Chemistry Reference
In-Depth Information
semicrystalline polymer, like polyethylene or a crystallizable polyester or nylon,
the crystallite structure will change during yielding. Initial spherulitic or row
nucleated structures will be disrupted by sliding of crystallites and lamellae, to
yield morphologies like that shown in
Fig. 4.7
.
Yielding and strain hardening are characteristic of some metals as well as
polymers. Polymer behavior differs, however, in two features. One is the tempera-
ture rise that can occur in the necked region as a result of the viscous dissipation
of mechanical energy and orientation-induced crystallization. The other feature is
an increase of the yield stress at higher strain rates. These opposing effects can be
quite significant, especially at the high strain rates characteristic of industrial ori-
entation processes for fibers and films.
4.9.1
Rate and Temperature Effects
Most polymers tend to become more rigid and brittle with increasing straining
rates. In tensile tests, the modulus (initial slope of the stress
strain curve) and
yield stress rise and the elongation at fracture drops as the rate of elongation is
increased.
Figure 4.24
shows typical curves for a polymer that yields. The work
to rupture, which is the area under the stress
strain curve, is a measure of the
toughness of the specimen under the testing conditions. This parameter decreases
at faster extension rates.
INCREASING
RATE OF
EXTENSION
NOMINAL STRAIN
FIGURE 4.24
Effect of strain rate on the tensile stress
strain curve of a polymer which yields at low
straining rates.
