ABSTRACT
The most common type of stress-strain tests is that in which the response (strain) of a sample subjected to a force that increases with time, at constant rate, is measured. The shape of the stress-strain curves is used to define ductile and brittle behavior. Since the mechanical properties of polymers depend on both temperature and observation time, the shape of the stressstrain curves changes with the strain rate and temperature. Figure I4.l illustrates different types of stress-strain curves. The curves for hard and brittle polymers (Fig. I4.1a) show that the stress increases more or less linearly with the strain. This behavior is characteristic of amorphous poly-
mers at temperatures well below the glass transition temperature (T Tg); these materials (e.g., polystyrene, Tg ^ 100°C) fail at low strains, leading to brittle fracture at room temperature. Semicrystalline polymers and thermo set resins at T Tg also exhibit the pattern shown in Figure 14.la. The curve in Figure 14.1b represents polymers showing a ductile behavior that yields before failure. The most ductile polymers undergo necking and cold drawing. Semicrystalline polymers are typical examples that display this behavior at temperatures intermediate between melting and glass transition (Tg < T < Tm) (e.g., polyethylene at room temperature). The curves in Figure 14.1c are characteristic of elastomers (T > Tg) (see Chapter 3). An inspection of the curves of Figure 14.1 shows that the brittle behavior is that displayed by a sample that fails due to fracture at the maximum stress with relatively small strains (< 10%), while ductile samples display a maximum in the stress, failing at higher strains. Although very ductile plastics, like polyethylene, can reach strains of up to 250% prior to final failure, some polymers fail immediately after yielding.
