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

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7.14 Macroscopic Deformation • 217

produces (1) a decrease in elastic modulus, (2) a reduction in tensile strength, and

(3) an enhancement of ductility—at 4◦C (40◦F) the material is totally brittle, while

there is considerable plastic deformation at both 50 and 60◦C (122 and 140◦F).

The influence of strain rate on the mechanical behavior may also be important.

In general, decreasing the rate of deformation has the same influence on the stress–

strain characteristics as increasing the temperature: that is, the material becomes

softer and more ductile.

7.14 MACROSCOPIC DEFORMATION

Some aspects of the macroscopic deformation of semicrystalline polymers deserve

our attention. The tensile stress–strain curve for a semicrystalline material, which

VMSE

Polymers

was initially undeformed, is shown in Figure 7.25; also included in the figure are

schematic representations of the specimen profiles at various stages of deformation.

Both upper and lower yield points are evident on the curve, which are followed by a

near-horizontal region. At the upper yield point, a small neck forms within the gauge

section of the specimen. Within this neck, the chains become oriented (i.e., chain axes

become aligned parallel to the elongation direction, a condition that is represented

schematically in Figure 8.28d), which leads to localized strengthening. Consequently,

there is a resistance to continued deformation at this point, and specimen elongation

proceeds by the propagation of this neck region along the gauge length; the chain-

orientation phenomenon (Figure 8.28d) accompanies this neck extension. This tensile

behavior may be contrasted to that found for ductile metals (Section 7.6), wherein

once a neck has formed, all subsequent deformation is confined to within the neck

region.

Concept Check 7.3

When citing the ductility as percent elongation for semicrystalline polymers, it is not

necessary to specify the specimen gauge length, as is the case with metals. Why is this

so?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

Stress

Strain

Figure 7.25 Schematic tensile

stress–strain curve for a

semicrystalline polymer. Specimen

contours at several stages of

deformation are included. (From

Jerold M. Schultz,Polymer Materials

Science,copyright©c1974, p. 488.

Reprinted by permission of Prentice

Hall, Inc., Englewood Cliffs, NJ.)