Variation of longitudinal modulus with twist for yarns composed of high modulus fibers

Variations of longitudinal and transverse moduli with twist for twisted yam s and twisted yam-resin composite strands composed of fibers covering a wid e range of tensile moduli are investigated using a sonic pulse propagation method. For twisted yams composed of fibers relatively low in tensile modul us, the longitudinal modulus decreases with increasing twist in accordance with the cos(2)alpha model. For twisted yams composed of high modulus fiber s such as aramid and carbon, however, the reduction of the longitudinal mod ulus with twist is abrupt, and deviates largely from the prediction of the cos(2)alpha model. The relationship between the longitudinal modulus of twi sted yams and the twist angle is derived on the basis of a model that takes into account the anisotropic elasticity of the filaments. The analytical r esults of the proposed model are quantitatively consistent with the experim ental results for various twisted yams and twisted yam-resin composite stra nds.