Abstract:
Method for fabricating a reinforcing coating for a part made of composite material. Pre-impregnated reinforcing fibers are used in the form of slivers and said slivers are laid on the part in two layers of longitudinal slivers by being placed thereon. This fabrication makes provision for using an angular orientation for the slivers of each layer respectively of 0° and of some other angle relative to said determined direction of the solid body for covering using an interlaced pattern type that gives layer crossing lines parallel to a longitudinal axis of the solid body for covering, which pattern type is selected to form interlacing patterns that limit the number of layer crossing lines along the longitudinal axis of the solid body for covering to a single line.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from French patent application FR 10/04713 filed on Dec. 3, 2010, which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The general technical field of the invention is fabricating parts made of composite material and in particular parts that include a reinforcing coating. Such parts are often desirable in technical applications and in particular in aviation applications, for example for airplanes, helicopters, or other aircraft. 
     The present invention relates more particularly to fabricating a reinforcing coating that uses “fiber-placement” technology. 
     (2) Description of Related Art 
     The term “fiber-placement” is used herein to cover both the technology that uses a winding mandrel associated with a laying arm, and also the technology of placing fibers by means of a robot arm that performs laying only. 
     Fiber-placement is a technology for laying fibers, slivers, rovings, or other similar reinforcing strands, whether dry or pre-impregnated with resin, on preferably convex or plane surfaces of a three-dimensional body to be covered. Such placement of reinforcing fibers on a solid surface of the body for covering is performed by applying and positioning slivers, rovings, or other similar reinforcing strands by means of a positioning device or placement head that includes at least one system for guiding fibers and applying pressure to them on the surface, such as a flexible roller fitted with a latex skid for guiding the slivers. The placement device thus serves to position the fibers on the surface by direct mechanical contact without any need for particular tension in the fibers. The placement head is itself driven by a drive system such as a robot arm or a positioning gantry, having its degree of freedom servo-controlled to match the shape of the surface of the body for covering. 
     The fiber-placement method thus makes it possible, in particular, to cover concave surfaces within the limit set by the size of the head, and also enables discontinuous fiber-laying operations to be performed automatically such as operations of starting, laying, or cutting reinforcement. The paths given to the fibers being laid may be very complex, but they remain limited in particular by the apparent stiffness in the plane of the sliver of the material being laid. A fiber-placement installation may be designed to have a plurality of placement heads that work simultaneously on a single part, in particular for the purpose of improving the productivity of the system. 
     Winding is performed on a solid body, preferably a body of revolution, and it is applied by performing helical type relative movements between the part for covering and the spool or reel of material for laying, after initially connecting one end of the fibers on the part for covering. The helical type movement is provided by a combination of at least one movement in rotation and one movement in translation, typically along an axis parallel to the axis of rotation. Additional other degrees of freedom of movement in translation or rotation are possible, depending on the complexity of the shapes for covering. 
     The reinforcing material is thus laid continuously and it is positioned and applied solely under the effect of tension that is controlled between the point of laying on the surface and the exit point from the system for supporting the spool of reinforcing material. This tension, and the location of the outlet point, typically constituted by an eyelet, are determined by controlling the winding machine to comply with the characteristics of the reinforcement that is to be made. The reinforcing material is thus not positioned by being applied to the surface by means of a roller or some equivalent element. 
     The above-described “simple” winding may also present variants that are “complex”, in particular for the purpose of improving the productivity of the operation. For example, it is possible to increase the number of slivers that are wound simultaneously on the part, either by associating them with one another to form a wider or thicker sliver, or by distributing them over a plurality of locations on the part in a longitudinal direction, i.e. parallel to the axis of rotation or by distributing them in a circular manner, i.e. in a ring around the rotating part. 
     The braiding method, another method of laying that is somewhat similar, differs from the winding method by the fact that two groups of slivers or rovings are laid simultaneously on a solid mandrel following two groups of coaxial helical paths in opposite directions, crossing in alternation over and under one another so as to lay the reinforcement in a manner that is comparable to closed and “tubular” weaving. 
     The invention is thus applicable in the field of making composite-material parts by placing fibers, e.g. for the purpose of making a coating on a helicopter blade. 
