Ward 3 Tensile Strength

Thermotropic polyesters are melt-spun from the nematic phase and orient easily in an elongational flow field (moderate drawdowns/forces are sufficient). In the fiber case, highly oriented fibers form easily with an initial modulus close to theory—typical values range from about 70 to 150 GPa. Ward [46] has shown that the tensile modulus may be described by an aggregate model, i.e., the modulus is a function of the inherent chain modulus, the molecular chain orientation, and the shear modulus (which described the stress transfer between chains). The tensile strength of LCP fibers follows the prediction of the lag-shear model [47]. Both the aggregate model and the lag shear model treat the LCP as though it... 

More recently, Wilczynski, Ward and Hine [24] 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 [25] to the determination of the elastic constants of a new organic fibre, poly 2,6-dimidazo[4,5-6 4 5 -e]pyridinylene l,4(2,5-dihydroxy)pheny-lene (PIPD). This fibre is a lyotropic liquid crystalline fibre with a very high Young s modulus of 285 GPa and a much higher tensile strength (5.21 GPa) and compressive strength (5OOMPa) than other polyaramid fibres such as Kevlar. 

The orientation of chopped fibers developed during flow/processing can strongly influence various properties of compounds, such as modulus and tensile strength. In 1974 Brody and Ward [79] described fiber orientation in uniaxial fiber filled composites using Hermans orientation factors [80 to 82] originally developed to measure polymer chain orientation in man-made fibers (see Fig. 2.5(a)). 

We now turn our attention from considerations of stiffness to stress-strain and tensile behavior. Variations in strength and modulus as a function of direction (Broutman and Krock, 1967, Chapter 12) have been treated by several investigators for example, Tsai (1965) and Brody and Ward (1971). Even though the polymer matrix typically has such a low modulus that it does not contribute much overall to the composite modulus, the matrix can by no means be neglected, because failure often involves catastrophic crack growth in the matrix (see below). Stress-strain curves for unidirectional composites are typically fairly linear up to failure for loading in the direction of the fibers (Broutman and Krock, 1967, p. 370), but quite nonlinear transverse to the fiber direction. The stress to rupture is also very low in the latter case, presumably due to a high concentration of stress in the matrix. 

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