Technical Field

This invention .relates to reinforcements ana has particular reference to reinforcements for comDosite structures.

Composite structures are well known. Typical com¬ posite structures include fibre-reinforced tubular resin structures. The fibres provide strength to the resin anc the resin serves to transfer stress into the fibres.

There are often Droblems associated with laying uc such comDosite structures. A typical structure would be oroαuced by forming a mould, laying onto the mould a mat of fibres and then impregnating the mat wit a suitable curable resin. Eecause of the need to ensure that tne fibres are uniformly distributed throughout the resin matrix, in more recent years efforts have been concentrateα on forming preforms which can be imDregnated with resin to enable a reinforced structure to be anufactureα. Such preforms enable the reinforcing material tc be located at just the right point in the comoosite and avoid crcblems with the moving cf fibres during the imDregnation stage.

The Dresent invention is Darticularly concerned with fibre-reinforced composites of a tubular shaDe and preforms for such composites. Heretofore, such comDosites have been manufactured by wrapping the fibres around a suitable mandrel and imoregnating the wrapped structure.

Summary of the Invention

By the present invention there is provided a reinfor¬ cement preform for an elongate tubular composite structure which has a different cross-sectional area bore at dif¬ ferent regions along its length, characterised in that the Dreform is formed by knitting an elongate tubular fabric and formino the different cross-sectional areas of the bore

by selectively varying the knitted structure at selected regions along the length of the knitted fabric.

The preform may be of tubular knitted .jersey construc¬ tion.

The preform may be formed by knitting a tubular fabric on a two. bed knitting machine. A change in cross-sectional area may be made by decreasing at least one stitch from one or more.courses of the knitting. The stitch or stitches may be transferred sideways on the two bed knitting machine. Preferably stitches are decreased in Dairs one at

- each of two opposed positions in selected courses to form an even .structure. The opDosed positions may be at the edges of the tubular structure as it is knitted on the two bed machine. Further stitches may be decreased from selected courses ana preferably, to keep tne Droduct symmetrical, the stitches are decreased two at a time at ODDOsed positions around the tubular structure. Further, preferably the Dairs of stitches are decreased evenly from around the tubular structure.

Additionally or alternatively, the stitch length may be reduced to form a region of cifferent cross-secticnal area. The yarn tex may be lowered to change the area of the bore. The yarn tex and the stitch length may be reduced simultaneously to change tne bore (e.g. to form a constriction).

The fibre may be a high modulus fibre such as glass fibre. carbon fibre, silicone carbide fibre, a polyamide fibre such as "Kevlar" (Registered Trade Mark) or a poly¬ ester or nylon fibre.

The resin may be any suitable resin material. The knitted preform may be provided with reinforcing yarns, the reinforcing yarns may be of the same material as the knitted yarn or may be of different material.

The tubular structure may be a simDle tube or may be an L-shape or may be a T- or Y- or K-shaDe.

The Dresent invention also provides a composite tubular structure comprising a matrix reinforced with a preform of the type set out above.

Brief Description of Drawings

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

. Figure 1 is a perspective view of a simDle reinforce¬ ment preform in accordance with the present invention,

Figure 2 is a persDective view of a tubular coupler in accordance with the Dresent invention,

Figures 3A and 3B are views of end regions of com- Donentε to be connected together via the couDler of Fioure

Figure 4 is a tubular couDler aαaσted to nterengage two comDonents of different diameters.

Figure 5 is a T-shaped reinforcement preform,

Figure 6 is a plan view of a flattened tubular rein¬ forcement preform incorporating three different knitted systems for localised reduction in cross-sectional area of the bore of the preform,

Figures 7A to 7J show stages in the knitting of a localised tubular constriction, and

Figures 8 and 9 show a fur-ther embodiment of reinfor¬ cement preform according to the invention, shown ready for

impregnating in Figure 8 and flattened in Figure 9.

Description of Preferred Embodiments

Referring to Figure 1, this shows a tubular preform 1 having a constriction 2 with a larger diameter portion on either side, shown as 3 and 4. The preform is shown ready to be impregnated with a suitable resin, such as a poly¬ ester resin, to form a resin-reinforced composite struc¬ ture. A typical material for the manufacture of the prefor 1 would be a yarn of carbon fibre or silicon carbide fibre or glass fibre. The preform 1 can be knitted with the inbuilt constriction 2 and this means that the - preform can readily adopt the shape shown in Figure 1 when ready for -resin impregnation.

