment in accordance with the 
invention of a formed convex non-extractable piece starts by producing a 
plain rectangular preform strip 10. This preform strip 10 may be embodied 
directly with the desired dimensions or obtained by cutting from a larger 
preform plate. In either case, there are virtually no discards. 
The strip or the preform plate is normally obtained by draping by 
superimposing a certain number of laps N1, N2, N3, N4 and local interlap 
reinforcements R1, R2, etc., in a deformable zone and having a crossed 
structure formed on non-woven fibers F1, F2, F3, F4, etc., disposed in 
each lap along a specific direction, the direction differing from one lap 
to another. As shown on FIG. 1, the fibers, such as F1 and F2 or F3 and F4 
of two successive laps N1, N2 or N3, N4, are preferably orientated from a 
given positive angle but in the opposite direction with respect to the 
longitudinal direction of the preform strip 10. It is also possible to 
provide a different angle of orientation, for example .alpha. and .alpha.' 
(FIG. 1a), but in the opposite direction, from one lap to another so as to 
obtain an asymmetrical crossing of the fibers. The value of this angle 
.alpha., which must strictly not be nil, is determined according to the 
mechanical characteristics of the piece it is desired to produce. These 
characteristics also determine the number of laps constituting the preform 
strip 10 and the nature of the material constituting the fibers F1, F2, 
etc., these fibers normally being of the same type. By way of example 
being in no way restrictive, the angle may be about 45.degree. and the 
fibers F1, F2, etc., may be carbon, glass, Kevlar (trade name for an 
aromatic polyamide), etc. 
The fibers constituting the various laps of the preform strip 10 are 
preimpregnated with resin, this resin normally being a polymerizable 
resin. When draping is carried out, the resin is at a given polymerization 
stage which leaves the preform strip a sufficient adherence characteristic 
enabling it to be draped and handled. 
In one embodiment variant (not shown), the laps of the non-woven fibers are 
replaced by preimpregnated fiber fabrics orientated along directions more 
or less .alpha., as in the previously described example. 
So that it may undergo the deformations linked to forming without it 
breaking up, the preform strip 10 thus obtained is mechanically compacted 
by applying pressure on its opposing faces. This compacting may in 
particular be obtained by placing the preform strip in a partial vacuum or 
by rolling the latter between pressing rollers. The mechanical compression 
effect added to the adherence characteristic of the product creates 
cohesive links which contribute in providing the preform strip with 
resistance during forming. 
As shown at 12 on FIG. 1, one or several longitudinal seam lines may 
further be embodied on the preform strip 10 at predetermined locations 
which make it possible to locally isolate one portion P1 of the strip, 
which must not be deformed during forming, from the rest P2 of the strip. 
This zone may also be reinforced by strips of unidirectional fibers 
disposed longitudinally with respect to the strip prior to the forming 
sequence. 
In the example shown, which corresponds to the production of a piece with a 
U-shaped section, the isolated portion P1 is found in the bottom portion 
of FIG. 1 below the seam line 12 and it corresponds, when the piece is 
completed, to one internal wing AI (FIGS. 7 and 8) of the piece. All the 
rest P2 of the strip 10 shall at the time of forming undergo a deformation 
resulting in an apparent increase of its diameter, the elongation, 
however, being limited to the strict minimum, as shall be seen 
subsequently. 
The seam line 12 is embodied with a yarn of the same type as the fibers F1, 
F2, etc., forming the preform strip 10. 
In one embodiment variant (not shown), the isolating of the portions P1 of 
the preform strip 10 not to be deformed is obtained, not by embodying one 
or several seam lines as shown on FIG. 1, but by placing on the 
corresponding portion of the preform strip one or several strips of fibers 
orientated longitudinally with respect to the preform strip. The fibers of 
these additional laps, which are of the same type as the fibers F1, F2, 
etc., behave as a hoop and prevent the deformation of the corresponding 
portion P1 at the time of the subsequent forming operation. 
The same result may also be obtained without any seam line and without any 
additional band of longitudinal fibers by carrying out a mechanical 
flanging of the corresponding portion of the preform strip when the latter 
is placed in the forming tool. In certain cases, this latter solution may 
be combined with one of the previous solutions. 
