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2.2. CONSTITUENT MATERIALS

2.2.1. Fibres

The main function of fibres within composite structures is as the load- bearing component and, as such, they are generally significantly stiffer and stronger when compared to the matrix component. For example, the elastic modulus of the polymer matrix in a PMC may be typically 3.0 GPa [102] whereas that of the reinforcing glass fibre may be 86.9 GPa [103]. Various types of fibres are available in the global commercial market, including glass, carbon, graphite, aramid, boron, silicon carbide, polyethylene, and natural fibres derived from plants or animals ([104], [15] (p. 8), [101] (p. 596) and [12] (p. 54)). The relative importance of these fibres in the global composite industry can be seen in Table 2.2.

Geometrically, fibres can be further classified into two main groups, namely short fibres (also known as chopped or discontinuous fibres) and continuous fibres. As mentioned earlier, fibres can be categorised as continuous if they possess an aspect ratio, i.e., the length to diameter ratio, of 1000 or greater. With regards to this, continuous fibre-reinforced composites (CFRCs) generally possess improved mechanical properties when compared to discontinuous fibre- reinforced composites (DFRCs) [6] (p. 5); for example, the flexural modulus and

Table 2.2. Global market for advanced composite fibres in 2006a) Fibre type Amount Value Tonnes ( 103 ) % US$ ( 106 ) % E-glass fibre 2400 97 244 13.9 Carbon fibre 27 1.1 1300 74.1 Other fibresb) 46 1.9 211 12.0 a)

Rearranged after Ref. [105].

b)

Includes: aramid, boron, R-, S- and T-glass, as well as HMPE and quartz fibres

flexural strength of chopped E-glass FRP composite containing 20 % fibre volume fraction are, respectively, 7 GPa and 140 MPa compared to 35 GPa and 840 MPa (i.e., more than three times higher) for a unidirectional E-glass FRP composite with 60% fibre volume fraction [12] (p. 93); the lower production costs and ease of manufacture of complex shapes for DFRCs ([106] and [15] (p. 121)) has led to a wider variety of applications.

In the following sections, detail discussion concerning the different types of fibres will be limited to those utilised in this work, i.e., glass fibre and carbon fibre.

2.2.1.1. Glass fibres

Amongst the reinforcing materials used in fibre-reinforced polymer (FRP) composites, glass fibres are the most widely utilised mainly due to their relatively low price, availability and reliability [107], as well as their reasonably high strength when compared to carbon fibres [108]. Comparison between these two types of fibres in terms of cost and properties has been presented in Table 2.3 below. The table shows standard modulus continuous carbon fibre to be approximately ten times more expensive when compared to continuous E-glass fibre whilst the former is just slightly stronger in tension than the latter.

Table 2.3. Cost and properties of E-glass and standard modulus carbon fibre

Fibre type Cost a) (US$/kg) Properties Densityb) (g/cm3) Tensile strengthc) (MPa) Elastic modulusd) (GPa) E-glass, continuous 1.90 – 3.30 2.58 3450 72.5 Carbon, continuous 31.50 – 41.50 1.78 3800  4200 230 a) [101] (p. A35) c)[101] (p. A9) b) [101] (p. A6) d)[101] (p. A15)

Glass fibre filaments are produced by forcing a molten glass through holes in the bottom of a platinum bushing. Following this, the glass filaments are collected, quenched by passing through a light water spray, coated with a protective binder, gathered to form a bundle (also known as a tow or strand), and finally wound onto a spool as shown in Figure 2.10. This complete manufacturing process has been illustrated in Figure 2.11. In addition to protecting the surface of the fibres, the binder has additional functions including holding individual fibres together for ease of processing, lubricating the fibre surface in order to reduce abrasion when in contact with equipment during composite manufacture, impart anti-septic agent for softening and to improve chopability, and provide chemical links for improved interface bonding between the fibre surface and matrix ([109, 110], [109: pp. 18-19, and 110]).

The various types of glass fibres may be classified based on their chemical composition that, furthermore, determines their properties and usages. There are four types of commonly used glass fibres [110], i.e. A-glass fibre with high alkali content leading to good chemical resistance; C-glass fibre produced from soda borosilicate, possessing excellent chemical resistance, E-glass fibre produced from

Figure 2.10. Spool of E-glass fibre E2350-11 supplied by Owens Corning Asia-Pacific. Net weight of the spool is 17 kgs.

aluminium borosilicate-based materials, possessing good electrical resistance, and S-glass fibres consisting of S- and S2-glass fibres which contain compounding elements such as aluminium silicate and magnesium which make them much stronger than E-glass fibres. Typical chemical compositions of these types of glass fibres are presented in Table 2.4.

In general, glass is an amorphous material. Polyhendrons, that are each composed of oxygen atoms covalently bonded to silicon, form long three- dimensional networks within the glass structure. Glass fibres are fairly isotropic [12] (p. 11) which is a consequence of its three-dimensional network structure. Glass fibres are generally inexpensive, very strong in tension, possessing high fracture toughness, as well as being relatively insensitive to chemical environments, moisture, and elevated temperature. Some important physical and mechanical properties of glass fibres have been presented in Table 2.5.

