Geometrical Effects of Filament Twist on the Modulus and Strength of Graphite Fiber-Reinforced Composites

Using basic theory previously appearing in the literature, equations are derived for determining the effects of yarn twist on the modulus and strength of graphite reinforced composites. In the strength analysis the variation of individual filament properties is considered. Results show that a few turns of twist per inch have little effect on either modulus or strength. However, these properties decrease in value rapidly beyond seven turns per inch. The theory was checked using data from a unidirectional composite reinforced with graphite yarns having approximately five turns of twist per inch. Since the processability of graphite filaments into a fiber reinforced composite is improved by using a twisted yarn, it seems advantageous from a geometrical standpoint to use yarns containing several turns of twist per inch. (Author)