Analytical model of nonlinear twist dependency for Kevlar yarn based on local filament strain

Kevlar KM2 600 denier yarn is often twisted to improve yarn load at failure. It is noted in ASTM-D-7269 that twisting KM2 past five twists per inch can cause a lower load at failure. Understanding the physics that govern the load decrease at higher twist ratios can lead to improved yarn designs, possibly stronger than the peak values currently seen at three turns per inch. Increasing yarn strength would enable enhanced usage of Kevlar yarn, improving chord and fabric structures. This paper describes the testing and finite element analysis methods used to probe yarn tenacity as a function of twist per inch. The performed tests indicate that strength decreases as the yarn is twisted past three twists per inch and are in agreement with previously conducted trials. The finite element model results were compared to test data performed in this study and ones reported in literature. Employing the validated simulation data, yarn damage and failure pictorials were produced. The frames showing the yarn at varying displacements illustrate the yarn failure propagation at varying twist-per-turn values. It is suggested that yarn softening as a function of twist per turn is attributed to higher strains of the outer filaments, at large amounts of twist, than of the core filaments. Previous work has shown a dependency of local filament strength to its yarn radius. The focus of this paper is to derive a comprehensive filament model, using finite element analysis, that incorporates the yarn strain gradient and is experimentally verified.