     In the context of the present invention, the following definitions should also be taken into consideration. 
     The term “laying” is used to cover an operation of placing one or more slivers of composite material of a given layer on a part. 
     The term “layer” means a partial or total covering of the reinforcing material on a given surface of the part in a thickness that is equal to the unit thickness of the laid material, typically fibers, slivers, rovings, or other equivalent strands. 
     The term “number of laying operations” corresponds to the number of laying operations needed to place all of the slivers of a layer on a part. 
     The term “pattern” should be understood as the visible design obtained by one or more crossing lines of slivers at the surface of the coating or in the thickness of the coating, in particular when they are covered by a surface fabric, e.g. in order to facilitate a paint finishing operation. 
     By definition, the term “pitch” is a laying empty space between two slivers laid side by side, either consecutively or simultaneously depending on the method, and in the same direction, as measured along a direction perpendicular to the sliver laying direction, with the convention that a pitch of n corresponds to the empty space left between two slivers being equal to (n−1) times the width of the sliver. 
     The term “pre-impregnated sliver” should be understood as having fibers, e.g. in the form of a sliver, that prior to being placed on the part, are impregnated with a resin type matrix. 
     The making of coatings, e.g. on a helicopter blade, generally consists in stacking pre-impregnated woven fabric that is said to be balanced in the warp and weft directions, the fabric being arranged at ±45° relative to the pitch so as to ensure that twisting moments are taken up. In other words, this orientation serves to optimize the mechanical behavior of the blade in twisting. Such an arrangement of woven fibers presents good behavior in the event of an impact, since damage propagation is limited by the interlacing of the fibers that, by definition, is always present in a fabric that is woven. It should be observed that under such circumstances, additional orientations are needed for taking up centrifugal forces and forces due to the blade bending, e.g. 0°/90°. 
     The present trend is to use composite preforms that are made by fiber placing in order to improve productivity. 
     The making of parts by placing fibers, as obtained using placement means of the robot arm or equivalent type, generally leads to surface appearances that are similar to those of superposed unidirectional sheets, since the fiber slivers are arranged edge to edge. Such surface appearance is unsatisfactory relative to ability to withstand damage insofar as damage propagates quickly in the fiber direction. 
     In the event of an impact on a surface made of composite material of the unidirectional sheet type, it is found that damage is relatively large since breaks (cracks) in the impregnation resin or matrix propagate very easily between the aligned fibers, with this continuing so long as the crack does not encounter a physical obstacle. 
     Various documents that approach the invention are mentioned below. 
     The NLR document dated Jun. 16, 2010 “NLR and TU Delft improve composite damage tolerance with new APPLY fiber architecture” describes the use of crossing layers of composite materials constituting placement patterns in order to increase damage tolerance. However the patterns described are not optimized for adapting to the various impacts and strikes to which a helicopter blade or a propeller blade can be exposed. 
     Document US 2004/0074592 describes a composite material with multiple orientations using an impregnated resin. Tapes of filaments are impregnated with a solid or semisolid resin. The resulting material possesses a varying number of layers and orientations within a sheet of the composite material that is formed. A preform is produced to determine the load characteristics of the composite component, and the number of layers and their orientations are selected as a function of those characteristics. An offset of 90° is provided between the filaments. 
     Document US 2009/0098337 describes articles such as a turbine cage, that are substantially cylindrical and made of composite material. Depending on the example, several plies of unidirectional fibers are interlaced in various orientations in the range 0° to 90°, including 45°, in certain circumstances. 
     Document U.S. Pat. No. 6,227,805 in the name of the Applicant describes a technique for making a blade out of composite material using resin transfer molding (RTM) in which a coating for the shell, the spars, and the attachment means of the blade are constituted respectively by at least one cutout of woven fabric that includes at least two crossed plies of crossed carbon fiber woven fabric, said plies being superposed on one another so as to form a predetermined angle between them. 
