WO1991015357A1

WO1991015357A1 - Preform and composite structure

Info

Publication number: WO1991015357A1
Application number: PCT/GB1991/000472
Authority: WO
Inventors: Frank Robinson
Original assignee: Courtaulds Plc
Priority date: 1990-03-30
Filing date: 1991-03-27
Publication date: 1991-10-17
Also published as: GB9007162D0

Abstract

A preform for a turbine blade is folded up from a fabric sheet to provide at least two layers of fabric over substantially the whole of a base portion (1) and the blade portion (2).

Description

PREFORM AND COMPOSITE STRUCTURE

Technical Field

This invention relates to preforms and composite structures, and has particular reference to preforms suitable for use in the manufacture of composite struc¬ tures for use as gas turbine engine blades.

The term "blade" as used herein is intended to cover not only rotor blades, but also stator blades such as stator vanes or guide vanes of gas turbine engines.

In the late 1960's and in the,early 1970's, develop¬ ment work was carried out to produce carbon fibre rein¬ forced rotor blades for gas turbines. These blaαes were intended to be fan blades on the large diameter first stage air compressor of a high by-pass engine. A very considerable amount of work was carried out to design and produce fan blades using a plastics matrix material incorporating carbon fibre as a reinforcement for the fan blade.

Unfortunately, however, the blades were unable to withstand impact. Typically, the fan blades of a gas turbine engine have to be able to withstand an impact from a bird which may be ingested during flight or take-off. Tests have been developed whereby chicken carcasses are fired at fan blades or compressor blades rotating in an engine. Typically a fan blade must be able to withstand the imDact from a 3 pounds (1 kilo) chicken, hitting the fan blade at a velocity of about 300 mph (just under 500 kilometres per hour). Unfortunately, it was found that the fan blades produced during the late 1960's and early 1970's were unable to withstand this impact. The project to design an engine using a composite carbon fibre rein¬ forced fan blade was suspended and the carbon fibre reinforced fan blades were replaced with non-reinforced titanium fan blades. There is however a need for light- weight reinforced composite fan or compressor blades.

Turning to a different need. but nevertheless an eαually important need. there is a need for imoroved turbine efficiency.

It has long been known that the operating efficiency of a gas turbine engine can be increased by increasing the operating temperature of the turbine of the engine. The efficiency can be proDortional to the third power cf the operating temperature of the turbine and therefore there is an incentive to increase the operating temDerature by as much as possible. Even small increases in temDerature are advantageous and if major increases are to be obtained it has long been realised that it may be necessary to introduce ceramic materials into the gas turbine cf the engine tc enable the engine to operate at temDeratures above those which can be accommodated by metal co oonents.

It has been very difficult to make fibre reinforced metal or ceramic turbine blades, particularly rotor blades that will withstand the very high temperatures and the high centrifugal and centripetal loads imposed on them by the high rotational sσeeds. Many problems have been encountered with obtained optimised orientation of the reinforcing fibres, in order to maintain the integrity of the rotor blades whilst keeping their weight as low as possible.

An object of the present invention is to provide a fabric preform for a composite structure blade (which includes a rotor bade or a stator vane) for a gas turbine engine. In the case of rotor blades the right selection of fabric preform provides enhanced reinforcement for rotor blades compared with that achieved in the past. The right selection of material of the fabric preform together with the right selection of matrix material in which the preform is embedded provides qood resistance tc hiqh -> temDeratureε in the turbine region of the engine.

Summary of the Invention

By the oresent invention there is provided a fabric preform for reinforcing a composite structure gas turbine engine blade as hereinbefore defined, whicn is charac¬ terised in that the gas turbine blade has a substantially planar base portion and an upstanding blade portion, the Dreform for the comDosite blade having at least two layers of fabric over substantially the whole of the base Dcrtion and the blade portion.

The fabric is preferably knitted but may be a woven material. The knitted material mav be a double jersey knitted material.

