A modeling and experimental study of the influence of twist on the mechanical properties of high-performance fiber yarns

Little data exist on how twist changes the properties of high-performance c ontinuous fiber yarns. For this reason, a study was conducted to determine the influence of twist on the strength and stiffness of a variety of high-p erformance continuous polymeric fiber yarns. The materials investigated inc lude Kevlar 29(R), Kevlar 49(R), Kevlar 149(R), Vectran HS(R), Spectra 900( R), and Technora(R). Mechanical property tests demonstrated that the initia l modulus of a yarn monotonically decreases with increasing twist. A model based on composite theory was developed to elucidate the decrease in the mo dulus as a function of both the degree of twist and the elastic constants o f the fibers. The modulus values predicted by the model have good agreement with those measured by experiment. The radial shear modulus of the fiber, which is difficult to measure, can be derived from the regression parameter of experimental data by the use of the model. Such information should be u seful for some specialized applications of fibers, for example, fiber-reinf orced composites. The experimental results show that the strength of these yarns can be improved by a slight twist. A high degree of twist damages the fibers and reduces the tensile strength of the yarn. The elongation to bre ak of the yarns monotonically increases with the degree of twist. (C) 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1938-1949, 2000.