     Document EP 1 035 963 describes an applicator and control system serving to create patterns by leaving a gap in a tape, without a clear definition of any type of pattern. One of its objects is to improve the structural performance of a composite part. The limitation associated with the fiber-placement means of the type comprising solely a robot arm, as is used for making the coatings in a female mold, is such that the resulting draping can be considered as comprising superpositions of unidirectional sheets, and therefore does not have behavior that is optimized in terms of damage tolerance. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is defined by the claims. 
     The invention proposes a method of fabricating a reinforcing coating that makes it possible to overcome the above-mentioned limitations. 
     With this method, a reinforcing coating presents optimized tolerance to damage. 
     The method is adapted to various different fiber-placement techniques. 
     In an implementation of the invention, the method of fabricating a reinforcing coating for a composite material part makes use of pre-impregnated or dry reinforcement fibers in the form of slivers or rovings and consists in laying said slivers or rovings on the part by fiber placement. The method consists in: 
     using two crossed layers of longitudinal slivers; 
     locating the portions of the part or of the environment of the part that constitute the origin(s) for laying slivers; 
     determining the sliver placement directions for each of the layers relative to a determined direction of the part, and determining the laying order for each of the slivers in one of the directions in order to define at least one direction of the crossing lines of the fibers; 
     determining a laying empty space, referred to as the “pitch” between two slivers laid consecutively in a given direction; 
     for the two layers, determining the number of slivers per laying operation, the pitch of the slivers laid in each laying operation, and the number of laying operations; 
     establishing the order in which the laying operations are performed for each sliver in each direction; and 
     performing the laying sequence in the order and the pitch using the established pitch in order to superpose or interlace the slivers of two successive laying operations so as to reconstitute two layers with one or more crossing lines forming particular patterns. 
     It is found that a ridge or crossing between two sheets constitutes a shape discontinuity that generates a barrier to damage propagation. The creation of a multitude of sheet crossings, as obtained by laying slivers in patterns, is thus particularly advantageous for improving performance in terms of tolerance to impacts and damage. 
     In contrast, a line of discontinuity is likely to create a limit on stiffness or mechanical strength in a direction perpendicular to the line of discontinuity. It is therefore important to control the length and the orientation of such lines so as to arrange them as well as possible, in particular for a blade coating in which the twisting force direction is oriented at ±45° relative to the longitudinal direction of the part, e.g. a blade. 
     It is preferable to orient the lines of discontinuity in a direction parallel to the longitudinal axis of the part, e.g. a blade. It is also preferable to limit the number or the length of such lines of discontinuity in order to avoid excessively disturbing the stiffness characteristics of the reinforcement. For example, it may be preferable and sufficient to lay only one fiber-crossing line located about halfway along the chord of the part, e.g. a blade, in a direction parallel to the longitudinal direction of the part. 
     Placing the slivers by particular control of the crossing lines of the layers of slivers thus makes it possible to obtain different visible placement patterns. 
     The method in accordance with the invention thus makes it possible to propose types of sliver placement patterns that improve the behavior of the coatings, e.g. of a part, e.g. a blade, in terms of tolerance to damage, and to do so by the particular crossings of the layers of slivers that create barriers to crack propagation. 
     By way of example, the method in accordance with the invention consists in using slivers or rovings of width lying in the range 6.35 millimeters (mm) to 100 mm, of thickness lying in the range 0.2 mm to 0.4 mm, and of length lying in the range 100 mm to 1000 mm. These values that are given as examples may naturally be adapted to the size of the part. 
     In another implementation, the method in accordance with the invention consists in using dry or pre-impregnated tapes, rovings, or fibers of glass, aramid, basalt, carbon, or other natural fibers such as fibers of flax, hemp, cotton, or silk for making the slivers. 
     In an implementation, the method in accordance with the invention consists in locating the portion of the part that is to constitute the origin for the laying operation in the plane of the rigging bushings of the blade and on the twisting line of the part, e.g. a blade. It is starting from this origin that the origin of each layer is located. 
     In an implementation, the method in accordance with the invention consists in crossing the slivers of the two layers by superposing them while selecting a pitch equal to 1, 2, 3, 4, 5, 6, 7, 8, or any integer or non-integer number of times the width of the slivers. 
