For a continuous filament yarn, external tension
can be applied directly to each of the filaments. But this is not the case in a staple yarn; the tensile load can only be exerted at both ends of the yarn and the stress has to then be transmitted to each of the individual fibers through a so-called “shear lag” mechanism, proposed by Cox,via interfiber friction generated through twist as shown in equations derived by Pan.This stress transfer also occurs in filament yarn during the so-called fragmentation process. Because of this stress transfer process, several issues have to be considered to determine the yarn properties.The staple fibers are assembled into a continuous strand by virtue of twist alone. The tension in the fiber is built up from zero at the fiber ends to the maximum somewhere along the fiber length, ideally at the center. The tension distribution along the fiber length is linear at the portion of fiber length where slippage takes place. But at the portion of fiber tightly gripped through interfiber friction, a hyperbolic tension distribution has been proposed by Pan, as in eq. 1. Other issues, including the distribution of the frictiongenerated shear stress within a yarn, the fiber length effects on yarn properties, and the critical twist level above which a self-locking mechanism is formed so that a staple yarn gains certain strength, were all analyzed previously.However, several complex problems have yet to be solved. First, in all of the existing analyses, a continuous fiber-to-fiber contact in a yarn is assumed. Yet, in a more realistic case, fibers are indiscrete point contact. This will completely alter the distributions of both the tension and shear stress in individual fibers. Also, several competing factors are involved in the prediction of the optimal twist level at which a staple yarn acquires the maximum strength, including the twistout from a bonded structure (nonwoven) and yarn pullout from a woven fabric have been studied by Pan,and Pan and Yoon
theoretically and, for the latter case, also experimentally. For both structures, the fiber–fiber or yarn–yarn interactions are proven to consist of two forms; one is of an adhesive (for nonwoven fabric) or cohesive (for woven fabric) nature and is independent of the pressure at the crossing point, and the other is frictional and is directly related to the normal pressure that can be generated from tensions applied to the system. Yet, the cohesive force for woven fabric has proved to be generally negligible.