The base portion may be substantially αuadπ lateral in Dlan (either rectangular or trapezoidal). In the case where the platform is rectangular and the blade portion is skewed relative to the base portion, in plan, the blade will not lie parallel to either of the edges cf the base portion. The blade portion may be hollow so as to have the two layers of the blade portion separated by an elongate gao.

The Dreform may be σroduced as a flat blank and folded to form the base Dortion and the blade portion. In the form of a preform blank it Dreferably comprises a central base area and a pair of blade areas on opposite sides of the base area. The central area mav be a paral¬ lelogram, with the acute angle θ° of the parallelogram being such that (90°-θ°) is eαual to the angle at which the blade portion is skewed on the base portion. The bulk of the blade portion may lie in a plane substantially at right angles to the plane of the base portion.

In the blank, the two blade areas may be substantial¬ ly rectanqular in shape. One blade area mav be sliqhtlv wider than the other blade area to enable it to form the longer outside layer of an aerofoil shaDe. The central base area may be wider than both of the blade areas, to permit the formation of a blade portion located within the boundaries of the base portion.

The edges of the blade areas where they contact one another in the folded-UD Dreform may be sewn to retain them in position.

The connection between each of the two b^":ade areas and the central area may be discontinuous at one side so as to form a slot between the blade area and the base area, the slot being parallel to the connection and extending part way along the line of the connection, the slot at the connection of one blade area being diagonally opposite the slot at the connection of the other blade area, the blade areas being displaced oppositely along the edges of the central area so that the blade areas car, be folded along the length of the blaαe area portions and the bent, edge of one blade area which is closest to a fcld abuts the edge of the other blade area which is furthest from a fold. The knitted Dreform blank may be a piece of double jersey knit with the wales of the double jersey extending parallel to the side edges of the blade areas. A wale may be omitted on cne side of the jersey knit, to ease folding of the blade area, and may extend frc the root of the slot to the distal edge of the blade area.

Reinforcing inlays may be laid into or inserted into the knitted fabric. Such reinforcing inlays may be suffi¬ ciently large and located in the blade areas to form snubbers.

The base portion may include a root-forming portion on the opposite side of the blade area. The root-forming portion may be a folded shape having a re-entrant portion to assist securinq the base portion of the blade to a mounting area in the engine. The root-forming portion may be of "fir tree" shape. The base portion may be in the form of a platform. The preform blank may be folded tc provide snubbers in the blade areas. The snubbers may coincide with and assist the action of reinforcmq inlays.

A knitted preform is preferably knitted from a ceramic material yarn. Preferred ceramic materials are carbon fibre yarn and most preferred are silicon carbide yarns.

The present invention further provides a knitted Dreform blank having a central area in the shape of a parallelogram and having two rectangular integral exten¬ sions, one on each of one pair of opposed sides, the parallelogram having an acute angle in the range 60° to 89°. The knitted preform blank may be of silicon carbide yarn or carbon yarn. There may be a slot extending part way into the connection between the parallelogram central area and each of the two rectangular integral extensions, the slot of each extension being or, a diagonally opposite side to that of the other. The knitted preform blank may be double jersey knitted.

The knitted preform may be folded into the shape of a blade and impregnated with a heat resistant compatible material. A silicon carbide material may be impregnated with silicon carbide. The impregnation may be by means cf vapour phase deposition. A carbon fibre composite may be impregnated with carbon by carbon vapour phase deposition.

The height of the blade may be greater than the width of the blade. The height of the blade may be greater than the smaller of the edges of the parallelogram when seen in p1an.

The blade preform blank is preferably of such a design that all of the edges of the preform are selvedge, there being no cut edges.