     In another implementation, the method in accordance with the invention consists in crossing the slivers of the two layers by interlacing them, selecting a pitch equal to 1, 2, 3, 4, 6, or 8, or any integer or non-integer number of times the width of the slivers. 
     In an implementation, the method in accordance with the invention consists in crossing a number “x” of slivers of the first layer simultaneously with a number “x” of slivers of the second layer by interlacing them, said slivers presenting a pitch equal to “y”. 
     In another implementation, the method in accordance with the invention consists in crossing two slivers of the first layer simultaneously with two slivers of the second layer while interlacing them, said slivers presenting a pitch equal to 2. 
     In another implementation, the method in accordance with the invention consists in crossing four slivers of the first layer simultaneously with four slivers of the second layer while interlacing them, said slivers presenting a pitch equal to 2. 
     In an implementation, the method in accordance with the invention consists in using an angular orientation for the slivers of each layer respectively of ±45° relative to a given direction of the part. 
     In an implementation, the method in accordance with the invention consists in using an angular orientation for the slivers of each layer, respectively at 0° and at 90° relative to a given direction of the part. 
     It is also possible to select layer patterns with other crossing angles. 
     In an implementation, the method in accordance with the invention consists in determining a spacing between the slivers of each of the layers. The pattern then presents empty spaces between the slivers at the end of laying. For this purpose, it suffices for example to select a pitch that is not an integer, e.g. equal to 1.5. 
     In an implementation, the method in accordance with the invention consists in selecting an interlaced pattern type that gives layer crossing lines parallel to the longitudinal axis of a blade. 
     In an implementation, the method in accordance with the invention consists in selecting interlacing patterns in such a manner as to limit the number of layer crossing lines along the longitudinal axis of a part, e.g. a blade, to only one. 
     In an implementation, the method in accordance with the invention consists in positioning said single layer crossing line on the chord of the part. For example, on a blade, this serves to limit losses of stiffness characteristics on the twist box zone. 
     In an implementation, the method in accordance with the invention consists in selecting patterns with crossing lines of a plurality of pairs of layers that are voluntarily mutually offset, parallel, or in two directions forming an arbitrary angle, in order to limit characteristics of stiffness in the coatings. This avoids reduced characteristics being concentrated. 
     The term “reduced characteristics” is used to mean a reduction in mechanical performance, in traction strength, and in stiffness in the fiber direction, for example, associated with the disturbance to the alignment of the fibers. 
     Such a layer offset may be obtained for example merely by offsetting the laying origin in one direction relative to the other (this may also be referred to as phase shifting). 
     The objects given to the invention are also achieved by a composite material part including a reinforced coating fabricated by the method as described above. 
     The objects given to the invention are also achieved by a blade for a main rotor of a rotorcraft or a helicopter including a reinforced coating made according to the method as specified above. 
     The objects given to the invention are also achieved by a blade for an anti-torque rotor of a rotorcraft or a helicopter, including a reinforced coating fabricated by the method as described above. 
     The objects given to the invention are also achieved by a blade for propelling an aircraft, including a reinforced coating fabricated by the method as specified above. 
     An advantage of the method of the in accordance with the invention lies in controlling the number and the positions of the crossing lines and the patterns as obtained thereby. This control makes it possible to improve mechanical behavior of parts provided with a reinforcing coating, and in particular to limit the propagation of cracks after an impact or damage. 
     Another advantage of the method in accordance with the invention lies in its suitability for application to any type of panel, shroud, spherical cap, truncated cone, etc. . . . 
     Another advantage of the method in accordance with the invention lies in the fact that it makes it possible to make coatings with specific sliver crossing patterns that may be selected to adapt in particular to the dimensions and/or the shapes of the part. 
     Another advantage of the method in accordance with the invention lies in the fact that it makes it possible to make coatings with layer crossing lines that are offset between the multiple pairs of layers making up the thickness of the part. It is important to avoid layer crossing lines being superposed between two pairs of successive layers since that could create a zone of mechanical weakness. 
     Thus, it is possible to mutually offset the positions of the layer crossing lines by modifying the position of the origin for placing a series in a direction relative to the underlying series in the same direction. 
