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

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2nd Revised Pages

References • 283

The mechanism of plastic deformation for spherulitic polymers was also pre-

sented. Tensile deformation occurs in several stages as both amorphous tie chains and

chain-folded block segments (which separate from the ribbon-like lamellae) become

oriented with the tensile axis. Also, during deformation the shapes of spherulites are

altered (for moderate deformations); relatively large degrees of deformation lead to

a complete destruction of the spherulites to form highly aligned structures. Further-

more, the predeformed spherulitic structure and macroscopic shape may be partially

restored by annealing at an elevated temperature below the polymer’s melting tem-

perature.

Factors That Influence the Mechanical Properties of Semicrystalline Polymers

The mechanical behavior of a polymer will be influenced by both in-service and

structural/processing factors. With regard to the former, increasing the temperature

and/or diminishing the strain rate leads to reductions in tensile modulus and ten-

sile strength and an enhancement of ductility. In addition, other factors that affect

the mechanical properties include molecular weight, degree of crystallinity, prede-

formation drawing, and heat treating. The influence of each of these factors was

discussed.

Deformation of Elastomers

Large elastic extensions are possible for the elastomeric materials that are amorphous

and lightly crosslinked. Deformation corresponds to the unkinking and uncoiling of

chains in response to an applied tensile stress. Crosslinking is often achieved during

a vulcanization process.

IMPORTANT TERMS AND CONCEPTS

Cold working

Critical resolved shear stress

Dislocation density

Drawing

Grain growth

Lattice strain

Recovery

Recrystallization

Recrystallization temperature

Resolved shear stress

Slip

Slip system

Solid-solution strengthening

Strain hardening

Viscosity

Vulcanization

REFERENCES

Hirth, J. P. and J. Lothe,Theory of Dislocations, 2nd

edition, Wiley-Interscience, New York, 1982.

Reprinted by Krieger, Melbourne, FL, 1992.

Hull, D.,Introduction to Dislocations,3rd edition,

Butterworth-Heinemann, Woburn, UK, 1984.

Kingery, W. D., H. K. Bowen, and D. R. Uhlmann,

Introduction to Ceramics, 2nd edition, John

Wiley & Sons, New York, 1976. Chapter 14.

Read, W. T., Jr.,Dislocations in Crystals, McGraw-

Hill, New York, 1953.

Richerson, D. W.,Modern Ceramic Engineering,

3rd edition, CRC Press, Boca Raton, FL,

2006.

Schultz, J.,Polymer Materials Science, Prentice Hall

PTR, Paramus, NJ, 1974.

Weertman, J. and J. R. Weertman,Elementary Dis-

location Theory, Macmillan, New York, 1964.

Reprinted by Oxford University Press, New

York, 1992.