Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1999-09-27
2003-07-01
Weisberger, Richard (Department: 2169)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S173000, C156S175000, C156S180000, C156S250000, C156S273100
Reexamination Certificate
active
06585842
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to making fiber sheets, and more particularly multiaxial sheets formed by superposing and linking together a plurality of unidirectional fiber sheets disposed in different directions.
A field of application of the invention lies in making multiaxial fiber sheets for forming reinforcing plies for preparing composite material parts. The intended materials are particularly those constituted by fiber reinforcement which can be organic or inorganic, or precursors therefor such as fibers of polymer, glass, carbon, ceramic, para-aramid, . . . , which reinforcement is densified by an organic matrix, e.g. a resin, or an inorganic matrix, e.g. glass, carbon, or ceramic.
STATE OF THE ART
It has been known for a long time to make multiaxial fiber sheets by superposing unidirectional sheets, i.e. made up of threads or fibers that are oriented essentially in a single direction, the unidirectional sheets being superposed in different directions.
A common technique consists initially in making the unidirectional fiber sheets, and in giving them sufficient cohesion to enable them to be handled without dispersing the elements making them up.
A commonly proposed solution consists in bonding together the elements forming the warp of the unidirectional sheets by threads extending in the weft direction. This inevitably results in undulations being formed which, when a plurality of sheets are superposed and pressed against one another, can cause fibers to be crushed and broken, thereby creating discontinuities. That degrades the multiaxial sheets made in that way and consequently degrades the mechanical properties of the composite material parts prepared from such multiaxial sheets.
To remedy that drawback, a well-known solution consists in using bonding threads of number and weight that are as small as possible. Document GB-A-1 190 214 (Rolls Royce Limited) concerning sheets of carbon precursor fibers, and document FR-A-1 469 065 (Les Fils d'Auguste Chomarat & Cie), concerning sheets of glass fibers, illustrate that approach. Nevertheless, it is clear that the above-mentioned drawback is diminished but not eliminated.
It is also proposed in document EP-A-0 193 478 (Etablissements Les Fils d'Auguste Chomarat & Cie) to use bonding fibers but made of a heat-fusible material. During the preparation of the composite material, the temperatures used can cause the bonding threads to melt at least in part, thereby reducing the extra thickness where they cross the warp elements. However it is necessary for the material of the bonding fibers to be compatible with the nature of the matrix of the composite material, which greatly limits the use of that method.
Another solution mentioned in document FR-A-1 394 271 (Les Fils d'Auguste Chomarat & Cie) consists in placing glass fiber threads parallel to one another and in bonding them together chemically, the binder used being soluble in the matrix. In that case also, the need for compatibility between the binder and the matrix limits applications of the method. Furthermore, no means is described to enable the threads to be placed parallel to one another, and it will readily be understood that making wide sheets on an industrial scale gives rise to real practical difficulties. Finally, the resulting sheet is not free from undulations resulting from the threads being placed side by side.
Yet another solution consists in spreading out a plurality of tow, bringing together the resulting unidirectional fiber strips in a side by side configuration to form a sheet, and in imparting transverse cohesion to the sheet by needling. Such a method is described in particular in document U.S. Pat. No. 5,184,387 (assigned to Aerospace Preforms Limited) where the tows used are made of carbon precursor fibers capable of being needled without being broken. Nevertheless, multiaxial sheets are not made by superposing those unidirectional sheets. According to that document, annular sectors are cut out from the unidirectional sheet to form annular plies which are superposed and needled.
To avoid the need to give even temporary cohesion to unidirectional sheets for making multiaxial sheets, it is known to make the multiaxial sheets directly by forming a plurality of unidirectional sheets and by superposing them in different directions without any intermediate handling. The superposed sheets can be connected to one another by bonding, by sewing, or by knitting.
Documents illustrating that technique are, for example, documents: U.S. Pat. Nos. 4,518,640, 4,484,459, and 4,677,831.
In document U.S. Pat. No. 4,518,640 (assigned to Karl Mayer) reinforcing threads are introduced into the sheet while it is being formed, thereby making it possible to provide bonding without piercing through the fibers. Nevertheless, that gives rise to openings being present in the multiaxial sheet, which openings produce surface discontinuities.
In document U.S. Pat. No. 4,484,459 (assigned to Kyntex Preform), each unidirectional sheet is formed by causing a thread to pass around spikes carried by two parallel endless chains, such that the portions of the threads that extend freely between the spikes are mutually parallel. Unidirectional sheets are formed by guiding the respective threads in different directions, and they are bonded to one another by sewing. With that technique it is not possible to have reinforcing threads in the longitudinal direction of the multiaxial sheet; unfortunately, it is often necessary to place reinforcing elements in that main direction. In addition, if a large amount of tension is exerted on the threads to guarantee parallelism in each sheet, then the portions of the threads extending between the spiked chains can tend to become rounded by the fibers tightening, thereby giving rise to openings in the multiaxial sheet. Finally, it will be observed that that technique does not make a very high production speed possible given the time required for forming each unidirectional sheet.
In document U.S. Pat. No. 4,677,831 (assigned to Liba Maschinenfabrik GmbH), the technique described consists in displacing a main unidirectional sheet longitudinally parallel to the direction of the elements which make it up, and in laying transverse unidirectional sheets thereon in directions that make predetermined angles with the direction of the main sheet (0°), for example +45° and −45° and/or +60° and −60°. The transverse sheets are laid by a laying process between two spiked chains situated on either side of the main sheet. That technique which does not necessarily require a main sheet to be present, also suffers from several drawbacks.
Thus, it is necessary to eliminate the marginal zones where the transverse sheets turn around the spikes. Unfortunately, the wider the transverse sheets, the larger the marginal zones, and the larger the losses of material due to their being eliminated, and it is also more difficult to turn the sheets on the spikes. This greatly limits the width that can be used for the transverse sheets. In addition, the above-mentioned drawback of possible irregularity in the multiaxial sheet is also to be found, in particular due to the formation of holes because of the tensions that it is necessary to apply to the elements of the transverse sheets in order to hold them parallel during laying.
In addition, relatively high stitch density is necessary immediately after laying in order to confer sufficient strength to the resulting multiaxial sheet. In addition to making it impossible to preserve a smooth surface state, this high stitch density affects the flexibility of the multiaxial sheet and limits its deformability in use, e.g. by draping.
Furthermore, when a main sheet (0°) is provided, it is necessary to support it while the transverse sheets are being laid, such that all of them are to be found on the same side of the main sheet. Reinforcing elements are indeed provided that extend in the main direction (0°), but the resulting multiaxial sheet is not symmetrical between its faces. Unfortunately, s
Aucagne Jean
Bompard Bruno
Bruyere Alain
Coupe Dominique
Duval Renaud
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten Schurgin, Gagnebin & Lebovici LLP
Weisberger Richard
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