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

GTBL042-07 GTBL042-Callister-v2 August 6, 2007 12:43

216 • Chapter 7 / Mechanical Properties

Stress

Strain

TS

y

Figure 7.23 Schematic stress–strain

curve for a plastic polymer showing

how yield and tensile strengths are

determined.

Polymers are, in many respects, mechanically dissimilar to metals and ceramic

materials (Figures 1.4, 1.5, and 1.6). For example, the modulus for highly elastic

polymeric materials may be as low as 7 MPa (10^3 psi), but may run as high as 4 GPa

(0.6× 106 psi) for some of the very stiff polymers; modulus values for metals are

much larger (Table 7.1). Maximum tensile strengths for polymers are about 100 MPa

(15,000 psi)—for some metal alloys they are 4100 MPa (600,000 psi). And, whereas

metals rarely elongate plastically to more than 100%, some highly elastic polymers

may experience elongations to greater than 1000%.

In addition, the mechanical characteristics of polymers are much more sensi-

tive to temperature changes near room temperature. Consider the stress–strain be-

havior for poly(methyl methacrylate) at several temperatures between 4 and 60◦C

(40 and 140◦F) (Figure 7.24). It should be noted that increasing the temperature

Strain

Stress (MPa) Stress (10

3 psi)

0

0

0.1 0.2 0.3

10

20

30

40

50

60

70

80

0

2

4

6

8

10

12

4 °C (40°F)

20 °C (68°F)

30 °C (86°F)

40 °C (104°F)

50 °C (122°F)

60 °C (140°F)

To 1.30

Figure 7.24 The influence of temperature on the stress–strain characteristics of

poly(methyl methacrylate). (From T. S. Carswell and H. K. Nason, “Effect of Environmental

Conditions on the Mechanical Properties of Organic Plastics,”Symposium on Plastics,

American Society for Testing and Materials, Philadelphia, 1944. Copyright, ASTM, 1916

Race Street, Philadelphia, PA 19103. Reprinted with permission.)