The tensile strength of both E and S grades of glass fibre is very sensitive to temperature, although S-glass fibre is slightly less sensitive. The tensile strength of E-glass fibre degrades by approximately 24% and 50% as the temperature is increased from 22 C to 371 C and 538 C, respectively, while that of S-glass

Table 2.4. Typical chemical composition of E- and S-glass fibres (%wt)

Element E-glass S-glass

Silicon oxide, SiO2 54.3b,c) 55.2a) 64.2b,c) 65.0a) Aluminium oxide, Al2O3 8.0 a) 15.2b,c) 24.8b,c) 25.0a) Calcium oxide, CaO 17.2b,c)

18.7a)

a) 0.01b,c) Magnesium oxide, MgO 4.6a)

4.7b,c)

10.0a) 10.27b,c) Sodium oxide, Na2O 0.3a)

0.6b,c) 0.27b,c) 0.3a) Boron oxide, B2O3 7.3 a) 8.0b,c) a) 0.01b,c) a) [12] (p. 8) b)[15] (p. 15) c)[110] (p. 134)

fibre degrades by approximately 18% and 47% for the same temperature increase [110] (p. 135) and [111].

2.2.1.2. Carbon and graphite fibres

These days, carbon fibre has been widely used in applications including sport and leisure, civil structures, ground, marine and air transportations, military equipment, as well as high performance aerospace structures. Among the advantageous of carbon and graphite fibres are their low density combined with

Figure 2.12. A roll of S2-glass fibre mat Unitex UT-S500 supplied by SP System, Newport, Isle of Wight, UK

Table 2.5. Typical properties of E- and S-glass fibresa)

Properties E-glass S-glass

Density (gcm3

) 2.54 2.48

Coefficient of linear thermal expansion

(×106C1) 4.7 5.6

Tensile strength at 22 C (MPa) 3348  Tensile modulus at 22 C (GPa) 72.4 85.5

Yield elongation (%) 4.8 5.7

a)

high strength, high modulus and high temperature resistance in non-oxidising environments [112]. When being embedded in polymer matrices, which also possess low density, they produce high specific strength and high specific stiffness fibre-reinforced polymer matrix composites. The most recent commercial passenger aircraft such as the Boeing 787, contain approximately 50 wt% of composite materials, most of which is carbon fibre reinforced polymer (CFRP) [113], whilst high performance bicycles, canoes, fishing rods, golf clubs, hockey sticks, snowboards, and tennis racquets are additional examples of sporting goods made from CFRP composite [114].

Both carbon and graphite fibres are composed of carbon atoms arranged in a specific structure. The carbon content of carbon fibres is 80% to 95% while that of graphite fibres is much higher at 99% or more [15] (p. 21). Three types of organic precursors are commonly used to produce carbon and graphite fibres, i.e., rayon fibres, pitch, and polyacrilonytrile (PAN) [15] (p. 21), [82] (p. 64), [12] (p. 20), and [115]. Carbon and graphite fibres are generally produced by the so-called thermal decomposition process. The strength and modulus of carbon and graphite fibres are spread over a wide range (Table 2.6) depending upon their precursor, processing procedure and parameters. Compared to glass and silicon carbide fibres, carbon fibres possess much lower density (see Tables 2.5, 2.6 and 2.8) leading to superior specific strength and specific stiffness.

Precursors. Continuous rayon fibres are extracted from wood pulp and processed

by wet spinning. Rayon fibres are a natural thermosetting polymer [12] (p. 23). Pitch precursor can be obtained from inexpensive sources such as asphalt, polyvinyl chloride (PVC), and tar [12] (p. 23), [115], while PAN fibre precursors are apparently those used in textile industries [115].

Thermal decomposition process. Prior to thermal decomposition processing, the

precursors generally undergo particulate removal and spinning for pitch precursor; and spinning, extraction, and stretching to improve longitudinal alignment of polymer molecules for PAN fibre precursor. There are three important stages

generally involved in thermal decomposition process to produce carbon fibres [116] and [12] (p. 20), i.e.

1. Stabilisation. This stage is carried out at 200260

C [115] for a few hours in air. After being stabilised, precursors will reduce their tendency to melt in the further higher temperature processes.

2. Carbonisation. During this process, most of the non-carbon elements that may contaminate the precursor are removed, resulting in a higher purity of carbon fibre. The required temperature for carbonisation is approximately 10001500 

C [115], and it must be within an inert atmosphere such as nitrogen. In order to avoid any distortion of the precursor structure, the heating rate must be kept low.

3. Graphitisation. In this stage, the properties of the carbon fibres yielded in the previous stage are improved by holding the fibre at above 1800 C [15] (p. 22), up to 3000 C for PAN-based carbon fibre, for a very short time,

e.g., one minute, in an inert atmosphere such as argon [117].

The degree of alignment and order of atomic structure of the carbon fibre along the fibre axis direction, leading to higher stiffness and lower linear thermal expansion coefficient in this direction [115] and [12] (pp. 27-29), strongly depends upon the graphitisation temperature. Although carbon (graphite) possesses relatively high strength and high modulus, it is brittle in nature without any plastic deformation prior to fracture. As a consequence, the presence of any defect such as micro-cracks and fibre diameter inconsistencies will generate a very high local stress.

Hawthorne and Teghtsoonian [118] studied the compressive behaviour of carbon fibres by embedding single carbon fibres in epoxy resin and applying a compressive load onto the specimens. They found that the higher the degree of anisotropy of fibre structure the more likely that micro-buckling and kinking failure modes occurred, in which failure started at the outer surface and propagated into the fibre centre. Compressive strength was found to vary from less than 1 GPa for high modulus carbon fibre to 2.2 GPa for moderately oriented carbon fibre, while the failure strain decreased from 2.5% to less than 1% as the modulus

increased. In addition, Oya and Hamada [119] reported that the compressive strength of several different carbon fibre types varied from 15.6% (M50J) to 30.8% (T700S) of their respective tensile strengths.

The wide variety of properties, which depend upon their precursor type and fabrication parameters, for several carbon fibres can be seen in Table 2.6.

Figure 2.13. Two rolls of 12k tow size PAN-based carbon fibre from two different manufacturers

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