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
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
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
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
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
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
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
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.
Woven Glass Yarn
Fabrics vs Woven Rovings
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
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:
-
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.
-
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.
There are two basic ways of manufacturing multiaxial fabrics:
Weave & Stitch
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
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
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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