Textile reinforcements are produced using high performance fibres, like glass, carbon/graphite, Kevlar, PES HM and HT, ceramic fibres, boron and silicon carbide fibres, etc. These yarns have superior mechanical behaviour that can meet the specific demands of composite applications that are illustrated in Table 2. They also have high bending rigidity that affects the knitting process and other characteristics that must be taken into consideration when designing a knitted reinforcement for composite materials (Miller, 1989). Glass fibres (yarns, rovings) are the most common high performance fibres used to reinforce composite materials. They are characterised by hardness, resistance to chemical agents, stability and inertness, low weight and processability (Muckhopadhyay, 1994). There are more types of glass fibres depending on their chemical composition: E-glass, with good strength and high electrical resistivity, most common in composite materials; S-glass, with high tensile strength, most common in military applications; and C-glass, characterised by chemical stability and corrosion resistance.
|
Fibre |
Relative density [g/cm3l |
Young’s Modulus [GPa] |
Tensile strength [GPa] |
|
|
1 |
Carbon (PAN) |
2.0 |
400 |
2.0-2.5 |
|
2 |
Boron |
2.6 |
400 |
3.4 |
|
3 |
E-glass |
2.5 |
70 |
1.5-2.0 |
|
4 |
S-glass |
2.6 |
84 |
4.6 |
|
5 |
Kevlar 29 |
1.44 |
60 |
2.7 |
|
6 |
Kevlar 49 |
1.45 |
60 |
2.7 |
|
Table 2. Main characteristics for some high performance fibres |
Mechanically, the glass fibres are characterised by high strength, low elongation, high bending rigidity and brittleness. Law and Dias (1994) and Savci et al. (2001) showed that the glass fibres can resist when bent around the needle hook and therefore can be processed through knitting. Due to their brittleness and their low resistance to friction, the glass yarns damage easily, thus affecting the knitting process and subsequently the real strength of the reinforcement. Knitting glass fibre therefore requires a preliminary stage to determine the optimum technological conditions that ensure minimum fibre damage while maintaining the fabric quality. The fabric density, essential for the fibre fraction volume of the composite reinforcement gives this quality, together with the amount of fibre damage. High fibre fraction volume is a sine-qua-non requisite for the performance of the composite materials.
