The role of the
reinforcement in a composite material is fundamentally one of increasing the
mechanical properties of the neat resin system. All of the different fibres used
in composites have different properties and so affect the properties of the
composite in different ways. The properties and characteristics of common fibres
are explained below.
Properties of
Reinforcing Fibres & Finishes
The mechanical properties
of most reinforcing fibres are considerably higher than those of un-reinforced
resin systems. The mechanical properties of the fibre/resin composite are
therefore dominated by the contribution of the fibre to the composite. The four
main factors that govern the fibre’s contribution are:
1. The basic mechanical
properties of the fibre itself.
2. The surface
interaction of fibre and resin (the ‘interface’).
3. The amount of fibre in
the composite (‘Fibre Volume Fraction’).
4. The orientation of the
fibres in the composite.
The basic mechanical
properties of the most commonly used fibres are given in the following table.
The surface interaction of fibre and resin is controlled by the degree of
bonding that exists between the two. This is heavily influenced by the treatment
given to the fibre surface, and a description of the different surface
treatments and ‘finishes’ is also given here.
Laminate Mechanical
Properties
The properties of the
fibres given above only shows part of the picture. The properties of the
composite will derive from those of the fibre, but also the way it interacts
with the resin system used, the resin properties itself, the volume of fibre in
the composite and its orientation. The following diagrams show a basic
comparison of the main fibre types when used in a typical high-performance
unidirectional epoxy prepreg, at the fibre volume fractions that are commonly
achieved in aerospace components.
These graphs show the strengths and
maximum strains of the different composites at failure. The gradient of each
graph also indicates the stiffness (modulus) of the composite; the steeper the
gradient, the higher its stiffness. The graphs also show how some fibres, such
as aramid (Kevlar), display very different properties when loaded in
compression, compared with loading in tension.
Laminate Impact
Strength
Impact damage can pose
particular problems when using high stiffness fibres in very thin laminates. In
some structures, where cores are used, laminate skins can be less than 0.3mm
thick. Although other factors such as weave style and fibre orientation can
significantly affect impact resistance, in impact-critical applications, carbon
is often found in combination with one of the other fibres. This can be in the
form of a hybrid fabric where more than one fibre type is used in the fabric
construction. These are described in more detail later.
Comparative Fibre Cost
The figures above are
calculated on a typical price of a 300g woven fabric. Most fibre prices are
considerably higher for the small bundle size (tex) used in such lightweight
fabrics. Where heavier bundles of fibre can be used, such as in unidirectional
fabrics, the cost comparison is slightly different.
Fibre Types
Glass
By blending quarry products (sand,
kaolin, limestone, colemanite) at 1,600 degrees C, liquid glass is formed. The
liquid is passed through micro-fine bushings and simultaneously cooled to
produce glass fibre filaments from 5-24um in diameter. The filaments are drawn
together into a strand (closely associated) or roving (loosely associated), and
coated with a “size” to provide filament cohesion and protect the glass from
abrasion.
By variation of the
“recipe”, different types of glass can be produced. The types used for
structural reinforcements are as follows:
1) E-glass (electrical) -
lower alkali content and stronger than A glass (alkali). Good tensile and
compressive strength and stiffness, good electrical properties and relatively
low cost, but impact resistance relatively poor. E-glass is the most common form
of reinforcing fibre used in polymer matrix composites.
2) C-glass (chemical) - best
resistance to chemical attack. Mainly used in the form of surface tissue in the
outer layer of laminates used in chemical and water pipes and tanks.
3) R, S or T-glass –
manufacturers’ trade names for equivalent fibres having higher tensile strength
and modulus than E glass, with better wet strength retention. Higher ILSS and
wet out properties are achieved through smaller filament diameter. S-glass is
produced in the USA by OCF, R-glass in Europe by Vetrotex and T-glass by Nittobo
in Japan. Developed for aerospace and defence industries, and used in some hard
ballistic armour applications. This factor, and low production volumes mean
relatively high price.
