GB2066308A - Three-dimensional woven structure - Google Patents

Abstract

A multiply woven fabric of high tensile fibres such as carbon fibres or glass fibres, in which fibres lie in three, preferably orthogonal, directions to provide strength in each direction has weft yarns 21 and warp yarns 22 normal to the weft direction, the warp yarns including portions 23 rising from the lower surface 220 to the upper surface 120 of the structure and portions 24 falling from the upper surface to the lower, the weft yarns being inserted between pairs of rising and falling warp yarns. <IMAGE>

Description

SPECIFICATION Three-dimensional woven structure The present invention relates to three-dimensional woven structures and in particular to such structures of high tensile fibres such as carbon fibres or glass fibres.

Hitherto high tensile fibres such as carbon or glass fibres have found most general application in one and two dimensional compos ites. Pultruded sections of fibre combined with resin binding have been produced in various shapes in which all the fibres are aligned axially in the section. These materials are known for their characteristically high strength to weight and stiffness to weight ratio in the fibre axis.

Lay-ups of resin and fibre layed in two dimensions are also widely used. The fibres with resin binding are for example layed in different directions in plane sheets: or alternatively are wound progressively onto a former to generate pressure vessels. Two dimensional sheets of woven fabric composites are also produced. In each of these cases after laying up the high tensile fibres and resins, the material is heated to cure the resin, and the characteristically high structural properties of the resulting composite material derive from the properties of the fibre raw material. Three dimensionally interwoven blocks of fibre have also been assembled, generally by a labour intensive process which makes the blocks very expensive to manufacture.

Although fibre composites have excellent structural properties in the direction of the fibre axes, the composite material frequently has a tendency to split or fibrillate in planes parallel to the fibres. The failure generally results from shear stresses or or mechanical impact and is a consequence of the low resin strength compared with that of the fibres.

Fibrillation is a common problem of one and two dimensional fibre composites, but as will be shown is less serious in the case of three dimensional woven fibre composites, because in this form composites have strength in each axis.

The present invention consists in a threedimensional woven structure having an upper surface and a lower surface and comprising multiple layers of weft yarns extending generwally parallel to the upper and lower surfaces and sets of warp yarns arranged in successive planes normal to the weft direction, the warp yarns in each plane including yarns rising from the lower surface to the upper surface and yarns falling from the upper surface to the lower surface of the structure, each yarn alternately rising and falling, each weft yarn being inserted in a space between pairs of rising and pairs of falling yarns.

Suitably, a weft yarn is inserted between every pair of rising and falling yarns.

The rising and falling yarns provide fibres orientated in two directions normal to the weft direction. The rising yarns are preferably substantially perpendicular to the falling yarns, in which case the woven structure has fibres arranged in three mutually orthogonal directions.

Since the fibres in two directions are provided by the rising and falling yarns, the structure can be woven on a loom, with the same warp yarns forming the rising and falling yarns. Thus the invention also includes a method of making a woven structure as defined above in which the structure is woven in a loom by inserting weft yarns in sheds formed by raising and lowering selected warp yarns in sections normal to the weft direction.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a scrap section through a three dimensional woven composite, taken in the X-Z plane, Figure 2 is a scrap section similar to Fig. 1 but in the X-Y plane, Figure 3 illustrates the relationship between tensile strength of the woven composite in the directions of the X, Y and Z axes and the proportion of the fibre present in each direction, Figure 4 shows the form of three dimensional fabric in accordance with the invention, Figure 5 is a diagrammatic illustration of a loom on which the fabric of the invention can be made, and Figures 6(a) and 6(b) show two stages in the weaving of fabric on the loom of Fig. 5.

The three dimensional woven fabric is made of yarns of high tensile fibres, interwoven in three directions at right angles. As shown in Figs. 1 and 2, the fibres even when packed or beaten up tightly do not fill the whole of the composite volume, but resin is introduced to fill the inter fibre spaces. In the fabric shown in Figs. 1 and 2, yarns 2 extending in the Z direction are shown with a larger cross-section than the orthogonal yarns 3 and 4 extending in the X and Y directions respectively. As a consequence the composite fabric strength in the Z direction arising from yarns 2 is greater than that in the orthogonal directions.

