1 - Structure and mechanics of yarns

Abstract

In classic terms, yarn can be defined as a long continuous and very flexible fibrous strand that can be mechanically manipulated in many ways to form a wide variety of textile structures including woven fabrics, knit fabrics, braids, ropes, and cords. In this chapter, the focus will be on yarn structure and mechanics. However, the objective is not to review the classic aspects of yarn structure or yarn mechanics, which have been discussed in numerous previous literatures including many classic books that are still used as reliable references of the subject. Instead, the focus will be on key practical aspects that are highly beneficial to the manufacturers and the designers of textile yarns. The chapter is divided into three main sections: (1) yarn classification, (2) yarn structure, and (3) yarn mechanics. The first section provides a review of different types of yarns. The point of this classification is to emphasize the fact that textile yarns do not require any external adhesive materials or chemicals to build their structures; they are essentially interfibrous-bonded structures. It is for this reason that textile yarns are flexible enough to be manipulated into different structural shapes. Another reason for classifying yarns is that there is no unique or unified method to describe the structures of all yarns. Different yarns may look approximately the same in an end product. They may also have the same thickness or yarn count, but their structural features will certainly be different. These differences determine the mechanical behaviors of different yarns and consequently their contribution to the fabric quality. The second section emphasizes three key components of yarn structure: fiber compactness, fiber arrangement, and fiber mobility. These components are discussed in relation to the different spinning systems used to produce yarns including extrusion spinning of continuous-filament yarns, and staple-fiber yarn spinning methods such as ring spinning, compact spinning, open-end (rotor) spinning, air-jet spinning, and friction spinning. Values of new measures of yarn structures are introduced. These include yarn density and yarn circularity. The third section deals with yarn mechanics with a focus on the tensile behavior of yarn, which is the most commonly considered mechanical parameter of textile yarns. Deviations from the classic theory of yarn mechanics are discussed and challenges facing the empirical prediction of yarn strength are addressed. In practice, yarn mechanics is an important subject in the context of two key objectives: (a) estimating yarn strength and (b) predicting yarn strength prior to making the yarn. Obstacles facing meeting these objectives and ways to overcome these obstacles are also addressed. These include technological obstacles and analytical obstacles.