     Thus, it is also possible to give the upper layer crossing line an orientation that is different from the direction of the under crossing line, e.g. by placing the part on a support that pivots under the installation for placing fibers. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEW OF THE DRAWINGS 
       The invention and its advantages appear in greater detail from the context of the following description of implementations given by way of non-limiting illustration, and with reference to the accompanying drawings, in which: 
         FIG. 1  shows an example of a reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers; 
         FIG. 2  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers; 
         FIG. 3  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers; 
         FIG. 4  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers; 
         FIG. 5  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers; 
         FIG. 6  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers; 
         FIG. 7  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers; 
         FIG. 8  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers; 
         FIG. 9  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers; 
         FIG. 10  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers; 
         FIG. 11  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers; 
         FIG. 12  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers of two slivers at a pitch of 2; 
         FIG. 13  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers of four slivers at a pitch of 2; 
         FIG. 14  shows an embodiment of a blade on which the reinforcing coating obtained by the method in accordance with the invention is to be applied; 
         FIG. 15  shows a blade on which the  FIG. 1  embodiment of the reinforcing coating is applied; 
         FIG. 16  shows a blade on which the  FIG. 7  embodiment of the reinforcing coating is applied; 
         FIG. 17  shows a blade on which the  FIG. 20  embodiment of the reinforcing coating is applied; 
         FIG. 18  shows a blade on which the  FIG. 5  embodiment of the reinforcing coating is applied; 
         FIG. 19  shows a blade on which the  FIG. 10  embodiment of the reinforcing coating is applied; 
         FIG. 20  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers; 
         FIG. 21  shows another element of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers; 
         FIG. 22  shows an example of crossing to define the term “superposed layers”; 
         FIG. 23  shows another example of crossing to define the term “superposed layers”; 
         FIG. 24  shows an example of crossing to define the term “interlaced layers”; 
         FIG. 25  shows another example of crossing to defined the term “interlaced layers”; and 
         FIG. 26  is a logic diagram to define the terms “superposed layers” and “interlaced layers”. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Elements that are structurally and functionally identical and that are present in more than one of the figures are given the same numerical or alphanumerical references in each of them. 
     The method in accordance with the invention consists in controlling the sequences with which slivers are laid by determining the position, the orientation, and the extent of layer crossing lines on a surface. The method in accordance with the invention thus makes it possible in controlled manner to reproduce different layer crossing configurations. 
       FIG. 1  shows an example of reinforcing coating obtained by the method in accordance with the invention. This method makes it possible to control the positions on the surface and the shapes of the layer crossing lines. The coating comprises two superposed layers A and B. Each layer A, B comprises respective slivers Ai and Bj, where i and j are integers. The slivers Ai and Bj present a pitch of 1 and they are oriented respectively at 0° and at 90° relative to a direction D. Touching slivers A 21  to A 40  are thus parallel to the direction D and they cover touching slivers B 1  to B 20  that are shown as being orthogonal to the direction D. 
       FIG. 2  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two superposed slivers A and B with a pitch of 2. Each layer A, B comprises respective slivers Ai and Bj, i and j being integers. The slivers Ai and Bj present a pitch of 2 and are oriented respectively at 0° and at 90° relative to a direction D. The slivers A 01  to A 10 , which are disjoint by a pitch equal to 2, are thus parallel to the direction D and they are put into place during a first laying operation on a part (not shown). The slivers B 11  to B 20 , which are disjoint by a pitch equal to 2, are subsequently put into place during a second laying operation at 90° relative to the slivers A 01  to A 10 , overlying them. The slivers A 21  to A 30  are disjoint by the pitch equal to 2, they are parallel to the direction D, and they are put into place during a third laying operation on the part (not shown) above the slivers that have already put been into place. The slivers B 31  to B 40  are disjoint by the pitch equal to 2, and they are subsequently put into place during a fourth laying operation at 90° over the previously-placed slivers. This produces the pre-impregnated coating in full. 