The relationship between tensile strength of the fabric in each direction to the proportions of the fibres packed into each of the axes of the three dimensional fabric is illustrated ih Fig. 3. The vertical axis represents the variable n, where the yarns in the X and Y directions both have equal cross-sectional dimensions 1/n of the woven cell dimension.

The horizontal axis shows the proportion of the X, Y or Z yarns in each direction for various values of n.

Thus when n = 1 all of the space is occupied by X, Y yarns and none remains for a Z yarn, which is the case for a two dimen sional lay-up. When n = 2 however the yarns X. Y and Z are equal in size, and the tensile strength in each direction is 1/4 the strength of the pultruded section of the same fibre. Since carbon fibre has about ten times the specific strength of steel (i.e. strength to density ratio) such a material has a specific strength about 2.5 times that of steel in each direction.

Generally, by selecting the count of the yarns in each direction, the three dimensional woven fabric properties can be tailored to contain most of the strength in a preferred direction, while retaining sufficient working strength in the other two axes. For n = 3.7 for example the specific strength relative to steel is about 5 in the Z direction and 1 in each of the X and Y directions, while for higher values of n a greater specific strength in the Z direction is possible at the expense of further weakness in the X and Y axes.

The form of the three dimensional woven fabric of the present invention is shown diagrammatically in Fig. 4 in a section normal to the Z axis. The fabric 20 consists of weft yarns 21 extending in the Z-direction generally parallel to the upper and lower surfaces 1 20 and 220 of the fabric, and warp yarns 22. Each warp yarn 22 consists of alternating rising yarn portions 23 and falling portions 24, extending through the full thickness of the fabric 20 between the upper surface 1 20 and lower surface 220, the rising and falling yarn portions being at right angles to one another.Each warp yarn 22 extends in a section generally normal to the weft direction (i.e. to the Z axis) and each is displaced in the direction normal to the Z-axis relative to the next adjacent warp yarn, so that adjacent rising yarn portions 23 in successive warp yarns are separated by a plurality of weft yarns 21, and each weft yarn is inserted between rising and falling warp yarn portions.

The rising portions 23 and falling portions 24 of the warp yarns make up the X axis and Y axis yarns of the three dimensional fabric.

After the fabric 20 has been formed, it is interspersed with a suitable resin, such as an epoxy resin, and cured. Sections of the three dimensional woven composite thus formed, such as the section AA shown in Fig. 4, can then be cut to form bar stock of the desired section and of length equal to the width of the loom on which the fabric is woven. If desired, the woven fabric can be preformed before the resin is cured. for example by bending the fabric into a curved shape.

The fabric 20 can be manufactured progressively on a loom which may be formed by adapting well known types of weaving loom.

A suitable form of loom is shown diagrammatically in Fig. 5. The loom comprises a warp yarn supply mechanism 30 from which warp yarn 22 are fed to a heddle device 27, a takeup device 31 for drawing the woven fabric 20 from the loom, a reed, comb or other suitable beating-up mechanism 32, and a weft yarn insertion device 33. For the sake of clarity, the loom shown is for weaving a fabric with only four warp yarns in each warp section.

The heddle device 37 shown in Fig. 5 comprises rotating plates such as those used in a tablet loom, each holding N warp yarns in each warp section, where N = 4 in the case shown in the drawings. The rotation of the plates by 7r/4 or 45 after each weft insertion causes the warp yarns to rise and fall in the block of woven fibres. In the position shown in Fig. 6(a) one weft yarn is inserted in to the path 38 between warp yarns 222 and 223. After rotation by 7T/4 in the direction of the arrow the rotating plates 37 move to the position of Fig. 6(b), and in this position two weft yarns can be inserted into paths 38' and 38" between warp yarns 221 and 222 and yarns 223 and 224 respectively. Between each insertion, beat-up is effected by the reed or comb 32 as is normal in weaving looms.

