Filaments tested at room temperature

Although the detailed development of mechanical anisotropy in these particular filaments must depend on their exact chemical composition and subsequent processing, several 

In all cases, the compliance 513 is low and appears to decrease fairly rapidly with increasing draw ratio, in a manner comparable with 533. Hence, the extensional Poisson s 

Hine and Ward [43] have used the ultrasonic immersion method (Section 6.3.2) to determine a full set of elastic constants for a range of fibres, by making uniaxially oriented fibre composites. In the first method, the oriented composites were produced by the Leeds hot compaction process. Here fibres are compacted under suitable conditions of temperature and pressure to form an homogenous oriented material in which only a small fraction of the original fibre is melted and re-crystallised to form the matrix of the fibre composite. This matrix fraction can be removed and results extrapolated to 100% fibre fraction. The measured elastic properties for a range of fibres are shown in Table 8.2. The overall patterns 

More recently, Wilczynski, Ward and Hine [44] have proposed an inverse calculation method where the elastic constants of a fibre can be estimated from fibre resin composite and the elastic constants of the resin. The method was confirmed by measurements on polyethylene/epoxy and carbon fibre/epoxy resin composites. It has been applied [45] to the determination of the elastic constants of an organic fibre, poly 2,6-dimidazo [4,5-6. 4 5 -e] pyridinylene - 1,4(2,5 dihydroxy)phenylene (PIPD). This fibre is a lyotropic liquid crystalline fibre with very high Young s modulus of 285 GPa and a much higher tensile strength (5.21 GPa) and compressive strength (500 MPa) than other polyaramid fibres such as Kevlar. 

The technique of Brillouin spectroscopy (Section 6.3.3) has been applied to determine the elastic constants of oriented polymer fibres. Early studies of this nature were undertaken by Kruger et al. [46,47] on oriented polycarbonate films, also determining the third-order constants, which define the elastic non-linear behaviour. Wang, Liu and Li [48,49] have described measurements on oriented polyvinylidene fluoride and polychlorotrifluoroethy-lene films. In the latter case the results were interpreted using an aggregate model differing in detail from that of Ward discussed in Section 8.6.2. 

In all cases the compliance X13 is low and appears to decrease fairly rapidly with increasing draw ratio, in a manner comparable with X33. Hence the extensional Poisson s ratio V13 = X13/X33 is rather insensitive to draw ratio and, with the exception of high-density polyethylene, does not differ significantly from 0.5. It is thus generally a valid approximation to assume that the filaments are incompressible. NB For anisotropic bodies is not confined to a maximum value of 0.5, but is limited solely by the inequalities necessary for a positive strain energy [2] 

There have been no similar attempts to determine comprehensive sets of elastic constants for oriented fibres or monofilaments. Kawabata [21] devised an apparatus that used a linear differential transformer to measure diametral changes of 0.05 iim in single fibres of diameter 5 fxm subjected to transverse compression. Equation (7.2) above was then used to calculate the transverse modulus El = l/ ii. Results were obtained for poly(p-phenylene terephthalamide) (Kevlar) and high-modulus polyethylene (Tekmilon) fibres. Values of E were in the range 2.31 -2.59 GPa for Kevlar and a value of 1 -2 GPa was found for Tekmilon. 

Further work in this area has been undertaken by Ward and co-workers [22], combining measurements of contact width and diametral compression on monofilaments of polyethylene terephthalate (PET), polyethylene and a thermotropic liquid crystalline poljoner based on hydroxybenzoic and hydroxynaphthoic acid in the ratio 73 27 (Vectra). Values for E were in the range 1.94-2.34 GPa for PET, 0.63-1.50 GPa for polyethylene and 0.96-1.01 GPa for Vectra. 

The technique of Brillouin spectroscopy (Section 5.3.3 above) has been applied to determine the elastic constants of oriented polymer fibres. Early studies of this nature were undertaken by Kruger and co-workers [26, 27] on oriented polycarbo- 

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