       FIG. 3  shows another example of a reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers A and B with a pitch equal to 3. Thus, during successive laying operations the following slivers are put into place: 
     slivers B 01  to B 07 ; 
     slivers A 08  to A 14 ; 
     slivers B 15  to B 21 ; 
     slivers A 22  to A 28 ; 
     slivers B 29  to B 35 ; and 
     slivers A 36  to A 42 . 
       FIG. 4  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers A and B with a pitch equal to 4. Thus, during successive laying operations, the following slivers are put into place: 
     slivers B 1  to B 5 ; 
     slivers A 6  to A 10 ; 
     slivers B 11  to B 15 ; 
     slivers A 16  to A 20 ; 
     slivers B 21  to B 25 ; 
     slivers A 26  to A 30 ; 
     slivers B 31  to B 35 ; and 
     slivers A 36  to A 40 . 
       FIG. 5  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers A and B with a pitch equal to 6. Thus, during successive laying operations, the following slivers are put into place: 
     slivers B 1  to B 3 ; 
     slivers A 4  to A 6 ; 
     slivers B 7  to B 9 ; 
     slivers A 10  to A 12 ; 
     slivers B 13  to B 15 ; 
     slivers A 16  to A 18 ; 
     slivers B 19  to B 21 ; 
     slivers A 22  to A 24 ; 
     slivers B 25  to B 27 ; 
     slivers A 28  to A 30 ; 
     slivers B 31  to B 33 ; and 
     slivers A 34  to A 36 . 
       FIG. 6  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two superposed layers A and B with a pitch equal to 8. Thus, during successive laying operations, the following slivers are put into place: 
     slivers B 1  to B 3 ; 
     slivers A 4  to A 6 ; 
     slivers B 7  to B 9 ; 
     slivers A 10  to A 12 ; 
     slivers B 13  to B 15 ; 
     slivers A 16  to A 18 ; 
     slivers B 19  to B 21 ; 
     slivers A 22  to A 24 ; 
     slivers B 25  to B 27 ; 
     slivers A 28  to A 30 ; 
     slivers B 31  to B 33 ; 
     slivers A 34  to A 36 ; 
     slivers B 37  to B 39 ; 
     slivers A 40  to A 42 ; 
     slivers B 43  to B 45 ; and 
     slivers A 46  to A 48 . 
     All of the embodiments of superposed layers may be made by winding. 
       FIG. 7  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers A and B with a pitch equal to 1. During successive laying operations, the sliver A( 2   i ) is put into place and then over it and in alternation, the sliver B( 2   i− 1), where i is an integer lying in the range 1 to 20. 
       FIG. 8  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers A and B with a pitch equal to 2. During successive laying operations, the sliver A( 2   i− 1) is put into place, then over it and in alternation, the sliver B( 2   i ), where i is an integer in the range 1 to 20, and incremented by unity on each laying operation. With each incrementation, the following sliver is put into place separated from the preceding sliver by the pitch of 2, and this applies to each of the layers A and B. 
       FIG. 9  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers A and B with a pitch equal to 4. During successive laying operations, the sliver B( 2   i− 1) is put into place, then over it and in alternation, the sliver A( 2   i ), with i being an integer lying in the range 1 to 20, being incremented by unity on each laying operation. With each incrementation, the following sliver is put into place separated from the preceding sliver by the pitch of 4, with this applying to each of the layers A and B. 
       FIG. 10  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers A and B with a pitch equal to 6. During successive laying operations, the sliver B( 2   i− 1) is put into place and then over it and in alternation, the sliver A( 2   i ), where i is an integer in the range 1 to 20, being incremented by unity for each laying operation. With each incrementation, the following sliver is put into place, separated from the preceding sliver by the pitch of 6, with this applying to each of the layers A and B. 
       FIG. 11  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers A and B with a pitch equal to 8. During successive laying operations, the sliver B( 2   i− 1) is put into place, and then over it and in alternation, the sliver A( 2   i ), with i being an integer in the range 1 to 20, being incremented by unity for each laying operation. With each incrementation, the following sliver is put into place, separated from the preceding sliver by the pitch of 8, with this applying for each of the layers A and B. 
