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Complete Guide to Composites, Part 4

Forget chopped mat...

courtesy of SP Composites

Click on pics to view larger images

At a glance...

  • Unidirectional fabrics
  • Stitched fabrics
  • Multiaxial fabrics
  • Chopped mat
  • Tissue
  • Braid
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In composite terms, a fabric is defined as a manufactured assembly of long fibres of carbon, aramid or glass, or a combination of these, to produce a flat sheet of one or more layers of fibres. These layers are held together either by mechanical interlocking of the fibres themselves or with a secondary material to bind these fibres together and hold them in place, giving the assembly sufficient integrity to be handled.

Fabric types are categorised by the orientation of the fibres used, and by the various construction methods used to hold the fibres together. The four main fibre orientation categories are: Unidirectional, 0/90 degrees, Multiaxial, and Other/random.

Unidirectional Fabrics

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A unidirectional (UD) fabric is one in which the majority of fibres run in one direction only. A small amount of fibre or other material may run in other directions with the main intention being to hold the primary fibres in position, although the other fibres may also offer some structural properties. While some weavers of 0/90 degree fabrics term a fabric with only 75% of its weight in one direction as a unidirectional, at SP Systems the unidirectional designation applies only to those fabrics with more than 90% of the fibre weight in one direction. Unidirectionals usually have their primary fibres in the 0 degrees direction (along the roll - a warp UD) but can also have them at 90 degrees to the roll length (a weft UD).

True unidirectional fabrics offer the ability to place fibre in the component exactly where it is required, and in the optimum quantity (no more or less than required). As well as this, UD fibres are straight and uncrimped. This results in the highest possible fibre properties from a fabric in composite component construction. For mechanical properties, unidirectional fabrics can only be improved on by prepreg unidirectional tape, where there is no secondary material at all holding the unidirectional fibres in place. In these prepreg products only the resin system holds the fibres in place.

Unidirectional Construction

There are various methods of maintaining the primary fibres in position in a unidirectional including weaving, stitching, and bonding. As with other fabrics, the surface quality of a unidirectional fabric is determined by two main factors: the combination of tex and thread count of the primary fibre, and the amount and type of the secondary fibre. The drape, surface smoothness and stability of a fabric are controlled primarily by the construction style, while the area weight, porosity and (to a lesser degree) wet out are determined by selecting the appropriate combination of fibre tex and numbers of fibres per cm.

Warp or weft unidirectionals can be made by the stitching process. However, in order to gain adequate stability, it is usually necessary to add a mat or tissue to the face of the fabric. Therefore, together with the stitching thread required to assemble the fibres, there is a relatively large amount of secondary, parasitic material in this type of UD fabric, which tends to reduce the laminate properties. Furthermore the high cost of set up of the 0 degrees layer of a stitching line and the relatively slow speed of production means that these fabrics can be relatively expensive.

0/90 degree Fabrics

For applications where more than one fibre orientation is required, a fabric combining 0 degree and 90 degree fibre orientations is useful. The majority of these are woven products. 0/90 degree can be produced by stitching rather than a weaving process and a description of this stitching technology is given below under Multiaxial Fabrics.

Woven Fabrics

Woven fabrics are produced by the interlacing of warp (0 degree) fibres and weft (90 degree) fibres in a regular pattern or weave style. The fabric's integrity is maintained by the mechanical interlocking of the fibres. Drape (the ability of a fabric to conform to a complex surface), surface smoothness and stability of a fabric are controlled primarily by the weave style. The area weight, porosity and (to a lesser degree) wet out are determined by selecting the correct combination of fibre tex and the number of fibres/cm. The following is a description of some of the more commonly found weave styles:

  • Plain

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    Each warp fibre passes alternately under and over each weft fibre. The fabric is symmetrical, with good stability and reasonable porosity. However, it is the most difficult of the weaves to drape, and the high level of fibre crimp imparts relatively low mechanical properties compared with the other weave styles. With large fibres (high tex) this weave style gives excessive crimp and therefore it tends not to be used for very heavy fabrics.

  • Twill

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    One or more warp fibres alternately weave over and under two or more weft fibres in a regular repeated manner. This produces the visual effect of a straight or broken diagonal to the fabric. Superior wet out and drape is seen in the twill weave over the plain weave with only a small reduction in stability. With reduced crimp, the fabric also has a smoother surface and slightly higher mechanical properties.

