GB2251001A

GB2251001A - Reinforcement preform of knitted fibres

Info

Publication number: GB2251001A
Application number: GB9027585A
Authority: GB
Inventors: Kenneth Richard Langley
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1990-12-20
Filing date: 1990-12-20
Publication date: 1992-06-24
Anticipated expiration: 2010-12-20
Also published as: GB9027585D0; US5330818A; GB2251001B

Abstract

A reinforcement preform for a carbon/carbon or the like component such as an airfoil guide vane comprises a knitted fabric formed with a hole or slot (12) in the vicinity of a junction between adjacent sections of the finished article and on assembly the reinforcement fabric (6, 8) for one of the sections is passed through the hole. In this way stresses can be better transmitted to a base section with reduced risk of tearing or delamination. In addition a rib may be formed on the part (4) and may be secured by stitching. The fabric may be of silicon carbide fibres. <IMAGE>

Description

4 1 22510M 1 - REINFORCEMENT PREFORM OF KNITTED FIBRES The invention

relates to a reinforcement preform of knitted fibres for a reinforced article.

In particular the present invention concerns the assembling of a one piece knitted fibre reinforcement preform for maximum strength and durability prior to its use in a transfer moulding process.

Conventional techniques of manufacturing carbon and glass fibre reinforced articles have used labour intensive wet layup or prepreg techniques using layers of woven cloth or matt. In attempts to speed up the process the reinforcing fibres have been chopped up and mixed with one of the components of a catalyst-cured resin system thereby allowing the fibres to be injected into a moulding die. The latter process while being quick and easy to handle suffers a fundamental drawback. The random orientation of the reinforcing fibres produces a cast structure of uniform strength in all directions but which is of inadequate strength in areas of high stress. While the layup and prepreg techniques can be used to provide the necessary strength results may vary and these are inefficient and unsuitable for economic production.

Reinforcement preforms can be knitted in the same way as socks and gloves. The basic preform is then folded and tacked or stitched together before being loaded into a die to be impregnated with injected resin. This arrangement still provides a ccr.ponent in which the reinforcement lacks sufficient strength in regions of lateral stress such as at a junction between two orthogonal sections. This invention seeks to overcome 2 - this drawback.

According to the present invention in its broadest aspect a reinforcing preform is knitted with a buttonhole located at a junction between adjacent sections of the finished article. In assembling the preform for maximum stress capability at the junction at least one section of reinforcing preform is passed through the buttonhole to provide reinforcement in an adjacent section.

The invention and how it may be carried into practice will now be described in greater detail with particular reference to the embodiment illustrated in the accompanying drawings in which:

Figure 1 shows in schematic form a known method of assembling a one-piece knitted fibre reinforcement preform for a'ceramic nozzle guide vane in a gas turbine engine, Figure 2 shows opened-out a reinforcement preform incorporating the invention, Figure 3 shows the preform of figure 2 folded in preparation for impregnation, and Figures 4a and 4b shows fragmentary views of an article similar to figure 3 illustrating how a further structural feature might be incorporated.

Referring now to figure 1, a reinforcement preform 2 for a ceramic airfoil guide vane comprises a length of knitted silicon carbide fibre fabric folded in the shape of a "T". The fabric preferably varies in width so that it has a broad centre section 4 flanked by narrower side sections 6,8. In assembly, the centre section 4 is folded back onto itself at two fold lines spaced equidistantly from a centre line so that the edges of the broader section meet on the centre line. These form the vane platform or base. The side sections are then bent upwards at right angles to the platform to form reinforcement for the airfoil portion.

The edges of the reinforcement piece are then stitched together before it is placed in a mould for impregnation. At least the edges 10,12 of the side sections are stitched together. However, the reinforcing fabric lacks inherent stiffness and stitching does not appreciably impart rigidity although it helps the finished piece retain its shape.

After impregnation the preform is loaded with, for example, a thermosetting resin. The rdsin is then burned off. The preform retains its stiffness and holds its shape while the carbon matrix is impregnated into the structure in a suitable process, for example, vapour deposition.

:ound, however, that in use stress It has been 4 concentrations around the junction of the airfoil section with the platform are sufficient to cause structural fatigue in the reinforcement matrix. Although the bulk of the component material is unaffected the localised failure is sufficient to curtail useful component life.

The present invention seeks to overcome these drawbacks in the manner illustrated in the remaining drawings. Basically the reinforcing piece is folded in a way which increases its inherent strength in the region of the airfoil platform junction so as to better absorb the 4 - stress encountered in use.

The centre section 4 of the preform as shown in figure 2 is formed with a "button hole" or slot 12 on the centre line of the middle section of the piece. Then as further shown in figure 3 the side sections 6,8 instead of being folded under the platform reinforcement are folded over it and then passed through the slot 12. As before the sections 6 and 8 are then stitched together along their abutting edges to comprise the airfoil reinforcement. By this means stresses in the airfoil section reinforcement will be better transmitted to the base section with reduced risk of tearing or delamination in the vicinity of their mutual junction.

Under certain loud conditions a symmetrical assembly described above may not prove ideal. If the vane is subject to a substantially unidirectional transverse loading the reinforcement layer on the side the force is subject to a tensile force and on the other side to a compressive force. If the layer on the compressive side has passed through slot 12 the force will tend to separate the double reinforcement in the adjacent platform section. The solution is to fold the platform reinforcement sections in opposite directions at opposite ends of the platform so that the section on the compressive side is not passed through slot 12. Thus only one section of reinforcement passes through the button hole slot.

The slot 12 or "button hole" is preferably formed by knitting in the first instance to preserve the integrity of the reinforcing fibres. In that way the woven fibres of the fabric remain capable of transmitting tensile stress. Interruption of fibre continuity as would be caused by a cut and stitched slot made after the reinforcing fabric was knitted would seriously reduce the inherent strength of the preform.

Figures 4a and 4b show the same basic component as figure 3 with the addition of an axial rib 14 formed on one face of the platform opposite the airfoil. An additional section is knitted into the basic preform piece, by increasing its length and varying the width as necessary. This is shown in figure 4a. During the assembly/folding stage, equivalent to figure 3, as shown in figure 4b the extra section is raised from the middle platform section to form the axially extending rib. This may be stitched either side on lines 16,18 to help it retain shape prior to the resin loading step described above.

The rib or a plurality of such ribs may be formed in the place of the vane, that is the airfoil section. The ribs may be connected with axial flanges-.

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Claims

A one-piece knitted fibre preform for a reinforced article having in its finished form a junction between adjacent sections subject to stress comprises in a portion of the preform for reinforcing a first of said sections a slot or hole defined by a knitted margin through which is passed a preform reinforcement for the second of said sections.
A reinforcement preform as claimed in claim 1 wherein the periphery of the buttonhole is stitched around.
3 A reinforcement preform as claimed in claim 1 or claim 2 wherein the buttonhole is elongate in one dimension to accommodate the width of the preform section to pass therethrough.
4 A reinforcement preform as claimed in any preceding claim wherein the preform. sections are formed contiguously.
A reinforcement preform as claimed in any preceding claim for an airfoil member of the kind having an airfoil section disposed perpendicularly to a platform section wherein the platform preform section is formed with a buttonhole and the airfoil preform section is formed contiguous with one arm of the platform preform section and is folded over said section and passed through the buttonhole.
6 A reinforcement preform as claimed in claim 5 further comprising a second airfoil preform section contiguous with a second arm of the platform preform opposite the first arm and is also folded over and passed through the buttonhole A reinforcement preform substantially as described with reference to fugres 2, 3 or 4 of the accompanying drawings.