       FIG. 12  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers A and B of two slivers laid simultaneously with a pitch of 2. During successive laying operations, the slivers B( 2   i− 1) and B( 2   i ) are put into place simultaneously, where i is an odd integer lying in the range 1 to 23 and is incremented by two for each laying operation, with the slivers A( 2   i− 1) and A( 2   i ) being put into place simultaneously over and in alternation with the previous slivers, i being an even integer in the range 2 to 20 and being incremented by two on each laying operation. With each incrementation, the following sliver is put into place separated from the preceding sliver by the pitch of 2, with this applying to each of the layers A and B. 
     Such a coating is suitable for being made rather by placing fibers or by braiding in a manner that is common-place in the state of the art. 
       FIG. 13  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers A and B of four slivers laid simultaneously, with a pitch of 2. During successive laying operations, the slivers B 1  to B 4  are put into place simultaneously, separated by a pitch of 2, and over and in alternation, the slivers A 5  to A 8  are put into place simultaneously while being separated by a pitch of 2. On each successive laying operation, the slivers in the same previously-placed layer are incremented by eight. Thus, during the third laying operation, the slivers B 9  to B 12  are put into place simultaneously and during the fourth laying operation, the slivers A 13  to A 16  are put into place simultaneously. 
     Such a coating is suitable for being made rather by placing fibers or by braiding in a manner that is common-place in the state of the art. 
       FIG. 14  shows an embodiment of a blade P presenting a longitudinal direction L and a chord C. The method in accordance with the invention serves to apply a reinforcing coating on such a blade P. 
       FIG. 15  is a diagram of the blade P having applied thereon the  FIG. 1  embodiment of the reinforcing coating. The slivers of the layers A and B are inclined at 45° relative to the longitudinal direction L of the blade P. Such a coating can be made by placing fibers and by winding. Under such circumstances, as for all layer crossings of the superposed type, fiber placing produces a result that is comparable to that which is obtained by winding. 
       FIG. 16  shows the blade P having the  FIG. 7  embodiment of the reinforcing coating applied thereto. The slivers of the layers A and B are inclined at ±45° relative to the longitudinal direction L. In addition, the stiffness in the leading edge box of the blade P is not degraded by the layer changes, providing the layer crossing line does not overlie it. The fibers are not disturbed by the layer crossings and they retain their mechanical characteristics in full. 
     Such a coating is suitable for being made rather by placing fibers or by braiding in a manner that is common-place in the state of the art. 
       FIG. 17  shows the blade P having applied thereon the embodiment of the reinforcing coating that comprises two interlaced layers with a pitch of 3, as shown in  FIG. 20 . The slivers of the layers A and B are inclined at ±45° relative to the longitudinal direction L. 
     Such a coating is suitable for being made rather by placing fibers or by braiding in a manner that is common-place in the state of the art. 
       FIG. 18  shows a blade P having applied thereto the  FIG. 5  embodiment of the reinforcing coating. The slivers of the layers A and B are inclined at ±45° relative to the longitudinal direction L. Such a coating may be made by placing fibers or by winding. 
     By way of example, when a winding mandrel is used, it is also possible to speak of placing fibers if the fibers are pressed against the mandrel by the effect of a laying head. 
       FIG. 19  shows the blade P having applied thereto the  FIG. 10  embodiment of the interlaced reinforcing coating. The slivers of the layers A and B are inclined at ±45° relative to the longitudinal direction L. 
     Such a coating is suitable for being made rather by placing fibers or by braiding in a manner that is common-place in the state of the art. 
       FIG. 20  shows another example of a reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers. 
       FIG. 21  shows another example of reinforcing coating obtained by the method in accordance with the invention and comprising two interlaced layers. 
       FIG. 22  shows an example of crossing illustrating the definition of the term “superposed layers”. 
       FIG. 23  shows another example of crossing illustrating the definition of the term “superposed layers”. 
       FIG. 24  shows an example of crossing to illustrate the definition of the term “interlaced layers”. 
       FIG. 25  shows another example of crossing to illustrate the definition of the term “interlaced layers”. 
