Aramid fibre strength

The long term reduction in tape strength with increasing load was taken to be the same as the reduction in aramid fibre strength, given by the logarithmic decay expression... 

Some three decades ago, scientists from the Du Pont company developed polycondensation methods which allowed the preparation of high molecular weight wholly aromatic polyamides. The first commercially produced wholly aromatic polyamide fibre was poly(m-phenyleneisophthalamide) (Nomex, Du Pont, 1967) [la, c]. Some years later, development of the preparation and processing of poly(p-phenyleneterephthalamide) (PPTA) led to the commercialization of the para product Kevlar (Du Pont) in the early seventies [lb, c]. While Nomex shows excellent thermal stability and flame-retardance, and indeed is referred to as a heat and flame resistant aramid fibre, Kevlar fibre also has similar properties, but in addition it has exceptional tensile strength and modulus, and is referred to as an ultra-high strength, high modulus aramid fibre. 

In this respect, (thermoset) plastics composites with discontinuous fibre products are already mostly used in the car body applications, where polyester/E-glass is predominating (mostly because of polyesters, economy, ease of processability and reasonable mechanical properties provided), followed by use of phenolics (when fire retardance is required, in friction linings and engine compartments), and epoxies. Replacement by carbon or aramid fibre reinforcements can reduce body mass by 40% (compared to steel) and with more added strength, but the cost is unfavourable at the moment, as mentioned previously [12, 13]. 

Aramid fibres are aromatic polyamides. Therefore, the presence of the polar and hydrolysable amide group introduces the potential for moisture absorption up to 5%. As a result, these fibres exhibit time-dependent loss of strength or static fatigue in aqueous environments [28,29]. The fibres are spun from sulphuric acid solution so that residuals might be responsible. These fibres are also highly susceptible to hydrolysis in alkaline environments. As with other polymers, aramid fibres are... 

The aramid fibres (Kevlar and Nomex are the trade names for two of the most common) are especially desirable for their strength-to-weight ratios being superior to metals. They are most often used in composites having polymer matrices (generally epoxies and polyesters). Since the fibres are relatively flexible and somewhat ductile, they may be processed by most common textile operations. 

The modulus and strength of the fibre are almost twice as high as aramid fibres, with other properties remaining similar. The fibre degrades by hydrolysis in warm and moist conditions, which makes the fibre unsuitable for applications that expose the material to warm and moist environments. 

Aramid fibres have the highest strength to weight ratio compared to the other fibres but broadly similar tensile strength to glass fibre. There are a number of manufacturers but Kevlar and Nomex are the familiar brand names. 

Aramide fibres. Man-made fibres of aromatic polyamides characterized by high strength, high elastic modulus, good chemical resistance, high thermal stability. Applications tyre cords, protective garments, electric insulators. Trade names Arenka (NL), Kevlar (USA), Nomex (USA). 

The reinforcement types which will be discussed are glass fibre, carbon fibre and aramid fibre. Each of these is not a single fibre type but more a family of fibres with a range of properties. Typical properties of a range of reinforcement fibres are given in Table 1.2 The strength and strain figures... 

Aromatic ether amide or aramid fibres are organic, man made fibres which are available in various forms for use in composites. They are characterised by having reasonably high tensile strength, a medium modulus and a very low density. Their composites fit well into a gap in the range of stress/strain curves left by the family of carbon fibres at one extreme and glass fibres at the other. 

Aramids, such as Kevlar 49, are chemically quite stable and have high resistance to neutral chemicals. But they are susceptible to attack particularly by strong acids and also by bases. However, Technora aramid fibre has extremely high strength retention in both acids and alkalis. 

A uni-directional aramid fibre composite with a similar volume of fibres will have a tensile strength of about 1400 N/mm However, its compressive yield strength is about one-skth of this at 230 N/mml This also affects the flexural performance, giving a value of about 300 N/mml... 

Although aramid fibres have high inherent tensile strength, particularly in uni directional construction, in composites they tend to have creep rates very much higher than similar glass or carbon composites (reference 1.9). 

As illustrated in Table 1, the interfacial shear strength of aramid fibre composites is generally low. Several attempts to improve the interfacial bond have been attempted. These involve grafting appropriate functional groups on to the surface of the fibres. However, the improvement is limited because the crystal structure of aramid fibres ensures that the transverse strength is low. Consequently, improvements in interfacial shear strength simply shift the locus of the failure from adhesive to cohesive within the fibre, in which case, mechanical performance is generally not improved. However, there is some benefit to the moisture sensitivity of the fibres and composites. 

Fig. 2. Fibre strength and modulus from Smit et al. (2000). Specifie stre.ss equals stres.s/density , N/tex equals GPa/(g/cnv ). If values in GPa were plotted, aramid with a density of 1.44 g/cnv and PBO at I..") g/cnv would appear about 50% higher in comparison with polyethylene (Dyneema) at 0.97 g/em. PES is high-tenaeity polye.ster as used in tyre cords, etc.

Yang attributes the difference to differences in fibre type and test conditions Pointed fibre breaks are often observed on Kevlar 49 aramid fibres [post-treated to increase initial modulus] at slow strain rate. It reflects a highly ordered lateral fibre structure and is generally associated with high fibre strength. ... 

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