Page 486 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 459

more complex and besides the amorphous and crystalline portions mechanical
behavior is strongly influenced by the presence of the TTM fraction.
Some of the concepts useful for amorphous polymers can be to some extent applied
to semicrystalline polymers (Bowden and Young, 1974). The multiphase structure of
these polymers often requires more complex models in which microstructural aspects,
such as tie chains, degree of crystallinity, and orientation are included.
In the first part of this chapter the basic types of mechanical behavior models of
fibers have been compared. For a description of stress-strain curves the parametric
and nonparametric models are shown. The second part is devoted to the modeling
of tensile strength and failure mechanisms of polyester fibers.

13.4.1 Models of mechanical behavior of fibers
The modeling and interpretation of mechanical behavior of polymeric fibers require
the creation of models characterizing at least the relationships between deformation,
stress, and time or temperature, respectively (phenomenological models). More com-
plex constitutive and multiphase models are based on the simplified assumption about
deformation of some structural elements (phases). Structural models are based on the
creation of a simplified fiber structure and description of its response to mechanical
actions. In polymer fibers modeling is complicated by the fact that:

• Validity of the linear viscoelastic behavior is very limited (strain limit for polymers in the
glassy state is less than 1%). Most experiments are naturally beyond this limit. Parameters
from classical viscoelastic models can no longer be considered as material constants and
Boltzman superposition law is not valid.
• Permanent structural changes of the fibers due to external forces occur. There are not only
changes in orientation but frequent changes of the various phases portions (crystalline phase,
the phase of taut tie chains). These structural changes obviously provoke changes in mechan-
ical behavior (see, for example, strain hardening, etc.).
• Polymeric fibers have a long deformation/temperature history, which is due to a “memory”
effect to some extent reflected in the mechanical behavior.
• Models comprising microfibrils containing alternate Structural changes, which are reflected
in the mechanical (viscoelastic) behavior of fibers, that are often not directly experimentally
measurable and can only be estimated. This leads to a situation where mechanical models
include a redundant number of parameters without proper physical interpretation.
It is thus clear that the mechanical description of nonlinear viscoelastic semicrystal-
line materials (including fibers) will always be an approximation of real processes.
According to this approximation, the mechanical models can be formally divided
into four basic categories:
1. Continuum models
2. Micromechanical constitutive models
3. Structural models
4. Multiphase models

These models are distinguished by the extent to which the fiber structure is taken
into consideration. For direct treatment of experimental data the models from group