Page 315 - Fiber Fracture

FRACTURE OF SYNTHETIC POLYMER FIBERS 297

Strain
Fig 9. Two possible routes for improving the tensile strength of a fiber with stress-strain curve indicated hy
the heavy line.

0 10 20
Average Misorientation Angle
Fig. IO. Calculated dependence of modulus on average misorientation angle of the macromolecules with
respect to the fiber axis.

Ultimate Tensile Strength of Oriented Fiber

In the present section, we will assume that the network of polymer chains has been
successfully drawn to a large value of the draw ratio, leading to a high degree of
orientation of the macromolecules along the fiber axis, see Fig. lb. We now focus on
how to further improve the mechanical properties of that fiber. To this end, let us assume
that the latter has a stress-strain curve as represented by the heavy line in Fig. 9. There
are two basic routes for improving tenacity. Route (1) consists in increasing the modulus
of the fiber, i.e. further improving the molecular orientation of the polymer chains.
In Route (2), the modulus stays constant but the elongation at break is increased by
removing the defects in the fiber.
Let us start by investigating the usefulness of Route (1). Fig. 10 shows model
predictions for the dependence of the fiber modulus on the average misorientation angle