Aramid fibre density

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. 

We have developed different nonwoven stmctures with para-aramid fibres, and produced thick 3D stmctures by associating several nonwoven monolayers with a consolidation treatment. Porous composites were afterwards manufactured by impregnating the fibrous stmctures with an epoxy resin. Both dry and impregnated materials were characterized in terms of stmcture (density, fibre volume fraction), and compression tests were used to evaluate mechanical properties. Equivalent pore size and distance between fibre contacts were determined using theoretical models. They help to provide some insight on the mechanical behaviour of the different stmctures. 

Aramid and ultra-high molecular weight polyethylene are used extensively as base materials for ballistic protection. As discussed earlier (section 7.3.1), these high performance fibres are characterised by high strength, high eneigy absorption, and low density. However, to meet the protection requirements for typical ballistic threats, approximately 13-50 layers of fabric are required, which results in a bulky and stiff armour. The bulk-iness limits its comfort and has resfiicted its appUcation primarily to torso protection. 

Staple fibre Kevlar yams with two different linear densities were sourced from Aramex-Game, Germany, and one of the yams was doubled locally without any twist, coded as K2. This is mainly to increase the linear density of the yam to enable the use of thicker yam. Tilsa is a para-aramid yam from Tilsatec, with a similar linear density to that of the Kevlar yams from Aramex-Game. Several dope-dyed para-aramid yams were sourced from Tilsatec in various linear densities. 

Staple fibre aramid yams were sourced from three different manufacturers in various linear densities. The stress/strain curves of the 100% aramid yams are shown in Fig. 7.17. It can be observed from the figure that initial modulus of all the staple fibre aramids is more or less the same. The exception being the Russian made Ruslan aramid, which is a continous filament yam with 200 filaments with 100 turns per metre. Though initial modulus for all the staple fibre aramid yams is the same, the yams made by Tilsatec exhibited very low tenacity and required the least effort (work of mpture) to break the yam. This was the case with the Ruslan yam, too, as it required just 1644 cNcm to break the yam as compared to 5281-7332 cNcm for the other staple... 

Table 10.4 shows the effect of two heat sources on various types of fabric. Woven and nonwoven fabrics of different area densities made from aramid and FBI fibre have been compared in terms of their respective thermal protective performance (TPP) indices and the results are shown in Table 10.5. TPP values are the times for a temperature gradient of 25 °C to be generated across the fabric thickness when exposed to a heat source as defined in ASTM D4108. The higher the TPP value, the better the thermal protective property. The original fabric constructional data were published in imperial units and they have been converted to SI units for consistency. Woven fabrics were designed as the outer shell material in firefighters turnout coats, and the needlefelt, nonwoven fabrics could be considered for use as a backing or thermal liner in thermally protective apparel. This work shows that nonwoven fabrics provide...

Most technical yams for reinforcement are based on continuous multifilament yams having linear densities typically in the 1100 dtex (1000 denier) region in singles or doubled configurations. The number of filaments per yam will vary between about 140 and 200 for melt-spun nylon 6.6 and polyester to >500 for high tenacity viscose and other wet-spun fibres, including the aramids. For instance, Kevlar 29, suitable for rubber reinforcement, is available as a 1670 dtex yam with 1000 filaments. "... 

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.

Table 9.4 presents typical ranges of variation for several physical and mechanical properties of FRP bars made of glass (GFRP), carbon (CFRP) and aramid (AFRP) reinforcement. For the typical fibre content, all FRP reinforcing bars present low density, about one-sixth to one-quarter that of steel bars (again, the main competitor), which facilitates transport and... 

An further alternative approach being developed worldwide is to replace the steel completely by fibre-reinforced plastics (FRP), which consist of continuous fibres as carbon, glass or aramid, set in a suitable resin to form a composite rod or grid. These materials have high tensile strength, low density and are non-magnetic they can be used both for new structures and for repair of existing ones. The mechanical properties of FRP are determined by the amount and type of fibre, while the durability will be a function of both the resin and the fibre. 

Figure 3.2 Density for glass, aramid, polyethylene, carbon and ceramic fibres compared with aluminium alloy and steel.

These fibres are representative of the high performance aramid products currently available. The anisotropy of unidirectional composites is apparent as is the poor compression strength of aramid composites. The composite densities are extremely low. 

Unlike other reinforcements there are over 100 types of carbon fibre available. Carbon fibres have densities betw een those of aramids and glass fibres. The fibres are strong, can have a very high modulus, good fatigue resistance (in composite form), may have good electrical and thermal conductivity and very low thermal expansion in the direction of the long fibre axis. They may however be rather brittle, if unprotected oxidize above 300-400°C, and are more expensive than the other types of reinforcement. 

Porous 3D composites have been developed from para-aramid nonwoven sandwiches composed of different monolayer structures. The dry and resin-impregnated materials have been characterized in terms of density, porosity and air permeability. In an attempt to understand the fibre arrangements inside the material, the theoretical pore size and distance between fibre contacts have been evaluated. The compression results obtained with the composites can be related to this fibre arrangement which defines more specifically the number of fibre contacts and/or interfacings. This parameter is however not relevant for the compressive behaviour of the dry nonwovens, which depends largely on the fibre bending modulus and inter-fibre frictions. 

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