As will be explained below, the preform 1 would normally be knitted on an inverted V-bed type flat knitting machine and would therefore be knitted as a flat tube which can be readily distorted into a tubular shaDe with an inbuilt constriction.

A typical use of a tubular member with an inouilt constriction is as a coupler and this is illustrated in Figures 2, 3A and 3B.

In Figure 2, a tubular memoer 5 has a central con¬ striction 6 and a pair of constructions 7. 8 adjacent to either end of the member 5. The constriction 6 has a greater restrictive effect than the constrictions 7 and 8. The connector shown in Figure 2 will readily interconnect two ends of the tubes 10 and 12 shown in Figures 3A and 3B. The tube end shown in Figure 3A has a further constricted portion 9 in the tube 10. When the tube 10 is pushed into one end of the coupler 5 from the left (as shown), the coupler deforms slightly to permit the tube 10 to enter and pass through the constriction 7 until the end 11 of the tube 10 engages the constriction 6. The position of the constriction 9 on the tube 10 is so arranged that in this

relative position of the tube 10 in the coupler 5, the constriction 9 coincides with the constriction 7 thereby holding the tube 10 firmly in position in the coupler 5. Thus the tube 10 shown in Figure 3A is a snap-fit into one end of the coupler 5.

Similarly, tube 12 with its constriction 13 is pushed into the coupler 5 from the right (as shown) so that the* end 14 engages the constriction 6, and the constriction 13 then coincides with the constriction 8. It will be appreciated that in this position the two tubes 10 and 12 will be firmly -held together by the coupler 5.

To mterengage tubes of different diameters, a knitted structure 15 as shown in Figure 4 can be produced, in which the structure 15 has a larger diameter end portion 16 and a smaller diameter end portion 17. Again, end pcrtion 16 is provided with a constriction 18 ana end portion 17 is provided with a constriction 19.

A preform 20 of T-shaoe can be produced as is shown in Figure 5. The preform 20 has two ends 21. 22 each provided with a constriction (respectively 23 and 24) and one plane end 25. Other forms of couplers such as L-shaced couplers, Y-shaped couplers and K-shaped couplers can be produced as reαuired. One or more of the tubular ends can be provided with regions of reduced bore cross-section as required.

By knitting the reinforcement preform of, for example, glass fibre, this means that the preform will exactly conform to the shape of the tubular composite member to be formed from the preform. Internal mandrels may be provided to" hold the preform to shape during the resin-impregnating stage. In the case of simple- shapes such as the shape shown in Figure 1, the mandrels may be designed with z. break line along the constrictive portion 2. With more complex shapes, such as those shown in Figure 2, multi-part mandrels may be required or collapsing mandrels can be

used

The knitting is preferably carried out on an inverted V-bed type flat knitting machine having inclined flat beds. However the knitting could be carried cut on a dial and cylinder cylindrical knitting machine using the lower tex yarn or reduced stitch length techniques to form the constriction.

A preferred method of knitting the tubular structure is to knit a tubular jersey structure without interconnec- tion between the two sides. The change in cross-section of the bore may be provided by shortening the length of the stitches .to compress the stitches and. form a reduction in the bore of the tube knitted on the machine. This can be done by altering the throw of the cam which moves the needles. Alternatively, or in addition, a lower tex yarn can be used which again forms a tighter stitch and forms an inbuilt change in cross-section of the bore of the tubular fabric. If required, shorter stitch length can be combined with a lower tex yarn.

A further alternative method of knitting-in a change in bore area is to decrease and transfer stitches on the bed.

Figure 6 shows a sample tubular material in a flat condition in which the three methods of producing a con- striction in bore area are illustrated. The tubular member 26 is knitted in the direction of arrow 27. At 28 is shown the effect of knitting with a shorter stitch by reducing the needle cam throw and using a lower tex yarn. The ends of the threads used are shown to the right of the tubular member with the reference number 51. Reference 29 shows the effect of knitting with a shorter stitch length only and it can be seen that the constrictive effect is less than the combination of shorter stitch length and lower tex yarn.

Reference 30 shows the effect of fabric narrowing by reducing the number of stitches as the material is knitteα.

Figures 7A to 7G show the way in which a fabric narrowing is knitted on the two bed machine.

