Fiber interactions in a twisted fiber structure under tension

This paper deals with the critical role fiber interactions play in determining the strength of twisted fiber structures such as yarns and ropes made of continuous filaments. Fiber interactions can be viewed from a wider perspective. First, the fact that the strength of a fiber bundle is in general lower than the fiber strength implies the influence among fibers through load sharing during the fracture of the fiber bundle, which reduces the strength of the bundle as a whole. On the other hand, the fiber fragmentation phenomenon taking place in a fiber structure, which is also attributed to the fiber interactions, has been proved to yield a synergetic effect in system strength. The fragmentation only occurs in a twisted structure in which lateral pressure generated by the tensile loading leads to multiple breaks in individual filament via inter-fiber friction. The ultimate strength of the system is determined by the minimum length (also called the critical length) of the filament fragments which in turn is controlled by many factors including the twist level and filament properties. In all of previous analysis, the critical length used is the value derived under a normally implicit assumption that the fragmentation process has reached the saturated stage. As a result, for a given twisted structure, this critical length will be constant, and the strength of the system thus becomes length-independent. The author has found that in the real cases, there are many variables which affect the fragmentation process adversely so that a saturated fragmentation state is rarely realized, hence resulting the strength-length dependence experimentally observed