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Man-made fibers must be drawn to orient their constituent molecules into an orderly arrangement parallel to the fiber axis, and this leads to higher tenacity. The greater the draw ratio, the greater the degree of molecular orientation (i.e. crystallization), and   higher the fiber tenacity. Most man-made fibers must be drawn to some degree to give them adequate tenacity to fit them for commercial applications, but that having been said, there is still room for producing a range of fibers of different tenacities from the same material to suit different end-use. For example, engineering uses require fibers of higher tenacity than normally needed for apparel fabrics or carpets. If one uses the analogy of yarn in which fiber are parallel to one another (Staple fiber yarns) and those in which fibers are very much crumpled and distorted (textured yarns) , it is not difficult to see why drawn, and therefore strong fibers with good molecular orientation are much less extensible than u...

The role of twist in yarns and the part it plays in the design of textile structures .He discusses the obvious necessity of twist in the natural and staple fibers by pointing out“ Twist is essential to provide a certain minimum coherence between fibers, without a yarn having a significant tensile strength cannot be made. This coherence is dependent on the frictional forces brought into play by the lateral pressures between fibers arising from the application of a tensile stress along the yarn axis. With the introduction of continuous filament yarns, however, the role of twist must be reconsidered. In continuous filament yarns, twist is not necessary for the attainment of tensile strength (in fact, it reduces it) but it is necessary for the achievement of satisfactory resistance to abrasion, fatigue, or other types of damage associated with stresses other than a simple tensile stress, and typified by the breakage of individual filaments, leading ultimately to total breakdown of th...