Figure 7A is a stitch diagram showing tubular jersey knitting being carried out on a normal diameter portion of the tubular structure. The knitting is carried out in the direction of arrows 32, 31 on the upper and lower beds of the machine. It can be seen that all of the needles are in operation.

Figure 7B shows the first of a four needle narrowing of the fabric by transferring some of the loops sideways so as to produce double loops on needles 33, 34, 25, 36 thereby decreasing stitches from needles 37, 38, 39 and 40. It will be appreciated that the loop on needle 37 has been transferred onto needle 33 and the four loops to the right of needle 34 have all been transferred to the left by one needle to form the empty needle 38 and a double looo on needle 34. Similarly the empty needles 40 and 29 are produced by a similar action on the lower beα.

A further course of the tubular structure is then knitted as a narrowed tube as shown in Figure 7C and a further, or second, narrowing is produced by the same process of transfer of loops to produce the condition shown in Figure 7D. It can be seen in this Figure that the four needles 37, 38, 39 and 40 are still out of operation but additionally needles 41, 42, 43 and 44 are also removed from the knitting.

Again the narrow tubular structure is knitted as shown in Figure 7E. This therefore produces the constriction or narrowing of the tubular reinforcement preform.

To wrden out the fabric again, the loops on needles 45, 46 and.47 are transferred by one needle to the right and the loops on needles 48, 49 and 50 are transferred by one needle to the left. ^' The selvedge needle 43 is then raised and the yarn is then again wrapped around the needle 43 as the knitting proceeds. The raising of the selvedge needle 43 thus widens out the tube again. The same se¬ quence occurs with needle 41. The centre is filled as additional yarn wraps around needles 45 and 48. and stitches are formed during the next course.. The tubular fabric is then knitted as shown in Figure 7G at a slightly wider diameter.

The .widening is then repeated a second time as shown in Figure 7H and finally the original diameter is knitted on as shown in Figure 7J.

The nine figures of Figure 7 show the formation of a constriction but it will be appreciated the courses 7B, 7C, 7D and 7E alone can be used as many times as required to obtain a desired narrowing of bore cross-sectional area and the courses shown at 7E to 7J alone can be used as many times as reαuired to obtain a desired widening cf the tubular fabric.

Figure 8 shows a tubular composite 50 adapted to reinforce a cylinder with integral hollow flanges. The cylinder (or shaft) is shown at 51, 54, one end flange at 52 and one mid-shaft flange at 53. The knitted reinforce¬ ment is tubular so it can be used to reinforce a tubular hollow member of the shape shown in Figure 8 or as the skin-re nforcement of a solid structure of the shaoe shown.

Figure 9 shows the knitted reinforcement 60 that erects into the composite 50 of Figure 8 but in a flat¬ tened condition. Moving from left to right in Figure 9, the first tubular region 54A is knitted (i.e. as at 7E) to have constant circumferential length until the line 61 is

reached. At this point, the leaαing edge 53L of flange 53 is knitted by increasing the number of stitches (e.g. following repeated seαuences according to Figures 7F to 7J ) until the circumferential length of the tubular knitting is appropriate for the cylindrical portion of the flange 53. This point is marked by line 62 in Figure 9 and marks the commencement of a new phase of knitting of constant circum¬ ferential length (region 53C).

When a sufficient length of knitting for the axial extent of flange 53 has been provided (line 63 in Figure

9) decreasing commences as the trailing edge 53T is knitted to take the circumference of knitting back down to that

_; reαuιred for the shaft 51 (commenced at line 64). Between lines 64 anσ 65 in Figure 9, a region 51A of constant circumferential length knitting occurs and this is followed by a region 52L of increasing circumferential length (which will form the leading annular end of the flange 52) and a final region 52C of the reinforcement 60 which is knitted at the circumferential length appropriate to the cylindπ- cal face of the flange 52.

In Figures 7B, 7D, 7F and 7H the stitches are in¬ creased or decreased (as the case may be) at four positions around the tubular fabric. This effectively oroαuces radially extending lines where the knitted fabric is decreased (or increased). If desired, one may decrease (or increase) stitches at different needles on each course or on selected courses so that the taoering effect of the change in area of the bore is more widely spread around the circumference of the tubular fabric.

It will be appreciated that in any constriction formed by stitch decreasing, the stitches can be decreased to such an extent that the constriction can fully blank off the tube.