       FIG. 26  is a logic diagram also serving to define the terms “superposed layers” and “interlaced layers”. By definition, a sliver Ru.v over Ri.j means that the sliver Ru.v crosses the sliver Ri.j by passing over the sliver Ri.j. By definition, a sliver Ru.v under Ri.j means that the sliver Ru.v crosses the sliver Ri.j by passing under Ri.j. 
     It is thus possible to illustrate the definition of superposed layer configurations. 
     Consider a surface S that is completely or partially covered by two groups G 1  and G 2  of reinforcing slivers, rovings, or other similar strands. The group G 1  is constituted by at least two rovings or slivers R 1 . 1  and R 1 . 2  that are distinct, in alignment, and parallel, touching or non-touching, and extending in a common direction D 1 . The group G 2  is constituted by at least two rovings or slivers R 2 . 1  and R 2 . 2  that are distinct, in alignment, and parallel, touching or non-touching, and arranged in a common direction D 2 . 
     D 1  and D 2  are at a non-zero angle to each other, preferably at an angle equal to 90° in the context of this description. 
     If, after observing for example that R 2 . 1  crosses and passes over R 1 . 1  and under R 1 . 2 , it is then observed, for example, that R 2 . 2  crosses and passes over R 1 . 1  and also crosses and passes over R 1 . 2 , then the two groups of rovings, R 1 . 1  &amp; R 1 . 2  and R 2 . 1  &amp; R 2 . 2  are said to be superposed. 
     In order to generalize the above definition to all crossing configurations of two groups of two rovings, the logic diagram of  FIG. 26  is provided on the basis of observation of  FIGS. 22 and 23 . 
     The two groups G 1  and G 2  are by extension constituted by layers that are said to be superposed if the above description is true for all the groups of four rovings of the same type as R 1 . 1 , R 1 . 2 , R 2 . 1 , R 2 . 2  that are identical in the thickness of the reinforcement covering the surface S. 
     These crossing configurations of rovings of type 1S and 2S can be made in particular by a winding method or by a fiber-placement method. 
     It is also possible to illustrate the definition of interlaced layer configurations. 
     Consider a surface S that is completely or partially covered by two groups G 1  and G 2  of reinforcement comprising slivers, rovings, or other similar strands. The group G 1  is constituted by at least two rovings or slivers R 1 . 1  and R 1 . 2  that are distinct, in alignment, and parallel, touching or non-touching, and extending in a common direction D 1 . The group G 2  is made up of at least two rovings or slivers R 2 . 1  and R 2 . 2  that are distinct, in alignment, and parallel, touching or non-touching, and arranged in a common direction D 2 . 
     D 1  and D 2  form a non-zero angle, and preferably an angle of 90° in the context of this description. 
     If, after observing for example that R 2 . 1  crosses and passes over R 1 . 1  and crosses and passes over R 1 . 2 , it is then observed for example that R 2 . 2  crosses and passes over R 1 . 1  and then crosses and then passes under R 1 . 2 , then the two groups of rovings R 1 . 1  &amp; R 1 . 2  and R 2 . 1  &amp; R 2 . 2  are said to be interlaced. 
     The two groups G 1  and G 2  are by extension constituted by layers that are said to be interlaced if the above-described conditions are satisfied for all of the groups of four rovings of the same type as R 1 . 1 , R 1 . 2 , R 2 . 1 , R 2 . 2  that are identifiable in the thickness of the reinforcement covering the surface S. 
     In order to generalize the above definitions to all crossing configurations of two groups of two rovings, there is provided the logic diagram of  FIG. 26 , based on observing  FIGS. 24 and 25 . 
     The interlaced roving crossing configurations of type 1E can be made in particular by a winding method or by a fiber-placement method. 
     The interlaced roving crossing configurations of type 2E cannot be made by a winding method or a fiber-placement method, but need to be made rather by methods such as weaving or braiding that are known as such and that are not described in greater detail in the present description. 
     Naturally, the present invention may be subjected to numerous variations as to its implementation. Although several embodiments and/or implementations are described, it will readily be understood that it is not conceivable to identify exhaustively all possible embodiments or implementations. It is naturally possible to envisage replacing any of the means described or any of the steps described by equivalent means or by an equivalent step, without going beyond the ambit of the present invention.