Brief Description of Drawings

By way of example, embodiments of the present inven- tion will now be described with reference to the accom¬ panying drawings, of which:-

Figure 1 is a schematic view of a preform suitaole for the reinforcement of a comDosite blade or vane assemb¬ ly of a gas turbine engine,

Figure 2 is a plan view of the folded base and blade portions of the preform of Figure 1,

Figure 3 is a plan view of a fabric in the unfolded state for making the preform of Figure 2,

Figure 4 is a plan view of a similar fabric to that of Figure 3,

Figure 5 is a schematic stitch diagram view of a portion of the fabric of Figure 4 along the line of arrow V,

Figure 6 is a view similar to Figure 5 but in a folded condition,

Figure 7 is a plan view cf a folded blade assembly utilising two folded portions according to Figure 6.

Figures 8 and 9 are stitch diagrams of alternative forms of reinforcement material.

Figures 10 and 11 are cross sectional views of two further preforms showing blade and base portions incor¬ porating an integral root, Figure 12 is a cross sectional view of a Dreform showing blade and base portions incorporating integral snubbers with inlays,

Figures 13 and 14 show, respectively, a fabric blank and preform for making an aerofoil blade which has radially inner and outer platforms, and

Figures 15 and 16 show, respectively, a fabric blank and a preform for reinforcement of a multi-blade nozzle guide vane assembly for a gas turbine engine.

Description of Preferred Embodiments

Figure 1 illustrates a blade and base assembly preform in the folded and assembled state. The preform essentially comprises a base portion 1 and a blace portion 2. It can be seen that the blade portion 2 is located substantially at right angles to the base portion 1 and that the base portion 1 is itself substantially planar. It may be that base portion 1 will be slightly curved because it is incorporated into the inner or outer wall of the gas flow passage of a gas turbine engine.

The blade portion 2 may be a rotor blade for a gaε turbine engine or may be a stator vane, such as a compres¬ sor stator vane or a turbine guide vane for the engine. The base 1 is sometimes referred to as a platform. Typically the preform shown in Figure 1 would be formed of a knitted fabric such as knitted silicon carbide or knitted carbon fibre yarn and would be formed into a composite by any suitable deposition or impregnation technique.

The preform shown in Figure 1 is embedded within a suitable matrix using conventional techniques such as, for example, resin impregnation, reaction transfer moulding, or by building-up the matrix by means of chemical vapour deposition or infiltration techniques. In the case where the fabric is knitted from carbon fibres or threads, the matrix may be a resin or may be carbon which is formed, either by impregnating the fabric preform with a carbonisable resin, moulding the preform to the desired shape and carbonising the resin (the so-called charred resin route) or by chemically vapour depositing carbon from a carbon bearing gas such as methane, or by a combination of both techniques.

Heretofore such composite blades would have been made by laying up individual strands or a mat of fibrous material in a mould and moulding a resin around the mat or strands. The present invention enables a preform to be prepared in which the fibrous reinforcing material is in the form of a fabric which is uniformly spread out through the mould, and because knitting, which is the preferred technique, enables inlays to be laid into the material, the knitted fabric of the preform can be reinforced with inlay material just where it is reouired. Inlaying may also be used to increase the density of reinforcing fibre material in the preform.

The use of a fabric preform enables the composite structure to be made reliably and repeatedly with the fibrous reinforcing fabric just in the correct places and in the correct amounts.

The blade portion 2 is shown as being of generally aerofoil shape, but although preferred blade portions would normally be of aerofoil shape, for ease of understanding the invention, the blade portions illustrated in Figures 2, 10, 11, 12, 14 and 16 are shown flat. If in fact flat blades are required, it may be possible to use a woven fabric structure for the preform. For more ccmolex shapes. however, knitted fabrics are preferred, both because it is possible to deform knitted structures as preforms and permit them to accommodate shapes not pos- sible with woven fabrics, but also because it is possible to knit complex shapes such that there is a continuous selvedge or edge around the preform and no cutting of the preform is necessary during the assembly of the blade. 5 This means that there are no free edges which can become unravelled or untangled. Furthermore because the sewing together can take place through the selvedges, the struc¬ ture is more secure.

Referring to Figure 2 this shows blade and base 0 portions of a preform in which the blade pcrtion 3 is skewed at an angle 4 to an edge 5 of the base portion 1.

