Fracture of highly oriented, chain-extended polymer fibres

Polymeric high-modulus, high-tenacity fibres have a structure consisting of highly oriented, chain-extended, linear macromolecules. Para-aramid, aromatic copolyester, PBO and PIPD fibres are made by liquid-crystal routes, but HMPE is gel-spun. Deviations from an ideal structure reduce stresses below the ideal values. Tensile failure is due to axial splitting. HMPE fails by creep rupture under moderate loads. These fibres suffer from surface abrasion, flex fatigue and axial compression fatigue. Failure in shear is described qualitatively. Time and temperature dependence has been modelled by statistical mechanics. In fibre assemblies, axial compression fatigue is modelled in terms of axial and transverse compressive forces on yams and axial slip. The same principles should apply to the formation of kink-bands within fibres, which leads to failure in cyclic flexing.