  • Satin

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    Satin weaves are fundamentally twill weaves modified to produce fewer intersections of warp and weft. The harness number used in the designation (typically 4, 5 and 8) is the total number of fibres crossed and passed under, before the fibre repeats the pattern. A crowsfoot weave is a form of satin weave with a different stagger in the repeat pattern. Satin weaves are very flat, have good wet out and a high degree of drape. The low crimp gives good mechanical properties. Satin weaves allow fibres to be woven in the closest proximity and can produce fabrics with a close tight weave. However, the style's low stability and asymmetry needs to be considered. The asymmetry causes one face of the fabric to have fibre running predominantly in the warp direction while the other face has fibres running predominantly in the weft direction. Care must be taken in assembling multiple layers of these fabrics to ensure that stresses are not built into the component through this asymmetric effect.

  • Basket

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    Basket weave is fundamentally the same as plain weave except that two or more warp fibres alternately interlace with two or more weft fibres. An arrangement of two warps crossing two wefts is designated 2x2 basket, but the arrangement of fibre need not be symmetrical. Therefore it is possible to have 8x2, 5x4, etc. Basket weave is flatter, and, through less crimp, stronger than a plain weave, but less stable. It must be used on heavy weight fabrics made with thick (high tex) fibres to avoid excessive crimping.

  • Leno

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    Leno weave improves the stability in open fabrics which have a low fibre count. A form of plain weave in which adjacent warp fibres are twisted around consecutive weft fibres to form a spiral pair, effectively locking each weft in place. Fabrics in leno weave are normally used in conjunction with other weave styles because if used alone their openness could not produce an effective composite component.

  • Mock Leno

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    A version of plain weave in which occasional warp fibres, at regular intervals but usually several fibres apart, deviate from the alternate under-over interlacing and instead interlace every two or more fibres. This happens with similar frequency in the weft direction, and the overall effect is a fabric with increased thickness, rougher surface, and additional porosity.

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    Woven Glass Yarn Fabrics vs Woven Rovings

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    Yarn-based fabrics generally give higher strengths per unit weight than roving, and being generally finer, produce fabrics at the lighter end of the available weight range. Woven rovings are less expensive to produce and can wet out more effectively. However, since they are available only in heavier texes, they can only produce fabrics at the medium to heavy end of the available weight range, and are thus more suitable for thick, heavier laminates.

    Stitched 0/90 degree Fabrics

    0/90 degree fabrics can also be made by a stitching process, which effectively combines two layers of unidirectional material into one fabric. Stitched 0/90 degree fabrics can offer mechanical performance increases of up to 20% in some properties over woven fabrics, due to the following factors:

  • Parallel non-crimp fibres bear the strain immediately upon being loaded.

  • Stress points found at the intersection of warp and weft fibres in woven fabrics are eliminated.

  • A higher density of fibre can be packed into a laminate compared with a woven. In this respect the fabric behaves more like layers of unidirectional.

  • Other benefits compared with woven fabrics include:

  • Heavy fabrics can be easily produced with more than 1kg/sqm of fibre.

  • Increase packing of the fibre can reduce the quantity of resin required.

  • Hybrid Fabrics

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    The term hybrid refers to a fabric that has more than one type of structural fibre in its construction. In a multi-layer laminate if the properties of more than one type of fibre are required, then it would be possible to provide this with two fabrics, each containing the fibre type needed. However, if low weight or extremely thin laminates are required, a hybrid fabric will allow the two fibres to be presented in just one layer of fabric instead of two. It would be possible in a woven hybrid to have one fibre running in the weft direction and the second fibre running in the warp direction, but it is more common to find alternating threads of each fibre in each warp/weft direction. Although hybrids are most commonly found in 0/90 degree woven fabrics, the principle is also used in 0/90 degree stitched, unidirectional and multiaxial fabrics.

    The most usual hybrid combinations are:

  • Carbon / Aramid The high impact resistance and tensile strength of the aramid fibre combines with high the compressive and tensile strength of carbon. Both fibres have low density but relatively high cost.

  • Aramid / Glass The low density, high impact resistance and tensile strength of aramid fibre combines with the good compressive and tensile strength of glass, coupled with its lower cost.

  • Carbon / Glass Carbon fibre contributes high tensile compressive strength and stiffness and reduces the density, while glass reduces the cost.

  • Multiaxial Fabrics

    In recent years multiaxial fabrics have begun to find favour in the construction of composite components. These fabrics consist of one or more layers of long fibres held in place by a secondary non-structural stitching tread. The main fibres can be any of the structural fibres available in any combination. The stitching thread is usually polyester due to its combination of appropriate fibre properties (for binding the fabric together) and cost. The stitching process allows a variety of fibre orientations, beyond the simple 0/90 degree of woven fabrics, to be combined into one fabric.