The assembled preform is shown in Figure 2 but when laid flat, the fabric appears as shown in Figure 3.

It can be seen from Figure 3 that the flat fabric has 5 a central area indicated generally by 6 which is of paral lelogrammatic shaDe. There is an acute angle 7 (shown as θ) between an edge 8 of the parallelogram and an edge

9. This central area 6 will, when folded, form the base portion of the preform. A pair of rectangular blade areas G 10, 11 are integral with the edge 9 and a third edge 12 of the paral lelogrammatic central area 6. To assemble the flat fabric into the blade and base preform structure of

Figure 2, the blade areas 10 and 11 are folded along dotted lines 13, 14 downwards into the paper as shown in 5 Figure 3. This folding is continued until the planes of the areas 10 and 11 lie at right angles to the main plane of the central area 6.

The fabric is then folded upwardly along dotted lines 15, 16 so that the areas 10 and 11 come up through the plane of the paper and abut one another so that a face 17 showing on area 10 abuts a face 18 showing on area 11.

It w ll then be found that the preform shape shown in Figure 2 is produced and the angle 4 at which the blade portion 3 lies to edge 5 corresponds to the angle 7 so the blade portion 3 is skewed on the base portion at an angle of (90°-θ°).

It will be appreciated that the fabric shown in

Figure 3 would have to be rotated in the plane of the paper through 90° to permit it to be viewed as shown in Figure 2.

To hold the preform in its folded-up position, sice edges 19 and 20 would be sewn together as would side edges 21 and 22. If required, a stitch could be sewn through the base portion adjacent to the blade pcrtion to secure the folded-up position.

With the preform shown in Figure 2 being formed from the material of Figure 3, it will be clear that the junc¬ tions between the two blade areas lie along the leading and trailing edge cf the blade portion. In some cases it may be desirable to have each junction between the two blade areas away from the leading and trailing edges.

Referring to Figure 4, this shows a modified version of the fabric blank illustrated in Figure 3 in which tne edges of the aerofoil areas are net coincident in the folded preform with the leading and trailing edges of the blade portion.

In the fabric blank shown in Figure 4, a central area

106 corresponds to the area 6 of Figure 3. Other items shown in Figure 4 which are the same as those shown in

Figure 3 have the same reference numerals as used in

Figure 3, with the addition of 100.

In the fabric blank illustrated in Figure 4, the blade area 110 is displaced upward towards edge 140 of the portion 106. A slot 141 is provided along the line of the connection with the blade area 110 and the central area

106. Similarly a slot 142 is provided between blade area 111 and the central area 106. Thus edge portion 143 on the side of the central area 106 is longer than edge portion 144.

The fabric blank shown in Figure 4 is knitted so that the wales of the knitting lie parallel to the side edges of the areas 110 and 111 such as side edge 145. The preform blank shown in Figure 4 is of double jersey knit and a wale on one side of the fabric is omitted along line 146 to create a line along which a fold will naturally occur. Similarly a wale is omitted along line 147 in the area 111.

Figure 5 shows the missing wale on the line 147 in the double jersey knit blade area 111.

The double jersey structure is shown in Figure 5 as having been knitted on an upper bed of needles 148 and a lower bed of needles 149. For ease of understanding the structure, each needle is shown as a dot, although, of course, the knitted structure would not incorporate any needles. It can be seen that the structure is knitted with one needle, 150, out of operation on the uooer bed.

When the structure is then folded in the direction of arrow 151, it produces the structure shown in Figure 6, where the two fold-adjacent stitches are the stitches

152,153 on the unfolded structure.

Now, when the fabric blank of Figure 4 is fcldeo so that the faces 117 and 118 abut one another, the flaps 154 and 155 can be folded around so that the join between the two blade areas 110 and 111 is along lines 156,157 (see Figure 7) .