As shown diagrammatically in detail on FIG. 2, when the preform strip 10 is 
used for the production of a non-closed piece, this strip is placed 
directly into a suitable forming tool along a cylindrical surface having 
one rectilinear generator, as well as one convex directrix curve which has 
the shape of the piece to be embodied. The shape of this convex directrix 
curve may therefore be any and include in particular rectilinear portions, 
as well as variable radius bent inward portions. Most frequently, the 
generator of the cylindrical surface is then perpendicular to the 
directrix curve, as shown on FIG. 2. 
In the case shown on FIG. 3 where the piece to be embodied is a closed 
piece, such as a closed frame of an aircraft fuselage, the two extremities 
of the preform strip 10 are interconnected with the aid of one or several 
zigzag seams 14. These seams are made with yarns compatible with the 
method for polymerizing the final piece. 
As in the case of a non-closed frame, the preform strip is then placed in a 
suitable forming tool along a cylindrical surface having one rectilinear 
generator and one convex directrix curve having the shape of the piece to 
be embodied. 
FIGS. 4a to 4c illustrate one first stage for using a forming tool adapted 
for the production of a piece having a U-shaped section. 
The forming tool, generally denoted by the reference 16, includes an 
internal dimensionally stable support piece 18 whose external surface is a 
cylindrical surface having a directrix curve parallel to the internal 
surface of the piece to be embodied. 
Around the bottom portion of the internal support piece 18, a shim is 
placed with a height equal to the height of the portion P1 of the preform 
strip 10. An inflatable bladder 22, initially deflated, is placed above 
the shim 20 around the support piece 18. The thickness of the shim 20 
corresponds to the thickness of the bladder 22 in its deflated state. The 
preform strip 10 is placed as described previously around the shim 20 and 
the inflatable bladder 22. 
A forming die 24 is placed around the preform strip 10 and at the level of 
the shim 20, this die being preferably pressed against the shim 20 so as 
to maintain the portion P1 at the time of forming. The upper face of the 
die 24, as well as its internal and external peripheral edges, define a 
formed surface 25 complementary to one of the faces of the formed piece to 
be embodied. More precisely, this formed surface has a U-shaped convex 
section complementary to the internal concave of the U sectionally formed 
by the piece to be embodied. 
The forming tool 16 shown on FIGS. 4a to 4c further includes a fixed 
horizontal plate 26 placed around the internal support piece 18 
immediately above the upper edges of the preform strip 10 and the 
inflatable bladder 22. 
In practice, the preform strip 10 is firstly placed inside the forming die 
24, then the bladder 24 is positioned, as well as the pieces 18, 20 and 26 
which block this bladder on three sides. 
The forming tool also includes a mobile hoop 28 suitable for moving 
vertically downwards and initially retracted above the lower face of the 
plate 26. This hoop 28 is situated slightly beyond the outer peripheral 
edge of the forming die 24 and may either have a shape complementary to 
the latter, or be adapted for moving along its outer edge. 
Of course, some of the elements of the forming tool 16 are embodied by 
several pieces so as to enable this tool to be assembled and disassembled 
and allow for the placing of the preform strip and removal of the formed 
piece. 
In the first stage successively shown by FIGS. 4a to 4c, the forming of the 
piece is obtained by inflating the bladder 22. To this effect, this 
bladder is connected to a compressed air source (not shown) via a pipe 30. 
When the bladder 22 begins to be inflated (FIG. 4b), this bladder is 
blocked on three sides by the internal support piece 18, the shim 20 and 
the upper plate 26. As a result, the inflation of the bladder is expressed 
by the latter being deformed outwardly, which has the effect of 
progressively cladding the portion P2 of the preform strip 10 situated 
above the portion P1 against the plain upper face of the forming die 24. 
The preform strip 10 is progressively brought to the desired section via 
apparent elongation, the fibers then assuming a trajectory identical to 
the one obtained by curving a preform strip inside its plane with the aid 
of conical rollers, as described in the document FR-A-88 10984. 
It is interesting to observe that, at the end of inflating the bladder 22 
(FIG. 4c), the upper portion of the preform strip 10 intended to form the 
outer wing AE (FIGS. 7 and 8) of the formed piece is found folded back 
upwards and cladded against the internal cylindrical face of an external 
support piece 32 so that the elongation of this section only exceeds the 
final elongation of the outer wing by the thickness of the hoop 28. 