E Glass Fibre Types
E Glass fibre is available in the
following forms:
Strand - a compactly
associated bundle of filaments. Strands are rarely seen commercially and are
usually twisted together to give yarns.
Yarns - a closely associated
bundle of twisted filaments or strands. Each filament diameter in a yarn is the
same, and is usually between 4-13um. Yarns have varying weights described by
their ‘tex’ (the weight in grams of 1000 linear metres) or denier (the weight in
lbs of 10,000 yards), with the typical tex range usually being between 5 and
400.
Rovings - a loosely
associated bundle of untwisted filaments or strands. Each filament diameter in a
roving is the same, and is usually between 13-24um. Rovings also have varying
weights and the tex range is usually between 300 and 4800. Where filaments are
gathered together directly after the melting process, the resultant fibre bundle
is known as a direct roving. Several strands can also be brought together
separately after manufacture of the glass, to give what is known as an assembled
roving. Assembled rovings usually have smaller filament diameters than direct
rovings, giving better wet-out and mechanical properties, but they can suffer
from catenary problems (unequal strand tension), and are usually higher in cost
because of the more involved manufacturing processes.
It is also possible to
obtain long fibres of glass from short fibres by spinning them. These spun yarn
fibres have higher surface areas and are more able to absorb resin, but they
have lower structural properties than the equivalent continuously drawn fibres.
Glass Fibre
Designation
Glass fibres are
designated by the following internationally recognised terminology:
Aramid
(Kevlar)
Aramid fibre is a
man-made organic polymer (an aromatic polyamide) produced by spinning a solid
fibre from a liquid chemical blend. The bright golden yellow filaments produced
can have a range of properties, but all have high strength and low density
giving very high specific strength. All grades have good resistance to impact,
and lower modulus grades are used extensively in ballistic applications.
Compressive strength, however, is only similar to that of E glass.
Although most commonly
known under its Dupont trade name ‘Kevlar’, there are now a number of suppliers
of the fibre, most notably Akzo Nobel with ‘Twaron’. Each supplier offers
several grades of aramid with various combinations of modulus and surface finish
to suit various applications. As well as the high strength properties, the
fibres also offer good resistance to abrasion, and chemical and thermal
degradation. However, the fibre can degrade slowly when exposed to ultraviolet
light. Aramid fibres are usually available in the form of rovings, with texes
ranging from about 20 to 800.
Carbon
Carbon fibre is produced by the
controlled oxidation, carbonisation and graphitisation of carbon-rich organic
precursors which are already in fibre form. The most common precursor is
polyacrylonitrile (PAN), because it gives the best carbon fibre properties, but
fibres can also be made from pitch or cellulose. Variation of the graphitisation
process produces either high strength fibres (@ ~2,600 degrees C) or high
modulus fibres (@ ~3,000 degrees C) with other types in between. Once formed,
the carbon fibre has a surface treatment applied to improve matrix bonding and
chemical sizing which serves to protect it during handling.
Carbon fibres are usually grouped
according to the modulus band in which their properties fall. These bands are
commonly referred to as: high strength (HS), intermediate modulus (IM), high
modulus (HM) and ultra high modulus (UHM). The filament diameter of most types
is about 5-7um. Carbon fibre has the highest specific stiffness of any
commercially available fibre, very high strength in both tension and compression
and a high resistance to corrosion, creep and fatigue. Their impact strength,
however, is lower than either glass or aramid, with particularly brittle
characteristics being exhibited by HM and UHM fibres.
Fibre Type Comparisons
Comparing the properties
of all of the fibre types with each other, shows that they all have distinct
advantages and disadvantages. This makes different fibre types more suitable for
some applications than others. The following table provides a basic comparison
between the main desirable features of generic fibre types. ‘A’ indicates a
feature where the fibre scores well, and ‘C’ indicates a feature where the fibre
is not so good.
