Callister - An introduction - 8th edition

15.2 Stress–Strain Behavior • 571
Stress (MPa)
60
50
40
30
A
B
10
8
6
4
Stress (10 3 psi)
Figure 15.1 The
stress–strain behavior for
brittle (curve A), plastic
(curve B), and highly
elastic (elastomeric)
(curve C) polymers.
20
10
C
2
elastomer
VMSE
Polymers
0
0
0 1
2 3 4 5 6 7 8
Strain
characterize some of these mechanical parameters. 1 The mechanical characteristics
of polymers, for the most part, are highly sensitive to the rate of deformation (strain
rate), the temperature, and the chemical nature of the environment (the presence
of water, oxygen, organic solvents, etc.). Some modifications of the testing techniques
and specimen configurations used for metals (Chapter 6) are necessary with
polymers, especially for the highly elastic materials, such as rubbers.
Three typically different types of stress–strain behavior are found for polymeric
materials, as represented in Figure 15.1. Curve A illustrates the stress–strain character
for a brittle polymer, inasmuch as it fractures while deforming elastically. The
behavior for a plastic material, curve B, is similar to that for many metallic materials;
the initial deformation is elastic, which is followed by yielding and a region of
plastic deformation. Finally, the deformation displayed by curve C is totally elastic;
this rubberlike elasticity (large recoverable strains produced at low stress levels) is
displayed by a class of polymers termed the elastomers.
Modulus of elasticity (termed tensile modulus or sometimes just modulus for
polymers) and ductility in percent elongation are determined for polymers in the
same manner as for metals (Section 6.6). For plastic polymers (curve B, Figure 15.1),
the yield point is taken as a maximum on the curve, which occurs just beyond the
termination of the linear-elastic region (Figure 15.2). The stress at this maximum is
TS
Figure 15.2 Schematic stress–strain
curve for a plastic polymer showing
how yield and tensile strengths are
determined.
y
Stress
Strain
1 ASTM Standard D 638, “Standard Test Method for Tensile Properties of Plastics.”