Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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7.13 Stress–Strain Behavior • 215

Stress (MPa) Stress (10

3 psi)

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8

6

4

2

0 0

60

50

40

30

20

10

01 234 5678

Strain

10

8

6

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0 0

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B

A

C

01 234 5678

Strain

Figure 7.22 The

stress–strain behavior for

brittle (curveA), plastic

(curveB), and highly

elastic (elastomeric)

(curveC) polymers.

Mechanical Behavior—Polymers

7.13 STRESS–STRAIN BEHAVIOR

The mechanical properties of polymers are specified with many of the same param-

eters that are used for metals—that is, modulus of elasticity, and yield and tensile

strengths. For many polymeric materials, the simple stress–strain test is employed

for the characterization of some of these mechanical parameters.^12 The mechanical

characteristics of polymers, for the most part, are highly sensitive to the rate of defor-

mation (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 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

VMSE

Polymers

materials, as represented in Figure 7.22. CurveAillustrates the stress–strain char-

acter for a brittle polymer, inasmuch as it fractures while deforming elastically. The

behavior for a plastic material, curveB, is similar to that for many metallic materi-

als; the initial deformation is elastic, which is followed by yielding and a region of

plastic deformation. Finally, the deformation displayed by curveCis totally elastic;

this rubber-like elasticity (large recoverable strains produced at low stress levels) is

elastomer displayed by a class of polymers termed theelastomers.

Modulus of elasticity (termedtensile modulusor sometimes justmodulusfor

polymers) and ductility in percent elongation are determined for polymers in the

same manner as for metals (Section 7.6). For plastic polymers (curveB, Figure 7.22),

the yield point is taken as a maximum on the curve, which occurs just beyond the

termination of the linear-elastic region (Figure 7.23). The stress at this maximum is

the yield strength (σy). Furthermore, tensile strength (TS) corresponds to the stress at

which fracture occurs (Figure 7.23);TSmay be greater than or less thanσy. Strength,

for these plastic polymers, is normally taken as tensile strength. Table 7.2 and Tables

B.2, B.3, and B.4 in Appendix B give these mechanical properties for a number of

polymeric materials.

(^12) ASTM Standard D 638, “Standard Test Method for Tensile Properties of Plastics.”