Tubular laminated structure for reinforcing a piece in composite material

The structure is produced by means of a tubular braid of which the wall is folded over to form corrugations or wrinkles which constitute layers of fibrous material, superposed in axial direction, each layer extending from the inside surface to the outside surface of the structure.

BACKGROUND OF THE INVENTION 
The present invention relates to the production of a tubular laminated 
structure, and more particularly to the production of a structure 
comprising layers of fibrous material, superposed in axial direction, each 
layer extending from the inside surface to the outside suface of the 
structure. 
The field of application of the invention is, more particularly but not 
exclusively, that of the production of fibrous structures of reinforcement 
constituting preforms for producing pieces in composite material, such as 
for example ablative thermal protection for solid propellant rocket 
motors. 
Various processes are already known for producing reinforcing preforms for 
axi-symmetrical pieces in composite material. 
One of these processes consists in stalking rings of fibrous material and 
optionally bonding them together, for example by needling, in order to 
produce a preform which is thereafter densified to obtain the desired 
axi-symmetrical piece. This process is suitable for producing pieces of 
relatively small thickness, such as brake discs, but it is no longer 
adapted when the pieces to be produced have a greater axial dimension. 
Another process consists in winding on a mandrel yarns, tapes or strips of 
fibrous material in superposed layers. The layers may be bonded together 
in different ways, such as needling or sewing, or else by means of pins 
implanted on the surface of the mandrel, either so as to actually 
constitute radial bonding elements, or to provide a passage for radial 
reinforcing elements which are inserted after the winding operation. 
OBJECT AND SUMMARY OF THE INVENTION 
The object of the present invention is to propose a new process which also 
enables the production of pieces of any axial dimension, but with an 
increased resistance to ablation, particularly when the pieces are 
subjeced to high temperature gas flows. 
This object is reached with a process which, according to the invention, 
consists in using a tubular braid and folding its wall so as to form 
corrugations or wrinkles which constitute the layers of fibrous material 
superposed in an axial direction. 
It is possible, due to the deformability of the braid, to obtain a 
structure in which the layers can have any required orientation with 
respect to the axis of the structure. In particular, in the case of an 
application for the production of composite thermal protections, the 
resistance to ablation may be improved by a "sleek-hair" orientation of 
the fibers with respect to the gas flow. 
Moreover, in each corrugation or wrinkle, the fibers extend from the inside 
to the outside of the tubular structure. Thus, even if the inside part of 
said structure is attacked by an intensely high temperature gas flow, the 
cold outside part of the fibers remains firmly embedded and the 
orientation thereof helps the discharge of the pyrolysis gases, resulting 
from the degradation of the inside part, without delamination of the 
structure. 
The invention will be more readily understood on reading the following 
description with reference to the accompanying drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring first to FIG. 1, this shows a tubular braid 10 which constitutes 
the starting product for the preparation of a structure according to the 
process of the invention. As illustrated in FIG. 2, the structure 11 is 
produced by folding the braid over itself by a translational movement 
parallel to its own axis (arrows F) so as to form corrugations or wrinkles 
12. Said latter are applied against one another and form layers which are 
superposed in axial direction, each layer extending from the inside 
surface to the outside surface of the structure 11. 
In the illustrated example, wrinkles 12 are arranged slantwise, meaning 
that they form an angle a with axis 13 of the structure 11, said angle a 
being variable between a few degrees and 90.degree.. The amplitude of the 
corrugations or wrinkles 12, determines with angle a, the thickness e of 
the structure 11, namely the distance between the inside surface and the 
outside surface. 
Depending on the geometrical properties of the braid (diameter, angles 
formed by the rows constituting the braid, etc . . . ) and making use of 
its ability to deform, it is possible to obtain a structure in which angle 
a and thickness e can be adjusted at will. For example, angle a can be 
equal to 90.degree., i.e. with wrinkles 12 perpendicular to axis 13. 
The process according to the invention is applicable to the preparation of 
tubular structures of which the cross-section is not necessarily circular, 
and of which the center of gravity of the cross-section is not necessarily 
on a straight line. Thickness e can also be made to vary along the axis of 
structure 11, by modifying the amplitude and/or the inclination of the 
wrinkles 12. 
As already indicated, the invention is more particularly applicable to the 
production of tubular structures constituting fibrous reinforcing preforms 
for the making of pieces in composite materials, and in particular 
ablative pieces. 
The material constituting the tubular braid 10 is selected as a function of 
the intended purpose of the structure. In the case of a reinforcement for 
a composite piece destined to withstand high thermal stresses, the 
selected braid will be produced from refractory fibers or precursors 
thereof, such as for example a braid in pre-oxidized PAN 
(polyacrylonitrile) fibers, which is a carbon precursor. The braid is 
produced with the selected fibers by any of the conventional processes for 
producing tubular braids. 
The tubular structure obtained after folding over the braid may be 
reinforced, for example by being subjected to a needling operation, the 
needling being carried out crosswise with respect to the wrinkles 12. 
Thereafter, in order to obtain the target composite piece, the structure 
is densified by through-deposition or infiltration, of a matrix-forming 
material such as a thermo-setting resin, carbon or other refractory 
material, for example of ceramic type. Various densifying methods may be 
used, such as the chemical vapor infiltration method, or the liquid 
impregnation followed by a heat treatment or else an infiltration preceded 
by a pre-impregnation. The material constituting the matrix, like the 
material constituting the reinforcing structure, is selected as a function 
of the desired application. It may be resin, pyrolytic carbon or another 
refractory material such as silicon carbide deposited by chemical vapor 
deposition according to a technique which is now well under control. 
The structure obtained according to the invention is well adapted to the 
production of thermal protections, particularly for the rear extensions of 
solid propellent rocket motors, since the resistance to ablation caused by 
the flow of high temperature combustion gases may be substantially 
increased due to a "sleek-hair" orientation of the wrinkles 12, hence of 
the fibers of the structure 11. Moreover, in each wrinkle 12 the fibers 
start from the inside surface of the structure and extend to the outside 
surface. Thus, when the hot inside part of the structure becomes degraded 
due to the hot gas flow, the fibers located in that part remain embedded 
in the cold outside part and the pyrolysis gases can easily escape towards 
the inside of the tube without affecting its resistance. 
Obviously, the process according to the invention is not limited to the 
production of reinforcing structures for ablative composite pieces, on the 
contrary, it can be applied to all cases in which a tubular reinforcing 
structure has to be prepared, provided that the necessary braid exists.