Mechanical properties of high performance fibers vis-a-vis applications in flexible structural composites

Some of the critical properties of high performance organic fibers and fiber assemblies have been addressed vis-à-vis their applications in flexible structural composites. These include: tensile properties; mechanical properties under complex modes of deformation; creep at high tensile loads; changes in physical properties due to thermo-mechanical/chemical treatments used in manufacturing of reinforced rubber goods.; The axial elastic modulus of fibers and tautly twisted filament assemblies of high performance organic polymers have been measured along with their crystalline orientation distributions. Based on well established procedures in continuum mechanics of axially symmetric structures, a quantitative relationship has been derived to relate the axial elastic modulus to the second and fourth moment of average crystalline orientation distribution. The latter was determined by X-ray diffraction measurements with yarns. This model, valid for single-phase materials, has been found to provide an excellent fit of data from twisted yams of aromatic polyamide and highly ordered polyethylene fibers, with a wide range of overall crystalline orientation distributions.; An important property of concern in engineering applications of polymeric filament assemblies of high performance organic fibers is creep. In this study, creep deformation data of gel-spun Ultra High Molecular Weight Polyethylne (UHMWPE) Spectra® 1000 yams have been fitted to a model obtained through an empirical mechanical analog of the viscoelastic process. The non-linear viscoelastic model composed of stress-dependent non-linear mechanical analogs qualitatively predicted the creep response to a series of step-loads applied on the UHMWPE yarns.; To understand the mechanical properties of high performance organic fibers under combined bending and extension, a simple pin-test procedure has been employed to characterize fibers and twisted yarns. The results obtained from the test have been interpreted with regard to the mechanisms through which fibers can exhibit superior performance characteristics under such deformation conditions.; An exploratory study of the changes in the mechanical properties of poly(p-phenylene terephthalamide) (PPTA) fibers due to the thermo-mechanical/chemical treatments used in commercial manufacturing of reinforced rubber goods has been conducted. The changes in tensile properties of PPTA yams have been inferred to be due to the process-induced changes in the overall crystalline orientation distribution.