Thus the joins between the two blade areas no longer coincide with the leading and trailing edges of the blades and, furthermore, the amount of reinforcing material in the leadinq and trailing edges is reduced (because of the omission of the wales on lines 146 and 147) and therefore reinforcing material can be used and located directly into the region of the leading edge and/or trailing edge (it will be appreciated that this is the location of greatest thermal and physical shock to the blade).

If required, those parts of the fabric which rein¬ force the leading edge and/or trailing edge cf the com¬ posite blade can be tapered even more, as is shown in Figures 8 and 9. It can be seen from Figure 8 that two needles 158,159 are put out of action, so that on folding, the stitches 160,161,162, adopt the sharp edged structure shown in Figure 9. This means that the reinforcement material can be incorporated further into the leading, and particularly, the trailing edge of the composite blaoe.

For rotor blades, the base portion may be provided with an integral root or "fir tree" configuration suitable for fixing into complementary shaped grooves or recesses in a rotor hub of an engine.

By increasing the amount of material between tne fold lines 15, 16, in the blank of Figure 3, excess mate^rial can be provided so that it can be folded as shown in Figure 10. For reasons of ease of understanding of the concept of incorporation of an integral root, the blade is not shown skewed in Figure 10.

It can be seen that blade portion 164 in Figure 10 is formed of two layers 165 and 166 and base portion 167 has an upper planar layer 168 and re-entrant edge portions 169,170. These re-entrant portions are interconnected by a lower portion 171. If required more than one pair of re-entrant portions 169, 170 could be provided so as to form a "fir tree" shape as shown in Figure 11.

Referring to Figure 11 the fabric preform has a blade portion 172 and a base portion 173 and is folded from a fabric blank similar to that shown in Figure 4 but is provided with even more material between the fold lines 115 and 116 and with additional fold lines formed alterna¬ tively on each face of the fabric to produce folds which define the re-entrant portions 169, 170 to give the "fir- tree" shape of the preform shown in Figure 11.

In some cases rotor blades require integral snubbers to damp out vibration of the blade. Thus in Figures 12 the blade portion 172 with its integral base portion 173 is provided with a snubber zone 174 which defines a platform which in use co-operates with snubbers on ad¬ jacent blades to provide vibrational damping of the blades. The blank for the snubbered blades will be similar to that shown in Figure 3 but would be widened over the blade portions 10, 11 at the location of the snubbers and then folded to the shape shown in Figure 12.

Because it is easy to inlay material into a knitted structure, inlays 175,176 can be provided in the region of the snubbers 174.

If it is required, the structure shown in Figure 3 could be knitted with the knitting direction running from the edge 8 towards the upper edge of area 6, so that inlays could be inserted to reinforce the fold zones 13,14 and 15,16.

From the foregoing it will be noted that each fibrous preform has a substantially planar base portion formed of two layers of fabric and an upstandinq blade portion also formed of two layers of fabric.

The knitted preform may be manufactured into a composite structure.

In many gas turbine engines the stator vane assembly comprises a Dlurality of aerofoil shaped vanes bounded at their radially inner and outer ends by platforms which co¬ operate with the platforms of adjacent blades to cefine a generally annular air passage. The stator vanes may be mounted in the engine casings from their outer ends or their inner ends in a manner well known per se. Referring to Figure 13 there is shown a knitted fabric blank for making a Dreform shown in Figure 14. The blank of Figure 13 is similar to that shown in Figure 3, and where ap¬ propriate the same reference numerals will be used in Figure 13 as were used in Figure 3 with the addition of 200. The blank of Figure 13 has a base region 206 which folds about lines 215 (F, F), 216 (E, E) to make an inner platform and about lines 213 (G. G) and 214 (D, D) to make aerofoil portions 210, 211. The difference is tnat the preform of Figure 14 has an additional outer platform which is formed by folding fabric, forming ears £60. 251, about their respective blades 210, 211 along fold lines H, H and C, C and then back again along fold lines I. I and B. B to bring the edges A, J into abutment.

