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

GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51

2nd Revised Pages

Summary • 281

Concept Check 8.9

For the following pair of polymers, plot and label schematic stress–strain curves on

the same graph.

• Poly(styrene-butadiene) random copolymer having a number-average molecular
  weight of 100,000 g/mol and 10% of the available sites crosslinked and tested at
  20 ◦C


• Poly(styrene-butadiene) random copolymer having a number-average molecu-
  lar weight of 120,000 g/mol and 15% of the available sites crosslinked and tested
  at –85◦C.Hint: poly(styrene-butadiene) copolymers may exhibit elastomeric be-
  havior.

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

Concept Check 8.10

In terms of molecular structure, explain why phenol-formaldehyde (Bakelite) will

not be an elastomer. (The molecular structure for phenol-formaldehyde is presented

in Table 4.3.)

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

SUMMARY

Basic Concepts

Slip Systems

On a microscopic level, plastic deformation corresponds to the motion of dislocations

in response to an externally applied shear stress, a process termed “slip.” Slip occurs

on specific crystallographic planes and within these planes only in certain directions.

A slip system represents a slip plane–slip direction combination, and operable slip

systems depend on the crystal structure of the material.

Slip in Single Crystals

The critical resolved shear stress is the minimum shear stress required to initiate dis-

location motion; the yield strength of a single crystal depends on both the magnitude

of the critical resolved shear stress and the orientation of slip components relative

to the direction of the applied stress.

Plastic Deformation of Polycrystalline Metals

For polycrystalline materials, slip occurs within each grain along the slip systems that

are most favorably oriented with the applied stress; furthermore, during deformation,

grains change shape in such a manner that coherency at the grain boundaries is

maintained.