The plates 37 continue to rotate and weft yarns are inserted alternately along the path 38 and the two paths 38' and 38". It will be apparent that each warp yarn is alternately raised once and then lowered once as the plates 37 rotate through 360o, so that the yarn alternately rises and falls through the fabric. With the use of rotating plates 37, the warp yarns associated with each plate also twist around each other in the warp section, but this would not be the case with a loom using heddles to move the warp yarns.

The weft insertion device 33 may take any suitable form, such as a rapier. Suitable mechanism may be provided to return some or all of the weft yarns at the fabric edges to form a selvedge.

To make a fabric such as that shown in Fig.

4 which is substantially deeper than the fabric in Figs. 6(a) and 6(b), rotating plates 37 having more holes in their periphery may be employed. To make the fabric illustrated in Fig. 4, N = 1 2 warp yarns in each warp section are required, so that each plate 37 requires 1 2 holes, the plates being rotated by sir/12 between weft insertions. In this case, the weft insertion device must be adapted two insert yarns alternately in five and six insertion paths, rather than in one and two paths as shown in Fig. 6. However, with such a deep fabric it may be preferable to use a heddle'- loom having in this case 24 shafts. A heddle loom enables a clearance for the weft path to be more easily formed in the warp system than do the rotating plates of a tablet loom.

It will be apparent that the fabric can be made in a desired thickness by selecting the number of weft yarns which are spaced through the fabric in a section normal to the warp direction, by selecting the number of weft insertion paths in the loom, and providing the correspondingly appropriate number of warp yarns in each section. In general there will be more than two layers of weft yarns in the fabric.

The various mechanisms of the loom shown diagrammatically in Fig. 5 can take various forms well known to those skilled in the art, and they are therefore not described further.

Claims (13)

1. A three-dimensional woven structure having an upper surface and a lower surface and comprising multiple layers of weft yarns extending generally parallel to the upper and lower surfaces and sets of warp yarns ar ranged in successive planes normal to the weft direction, the warp yarns in each plane including yarns rising from the lower surface to the upper surface and yarns falling from the upper surface to the lower surface of the structure, each yarn alternately rising and fall ing, each weft yarn being inserted in a space between pairs of rising and pairs of falling warp yarns.

2. A three-dimensional woven structure as claimed in claim 1, in which a weft yarn is inserted between every pair of rising and falling yarns.

3. A three-dimensional woven structure as claimed in claim 1 or claim 2, in which the rising yarns are substantially perpendicular to the falling yarns.

4. A three-dimensional woven structure as claimed in any preceding claim, in which all the warp yarns alternately rise and fall in their path through the structure.

5. A three-dimensional woven structure as claimed in any preceding claim, in which the rising warp yarns and the falling warp yarns are at a substantially equal angle to the upper and lower surfaces.

6. A three-dimensionai woven structure as claimed in any preceding claim, in which some at least of the weft yarns are returned at each edge of the structure to form a selvedge.

7. A three-dimensional woven structure as claimed in any preceding claim, in which the relative counts of the weft and warp yarns are selected in accordance with the relative strength requirements of the structure in the weft direction and in directions normal to the weft direction.

8. A three-dimensional woven structure as claimed in any preceding claim, in which the 'structure is filled with resin and cured.

9. A three-dimensional woven structure as claimed in claim 8, in which the structure has been preformed before curing.

1 0. A three-dimensional woven structure as claimed in claim 8, in which the woven and cured structure is cut into sections of predetermined dimensions.

11. An article of manufacture formed from three-dimensional woven structure as claimed in any preceding claim.

12. A method of making a three-dimensional woven structure as claimed in any one of claims 1 to 10, in which the structure is woven in a loom by inserting weft yarns in sheds formed by raising or lowering warp yarns in sections normal to the weft direction.

13. A method as claimed in claim 10, in which each warp yarn is progressively raised after each weft insertion to form the rising warp yarn and is then progressively lowered after each weft insertion to form the falling warp yarns.

1 4. A three-dimensional woven structure substantially as described with reference to, and as shown in, the accompanying drawings.

1 5. A method of making a three-dimensional woven structure substantially as de scribe.