Page 517 - Handbook of Properties of Textile and Technical Fibres

490 Handbook of Properties of Textile and Technical Fibres

microfibrils and fibrils, where the axial material connection by taut tie molecules with
the rest of the sample is interrupted. Hence, the failure of the material is initiated at the
ends of microfibrils. Here the usual axial connection by a great number of TTM is
interrupted and the amorphous layer exhibits a much smaller elastic modulus than
the normal amorphous phase. Very likely this effect is preceded by the opening of a
microcrack at the free end, which requires at least the rupture of a smaller number
of chains than in a normal cross-section of a microfibril. The rupture of an adjacent
microfibril leads to a microcrack extension in the lateral direction. Since the majority
of free ends are located in the outer boundary of the fibrils, the microcracks formed by
opening the point defects at the ends of microfibrils can coalesce by longitudinal
growth along this boundary (Peterlin, 1975). Interestingly, the ruptured molecules
yield free radicals that can be monitored by electron spin resonance. The microcracks
or microvoids can be visualized by a special staining technique using OsO 4 (Galeski
et al., 1989). As soon as one of the cracks reaches critical dimensions the catastrophic
crack propagation leads to macroscopic failure (Peterlin, 1978b).
Fracture studies on semicrystalline PET were published by Ward and coworkers
(Stearne and Ward, 1969; Foot and Ward, 1972), who observed that in semicrystalline
PET, fracture occurred by inherent flaws between amorphous and crystalline bound-
aries. Pecorini and Hertzberg (Pecorini and Hertzberg, 1993) studied the effect of
annealing and drying conditions on the fracture and fatigue behavior of PET and corre-
lated their results qualitatively to the changes in the tie molecule density.
Gupta et al. (1993) monitored the structural changes occurring during elongation up
to fracture of uniaxially drawn, heat-set PET film samples by rapid scanning Fourier
transform infrared spectroscopy, and identified chain uncoiling (lamellar separation)
in the amorphous regions and longitudinal slip in the crystalline regions as the main
deformation mechanisms in these materials. More recently, Van Den Neuvel et al.
(1993) identified molecular uncoiling by gauche-trans transitions as the major defor-
mation mechanism when a technical PET yarn was axially stretched.
The tensile failure of PET fibers at slow rates of deformation can be decomposed to
the two distinct stages (Lechat et al., 2006; Le Clerc et al., 2006):
• Initiation on (or near) the surface of the fiber, followed by slow growth of crack normal to the
fiber axis with plastic deformation leading to crack opening (“V-crack”)
• Quick catastrophic failure of the remaining area perpendicular to the fiber axis.
In Fig. 13.30 the breaking zone of nonannealed and annealed recycled PET fiber is
shown (Vaní cek and Militký, 2006).
The V notch is visible in the breaking zone of annealed sample. Walker et al. (1979)
found that extension of ductile thermoplastic sheets leads to either V notches or char-
acteristic “diamond” cavities. Both features showed stable growth until a fracture.
Under fast rates of deformation, using a pendulum, the broken ends left behind swell
and have a mushroom shape. Hearle and coworkers explained the appearance of this
shape by localized adiabatic deformation (Hearle et al., 1998). Under specific cyclic
loading conditions, quite different breaking zone morphology has been observed
(Lechat et al., 2006).