Aramid fibre modulus

For the global advanced composites market, the average cost of high-performance fibre reinforcements (carbon, aramid, high modulus polyethylene, boron, R/S/T-glass and some E-glass) is estimated from 5.5 to 6 per kg. This moderate price is due to the decrease in the carbon fibre price. Some grades could fall to less than 10/kg in the short or medium term. 

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. 

The weak van der Waals bonds between the molecules in UHMWPE give it very poor heat resistance. The fibres melt at 150°C and their properties deteriorate as the temperature inaeases above room temperature. Under high stress, the fibres tend to creep extensively and can break after a short time under load. A secondary slow heating, under tension, when approaching the melting point, increases modulus and reduces CTeep. It is extremely resistant to chemical and biological attack and has better abrasion and fatigue resistance than aramid fibres. ... 

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. 

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). 

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. 

There are two distinct types of aramid fibre, those in which the elastic modulus is about the same as for glass fibre, typically 60-70 kN/mm, and those with a modulus of about twice this level. 

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. ... 

The global market for advanced composites consumes over 140,000 tonnes/year of fibre reinforcements (of carbon, aramid, high modulus PE, boron, various types of glass fibres). 

It appears that the increase in compressive strength of plain HP-PE composites incorporating treated fibres is caused by the change in failure mode. It seems likely that compressive failure processes in the fibre itself are strongly determined by the low shear modulus and shear strength. These failure processes are comparable with those found in more detailed studies for aramid fibres (6). 

Aramid fibres, such as Kevlar and Twaron, are currently used in many different ballistic applications including body armour systems and armed vehicles. Aramid fibres/yams exhibit a fairly linear elastic behaviour up to failure under tension (Cepus, 2003). Aramid fibres have high tensile modulus (up to 120 GPa), are chemically stable under a wide variety of exposure conditions (except for UV exposure) and have a high melting point (550 °C). 

High-strength polymer fibres with drawn chain molecules can be produced by spinning. Aramid fibres with drawn molecules, for example, can have a Young s modulus in fibre direction of up to 450 GPa and an axial tensile strength of 4700 MPa. These fibres are frequently used in composites (see chapter 9). 

There are two types of aramid (fully aromatic polyamide) fibre those with high thermal stability and flame resistant properties but mechanical properties similar to those of standard textile fibres such as polyester, sometimes referred to as meta-aramid fibres because the main commercial products are of this chemical structure the others with additionally exceptionally high tensile strength and modulus, sometimes referred to as para-aramid fibres on similar grounds. The former type is not widely used in composite structures, but some of the most important uses of the latter lie in this area. 

FigveRS Depaidenoeof the rate of shift per unit strainforthel610ciii Raman band upon fibre modulus for the five different aramid fibres shown in Figure 8.4. The symbols have the same meaning as in Figure 8.4 (after [38])... 

Figure 8.5 give a plot of dAv/de versus fibre modulus Ef, and it can be seen that there is an approximately linear relationship consistent with Equation (1). This is an indication that the Raman technique is probing molecular stretching directly [37] and is consistent with the theoretical work of Northolt and coworkers [39-41]. They suggested that, for the deformation of aramid fibres such as Kevlar, the total strain is the sum of the strains due to two deformation processes, stretching and rotation, such that... 

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