Other Fibres
There are a variety of
other fibres which can be used in advanced composite structures but their use is
not widespread. These include:
-
Polyester A low density, high
tenacity fibre with good impact resistance but low modulus. Its lack of
stiffness usually precludes it from inclusion in a composite component, but it
is useful where low weight, high impact or abrasion resistance, and low cost are
required. It is mainly used as a surfacing material, as it can be very smooth,
keeps weight down and works well with most resin types.
-
Polyethylene In random
orientation, ultra-high molecular weight polyethylene molecules give very low
mechanical properties. However, if dissolved and drawn from solution into a
filament by a process called gel-spinning, the molecules become disentangled and
aligned in the direction of the filament. The molecular alignment promotes very
high tensile strength to the filament and the resulting fibre. Coupled with
their low specific gravity (<1.0), these fibres have the highest specific
strength of the fibres described here. However, the fibre’s tensile modulus and
ultimate strength are only slightly better than E-glass and less than that of
aramid or carbon. The fibre also demonstrates very low compressive strength in
laminate form. These factors, coupled with high price, and more importantly, the
difficulty in creating a good fibre/matrix bond, means that polyethylene fibres
are not often used in isolation for composite components.
-
Quartz A very high silica
version of glass with much higher mechanical properties and excellent resistance
to high temperatures (1,000 degrees C+). However, the manufacturing process and
low volume production lead to a very high price.
-
Boron Carbon or metal fibres
are coated with a layer of boron to improve the overall fibre properties. The
extremely high cost of this fibre restricts it use to high temperature aerospace
applications and in specialised sporting equipment. A boron/carbon hybrid,
composed of carbon fibres interspersed among 80-100um boron fibres, in an epoxy
matrix, can achieve properties greater than either fibre alone, with flexural
strength and stiffness twice that of HS carbon and 1.4 times that of boron, and
shear strength exceeding that of either fibre.
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Fibre Finishes
Surface finishes are
nearly always applied to fibres both to allow handling with minimum damage and
to promote fibre/matrix interfacial bond strength. With carbon and aramid fibres
for use in composite applications, the surface finish or size applied usually
performs both functions. The finish is applied to the fibre at the point of
fibre manufacture and this finish remains on the fibre throughout the conversion
process into fabric. With glass fibre there is a choice of approach in the
surface finish that can be applied.
Glass fibre rovings that are to be
used in direct fibre processes such as prepregging, pultrusion and filament
winding (more on these later in this series), are treated with a ‘dual-function’
finish at the point of fibre manufacture. Glass fibre yarns, however, when used
for weaving are treated in two stages. The first finish is applied at the point
of fibre manufacture at quite a high level and is purely for protection of the
fibre against damage during handling and the weaving process itself. This
protective finish, which is often starch based, is cleaned off or ‘scoured’
after the weaving process either by heat or with chemicals. The scoured woven
fabric is then separately treated with a different matrix-compatible finish
specifically designed to optimise fibre to resin interfacial characteristics
such as bond strength, water resistance and optical clarity.
Finishes, or sizes, for
carbon fibres used in structural composites are generally epoxy based, with
varying levels being used depending on the end use of the fibre. For weaving the
size level is about 1-2% by weight whereas for tape prepregging or filament
winding (or similar single-fibre processes), the size level is about 0.5-1%. The
chemistry and level of the size are important not only for protection and matrix
compatibility but also because they effect the degree of spread of the fibre.
Fibres can also be supplied unsized but these will be prone to broken filaments
caused by general handling. Most carbon fibre suppliers offer 3-4 levels of size
for each grade of fibre.
Aramid fibres are treated with a
finish at the point of manufacture, primarily for matrix compatibility. This is
because aramid fibres require far less protection from damage caused by fibre
handling. The main types of fibre treatment are composite finish, rubber
compatible finish (belts and tyres) and waterproof finish (ballistic soft
armour). Like the carbon fibre finishes, there are differing levels of composite
application finish depending on the type of process in which the fibre will be
used.
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Next week we’ll look
at the ways in which these fibres can be woven into fabrics having very
different designs and properties.
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