FIGS. 5a to 5c show a variant for implementing the first stage of the 
forming method of the invention. In this variant, the forming tool also 
includes an internal support piece 18, a shim 20, an inflatable bladder 
22, a forming die 24, an upper plate 26, a mobile hoop 28 and an external 
support piece 32. However, the bladder 22 is inflated before the start of 
forming and the plate 26, instead of being fixed, is mobile so as to be 
able to move downwards, thus drawing close to the forming die 24. 
Forming is then effected, not longer by inflating the bladder 22, but by 
the plate 26 moving downwards, as successively shown on FIGS. 5a to 5c. At 
the end of this first stage, the central portion of the strip 10 is 
cladded onto the upper face of the forming die 24 and the upper edge of 
the strip 10 is orientated upwards and cladded against the internal 
surface of the support piece 32 (FIG. 5c). 
Once the first forming stage has ended, the preform strip 10 thus has an 
approximately Z-shaped section, irrespective of the device used to arrive 
at this shape. So as to obtain a piece having a U-shaped section, it is 
thus necessary during a second stage to turn downwards the upper edge of 
the preform strip 10, as shown on FIGS. 4c and 5c. 
In the embodiment shown on FIGS. 6a to 6c, this operation is effected by 
moving the mobile hoop 28 downwards after having deflated, at least 
partially, the bladder 22. As shown by the figures, the mobile hoop 28 
then passes inside the upper portion of the preform strip 10 which is 
cladded against the external support piece 32 so that it comes into 
contact with the preform strip in the hollow formed between this upper 
portion and the central portion cladded onto the upper face of the forming 
die 24. Then the upper portion of the strip is gradually turned downwards 
without the apparent evolute ever increasing beyond the dimensions reached 
by this portion at the end of the first stage. 
When the second stage has been completed (FIG. 6c), the upper portion of 
the preform strip 10 is thus cladded downwards against the outer edge of 
the forming die 24 so as to form the outer wing AE of the piece to be 
embodied. 
Even at the level of the outer wing AE, the fibers thus undergo merely an 
extremely limited area contraction. 
The method of the invention thus causes the undulations to disappear which 
occur with existing methods and makes it possible to automatically form a 
complex shaped formed piece, whilst benefitting from the best possible 
mechanical properties since the deformations sustained by the fibers are 
also limited as far as possible. 
FIGS. 7 and 8 show the working of the formed pieces PP1 and PP2 obtained by 
applying the forming method of the invention respectively in the case of 
an open frame and a closed frame, these pieces having in both cases a 
U-shaped section comprising one internal wing AI and one external wing AE 
obtained as described earlier. 
So as to stiffen the pieces obtained, the latter are normally placed in an 
oven inside which polymerization of the resin is completed. 
One can readily understand that the forming method of the invention is not 
merely limited to the production of pieces having a U-shaped section. Thus 
and solely by way of example, the section of the pieces obtained may also 
have the shape of an L, a Z, an I, etc. 
The production of a piece with an L-shaped section is embodied in a forming 
tool similar to the one shown on FIGS. 4a to 4c and 5a to 5c, except for 
the mobile hoop 28 and the external support piece 32 which may both be 
suppressed. Forming is then limited to placing of the upper portion P2 of 
the preform strip 10 on the upper face of the forming die 24 in either of 
the ways described with reference to FIGS. 4a to 4c and 5a to 5c. When 
this forming stage is completed, the entire upper portion of the preform 
strip is folded down onto the upper face of the forming die 24 so that the 
piece clearly has the desired L-shaped section. 
In the case where the section of the piece intended to be produced has a Z 
shape, the forming tool and its use are identical to those described above 
for the production of pieces with an L-shaped section by adding an 
external support piece 32. However, the width of the portion of the 
preform strip 10 situated above the forming die 24 is much larger than the 
width of the upper face of the latter. The forming operation obtained 
either by inflating the bladder 22 or by moving the upper plate 26 thus 
has the effect of cladding the upper extremity of the preform strip 
against the internal face of the external support piece, which clearly 
provides the piece with the desired Z-shaped section. 
So as to obtain a piece having an I-shaped section, two symmetrical pieces 
are produced with a U-shaped section which are then assembled back to 
back. 