Referring to Figures 15 and 16 there is shown respectively a Knitted fabric blank for making a multivane nozzle guide vane (NGV) assembly for a turbine section of a gas turbine engine.

Referring to Figure 16 the fold lines of the NGV assembly preform are identified by the letters B tc T and these fold lines are indicated on the fabric blank shown in Figure 15. The fabric is knitted from silicon carbide fibres or yarns as a double jersey knitted structure. Selected courses of selected faces of the fabric are omitted to define 90° or 180° fold lines B, B to T, T as the case may be. The fabric blank of Figure 15 is folded to make the preform of Figure 16 and a matrix of silicon carbide is then built-uo on the folded-uo blank using a chemical vapour infiltration and deposition technique.

The chemical vaDour deoosition techmoues referred to above would build up silicon carbide and replicate the shape of the preform, but would require the final shape of the aerofoil surfaces and platforms to be machined. This could be achieved, for example by electro-chemical machin- ing (ECM) using pairs of electrodes which have machining surfaces shaped to form the aerofoil surfaces. One such method of ECM machining and design of electrodes which may be suitable is described in GB-B-2 ,021 , 645.

An alternative method of manufacture would be to knit the fabric blank of Figure 15 from carbon fibres and threads, injection mould the folded-up preform of Figure 16 with a carbonisable medium (e.g. phenolic resin), and then carbonise the medium to form an accurately sized composite component. Successive impregnation and carbonis- ing may be carried out to increase the density of tne matrix and if desired this may be followed by chemically vapour depositing silicon carbide on all the exposed surfaces of the composite.

Claims

1. A fabric preform for reinforcing a composite structure gas turbine engine blade as hereinbefore defined characterised in that the gas turbine blade has a substan- tially Planar base portion and an upstanding blade por¬ tion, the preform for the composite blade having at least two layers of fabric over substantially the whole of the base portion and the blade portion.

2. A preform as claimed in claim 1, characterised in that the blade has a platform at each end of a blade portion, the preform being created from a folded-uo fabric blank having regions defining the platforms and other regions defining the blade portion.

3. A fabric preform as claimed in claim 1 further comprising an integral root portion formed by shaDing the fabric.

4. A preform as claimed in claim 1. in which the fabric is knitted.

5. A preform as claimed in claim 4, in which the fabric is double jersey knitted.

6. A preform as claimed in claim 1, in which the base portion is substantially quadrilateral in plan and the blade portion is skewed relative to the base portion so that, in plan, the blade does not lie parallel to one or other of the edges of the base portion.

7. A preform as claimed in claim 1. in which the blade portion is hollow so as to have the two layers of the blade portion separated by a gap.

8. A preform as claimed in claim 1, folded uo from a fabric blank which in the flat state has a central base area and a pair of blade areas on opDosite sides of the base area.

9. A preform as claimed in claim 8, in which the central base area of the blank is a parallelogram with the acute angle θ° of the parallelogram being such that (90°-θ° ) is equal to the angle at which the blade portion is skewed on the base portion.

10. A preform as claimed in claim 9, in which the connection between each of the two blade areas and the central area of the blank is discontinuous at one side so as to form a slot between the blade area and the base area, the slot being parallel to the connection and extending part way along the line of the connection, the slot at the connection of one blade area being diagonally opposite the slot at the connection of the other blade area, the blade areas being oppositely displaced along the edges of the central area so that the blade areas are foldable along the length of the blade area portions and the edge of one blade area which is closest to a fold abuts the edge of the other blade area which is furthest from a fold.

11. A preform as claimed in claim 1 , in which reinforcing inlays are provided in the knitted materials.

12. A reinforced composite incorporating a preform as claimed in claim 1.

13. A preform for a multi-blade assembly for a gas turbine engine comprising a plurality of blade portions interconnected by platforms at each end of the blade portions, the preform being created from a fabric blank having regions defining the blade portions and other regions defining the platforms and which is folded to define the preform shape.