    Multiaxial fabrics have the following main characteristics:

  • Advantages

  • The two key improvements with stitched multiaxial fabrics over woven types are:

    1. Better mechanical properties, primarily from the fact that the fibres are always straight and non-crimped, and that more orientations of fibre are available from the increased number of layers of fabric.

    1. Improved component build speed based on the fact that fabrics can be made thicker and with multiple fibre orientations so that fewer layers need to be included in the laminate sequence.

  • Disadvantages

  • Polyester fibre does not bond very well to some resin systems and so the stitching can be a starting point for wicking or other failure initiation. The fabric production process can also be slow and the cost of the machinery high. This, together with the fact that the more expensive, low tex fibres are required to get good surface coverage for the low weight fabrics, means the cost of good quality, stitched fabrics can be relatively high compared to wovens. Extremely heavy weight fabrics, while enabling large quantities of fibre to be incorporated rapidly into the component, can also be difficult to impregnate with resin without some automated process. Finally, the stitching process, unless carefully controlled as in the SP fabric styles, can bunch together the fibres, particularly in the 0 degree direction, creating resin-rich areas in the laminate.

    Fabric Construction

    The most common forms of this type of fabric are shown here.

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    There are two basic ways of manufacturing multiaxial fabrics:

    Weave & Stitch

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    With the ¡¥Weave & Stitch¡¦ method the +45 degree and -45 degree layers can be made by weaving weft Unidirectionals and then skewing the fabric, on a special machine, to 45 degrees. A warp unidirectional or a weft unidirectional can also be used unskewed to make a 0 degree and 90 degree layer. If both 0 degree and 90 degree layers are present in a multi-layer stitched fabric then this can be provided by a conventional 0/90 degree woven fabric.

    Due to the fact that heavy rovings can be used to make each layer, the weaving process is relatively fast, as is the subsequent stitching together of the layers via a simple stitching frame. To make a quadraxial (four-layer: +45 degree, 0 degree, 90 degree, -45 degree) fabric by this method, a weft unidirectional would be woven and skewed in one direction to make the +45 degree layer, and in the other to make the -45 degree layer. The 0 degree and 90 degree layers would appear as a single woven fabric. These three elements would then be stitched together on a stitching frame to produce the final four-axis fabric.

    Simultaneous Stitch

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    Simultaneous stitch manufacture is carried out on special machines based on the knitting process, such as those made by Liba, Malimo, Mayer, etc. Each machine varies in the precision with which the fibres are laid down, particularly with reference to keeping the fibres parallel. These types of machine have a frame which simultaneously draws in fibres for each axis/layer, until the required layers have been assembled, and then stitches them together, as shown in this diagram.

    Other/Random Fabrics

  • Chopped Strand Mat

  • Chopped strand mat (CSM) is a non-woven material which, as its name implies, consists of randomly orientated chopped strands of glass which are held together - for marine applications - by a PVA emulsion or a powder binder. Despite the fact that PVA imparts superior draping handling and wetting out characteristics users in a marine environment should be wary of its use as it is affected by moisture and can lead to osmosis like blisters. Today, chopped strand mat is rarely used in high performance composite components as it is impossible to produce a laminate with a high fibre content and, by definition, a high strength-to-weight ratio.

  • Tissues

  • Tissues are made with continuous filaments of fibre spread uniformly but randomly over a flat surface. These are then chemically bound together with organic based binding agents such as PVA, polyester, etc. Having relatively low strength they are not primarily used as reinforcements, but as surfacing layers on laminates in order to provide a smooth finish. Tissues are usually manufactured with area weights of between 5 and 50g/sqm. Glass tissues are commonly used to create a corrosion resistant barrier through resin enrichment at the surface. The same enrichment process can also prevent print-through of highly crimped fabrics in gelcoat surfaces.

  • Braids

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    Braids are produced by interlacing fibres in a spiral nature to form a tubular fabric. The diameter of the tube is controlled by the number of fibres in the tube¡¦s circumference, the angle of the fibres in the spiral, the number of intersections of fibre per unit length of the tube and the size (tex) of the fibres in the assembly. The interlacing can vary in style (plain, twill, etc.) as with 0/90 degree woven fabrics. Tube diameter is normally given for a fibre angle of 45 degrees but the braiding process allows the fibres to move between angles of about 25 degrees and 75 degrees, depending on the number and tex of the fibres. The narrow angle gives a small diameter whereas the wider angle gives a large diameter. Therefore along the length of one tube it is possible to change the diameter by variation of the fibre angle - a smaller angle (relative to zero) giving a smaller diameter and vice versa. Braids can be found in such composite components as masts, antennae, drive shafts and other tubular structures that require torsional strength.

    Next week we'll look at core materials, ranging from foam to honeycombs to wood.

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