Of course, these general section shapes exhibited by the piece obtained 
according to the method of the invention are merely given by way of 
illustration, the upper face of the forming die 24 not needing to be 
perfectly plain when the core of the formed piece to be embodied needs to 
have a section with a particular shape. 
FIG. 9 diagrammatically shows a forming tool 16a making it possible to 
embody in accordance with the invention non-extractable convex formed 
pieces made of a composite material having a section with the shape whose 
concavity is orientated outwardly. The left and right halves of FIG. 9 
show the tool 16a respectively before and after it is implemented. 
This forming tool 16a includes an internal forming die 24a which has as a 
section a formed external surface 25a whose shape is complementary to the 
shape of the piece to be obtained. Around the die 24a, the preform strip 
10 is initially placed in the position described previously, that is along 
a cylindrical surface having one rectilinear generator and one convex 
directrix curve which has the shape of the piece to be embodied. An 
inflatable bladder 22a, initially deflated, is placed around the preform 
strip 10 followed by an external support piece 32a, whose internal surface 
is a cylindrical surface complementary to the surface initially formed by 
the preform strip 10, with the thickness of the almost deflated bladder 
22a. 
In this case, forming is obtained by inflating the bladder 22a so as to 
clad the preform strip against the formed external surface of the forming 
die 24a, as shown on the right half of FIG. 9. 
So as to embody a piece having such a section, each of the lateral edges of 
the strip 10 is maintained, either by stitching or with the aid of a strip 
of longitudinal fibers, before placing it into the forming tool. On the 
other hand, at least one of these lateral edges is left free inside the 
forming tool so as to allow the fibers to be deformed in order to be 
cladded against the formed surface of the forming die 24a. 
At the end of forming, a formed piece is obtained which may in particular 
assume the shape of an open frame or a closed frame, such as the one shown 
at PP3 on FIG. 10. This section of this frame has the shape whose 
concavity is orientated outwardly. 
In order to produce a formed piece with a section having the shape whose 
concavity is orientated inwardly, a forming tool 16b (FIG. 11) is used 
similar to the one described above with reference to FIG. 9 but in which 
the shapes presented by the surfaces in relation to the pieces 24a and 32a 
are inverted. 
More precisely, FIG. 11 shows that the tool 16b includes an internal 
support piece 18b which has an external cylindrical surface having one 
rectilinear generator and one convex directrix curve which corresponds, 
having almost the thickness of the inflatable bladder 22b, to the convex 
directrix curve formed by the preform strip 10 when the latter is placed 
in the tool. The tool 16b further includes a forming die 24b whose formed 
internal surface 25b is complementary to the external surface of the piece 
to be embodied. 
The production of the piece is moreover identical to the one described 
previously with reference to FIG. 9, that is forming is obtained directly 
by inflating the bladder 22b placed between the support piece 18b and the 
preform strip 10, as shown on the right half of FIG. 11. 
FIG. 12 represents the formed piece PP4 obtained at the end of forming 
effected with the aid of the forming tool 16b of FIG. 11. 
Of course, in order to produce pieces having different sections, the 
forming tool shall be adapted to this section without departing from the 
context of the invention. 
It is also to be noted that if the use of one or several inflatable 
bladders constitutes one particularly advantageous solution to implement 
the forming method of the invention, this solution must not be regarded is 
restrictive. In fact, the forming method of the invention rests 
essentially on the use of the aptitude of the preimpregnated fibrous laps, 
preferably formed of crossed fibers but not limitatively, more or less, 
for deforming when they are subjected to pressures perpendicular to their 
surfaces. These pressures enabling the preform strip initially placed 
along a cylindrical surface to be shaped may therefore be obtained by any 
device and in particular by means of rolling or by directly exerting a 
pneumatic pressure on the preform strip. 
As already mentioned, the forming method of the invention, as opposed to 
existing methods, makes it possible to obtain quickly, simply, and 
possibly in automated fashion, all non-extractable convex formed pieces 
made of a composite material by only carrying out initially on the preform 
strip those deformations strictly required for forming. In particular, 
this makes it possible to suppress any singeing of fibers, this singeing 
originating from undulations, and results in the stratified pieces 
obtained in the loss of all the stiffness and resistance characteristics 
of the fibers. 
It shall also be observed that implementation of the method of the 
invention may be effected either continuously or discontinuously and that 
the forming tool may include heating means so as to increase the rate of 
production.