222 Multilayer or 3D Weaving
The first major difference between conventional weaving and multilayer weaving is the requirement to have multiple layers of warp yarns. The greater the number of layers required (and thus the thickness of the preform) or the wider the fabric produced, means a larger number of individual warp yarns that have to be fed into the loom and controlled during the lifting sequence. Therefore the source of the warp yarn for multilayer weaving is generally from large creels in which each warp yarn comes from its own individual yarn package. Multiple warp beam systems have also been used although this is not as common. This requirement for a large number of warp ends raises the first disadvantage of weaving, namely that the cost of obtaining (generally) thousands of yarns packages and the time required to set up the large number of warp ends within the loom can be extremely expensive. This non-recurring cost becomes less significant as the length of the fabric being woven increases but having to weave large volumes of the same material restricts the flexibility of the process. Most multilayer weaving is therefore currently used to produce relatively narrow width products, where the number of warp ends is relatively small, or high value products where the cost of the preform production is acceptable.
As most 3D composites are produced from high performance yarns (carbon, glass, ceramic, etc) standard textile tensioning rollers are unsuitable and tension control on the individual yarns during the weaving is critical in obtaining a consistent preform quality. This is generally accomplished through spring-loaded or frictional tension devices on
Figure 2.3 Multilayer weaving loom (courtesy of the Cooperative Research Centre for Advanced Composite Structures, Ltd)
the creel or through hanging small weights on the yarns before entering the lifting device. Figure 2.3 illustrates the use of multiple warp beams and hanging weights in multilayer weaving. The lifting mechanisms are the same as used in conventional weaving although the heddle eyes through which the yarn passes tend to be smoothed and rounded to minimise friction with the more brittle high performance fibres. Jacquard lifting mechanisms tend to be used more frequently as their greater control over individual warp yarns offers more flexibility in the weave patterns produced. The weft insertion is accomplished with standard technology (generally a rapier mechanism) inserting individual wefts between the selected warp layers. Variations in the lifting and weft insertion mechanisms to allow multiple sheds to be formed and thus multiple simultaneous weft insertions have also been developed and would allow a faster preform production rate. This type of technology is often regarded as the true 3D weaving.
Figure 2.3 Multilayer weaving loom (courtesy of the Cooperative Research Centre for Advanced Composite Structures, Ltd)
It is through the design of the lifting pattern that the three-dimensional nature of the weave architecture is produced in multilayer weaving. Commonly the bulk of the warp and weft yarns are designed to lay straight within the preform and thus maximise the mechanical performance. In order to bind the preform together, selected warp yarns, coming from a separate beam if warp beams are used, are lifted and dropped so that their path travels in the thickness direction thus binding the layers together (Figure 2.4).
Figure 2.4 Illustration of multilayer weaving
Figure 2.5 Typical multilayer yarn architectures
Such a multilayer weaving loom is described by Yamamoto et al (1995). Examples of such weave architectures that are currently capable of being manufactured using multilayer weaving are illustrated in Figure 2.5. It should be noted that the illustrations in Figure 2.5 show idealised architectures and often these can be very different from the resultant preform architecture (Bannister et al 1998). Tension within and friction between the yarns, together with the initial weave parameters (yarn size and twist, yarn spacing, number of layers, etc) can all affect the final architecture and thus the composite performance. As with conventional weaving, multilayer weaving is only capable of producing fabrics with 0° and 90° in-plane yarns, although the binder yarns can be oriented at an angle. This tends to limit the use of these preforms as their shear and torsional properties can be relatively low. Various 3D weaving techniques can produce preforms with yarns at other angles although this requires the use of highly specialised equipment, which will be discussed later.
Figure 2.4 Illustration of multilayer weaving
Figure 2.5 Typical multilayer yarn architectures
Flat, multilayer fabrics are not the only structures that can be woven on standard looms. By correctly programming the sequence in which the warp yarns are lifted it is possible to weave a fabric with slits that can be opened out to form a complex three-dimensional structure. This concept is illustrated in Figure 2.6, which demonstrates how I-beams and box structures can be formed from, initially, flat fabric. An example of such an integrally woven I-beam is shown in Figure 2.7 and these types of components have already been used in the civil engineering field (Miiller et al., 1994). A reasonable range of shaped products can be formed in such a way however more advanced forms of 3D weaving are capable of producing more complex preforms.
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