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

GTBL042-07 GTBL042-Callister-v2 August 9, 2007 13:52

222 • Chapter 7 / Mechanical Properties

as for metals (Chapter 9); that is, a stress (normally tensile) is applied instantaneously

and is maintained at a constant level while strain is measured as a function of time.

Furthermore, the tests are performed under isothermal conditions. Creep results are

represented as a time-dependentcreep modulus Ec(t), defined by^13

Ec(t)=

σ 0

(t)

(7.24)

whereinσ 0 is the constant applied stress and(t) is the time-dependent strain. The

creep modulus is also temperature sensitive and diminishes with increasing temper-

ature.

With regard to the influence of molecular structure on the creep characteristics,

as a general rule the susceptibility to creep decreases [i.e.,Ec(t) increases] as the

degree of crystallinity increases.

Concept Check 7.4

Cite the primary differences among elastic, anelastic, viscoelastic, and plastic defor-

mation behaviors.

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

Concept Check 7.5

An amorphous polystyrene that is deformed at 120◦C will exhibit which of the be-

haviors shown in Figure 7.26?

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

Hardness and Other Mechanical

Property Considerations

7.16 HARDNESS

hardness Another mechanical property that may be important to consider ishardness,which

is a measure of a material’s resistance to localized plastic deformation (e.g., a small

dent or a scratch). Early hardness tests were based on natural minerals with a scale

constructed solely on the ability of one material to scratch another that was softer. A

qualitative and somewhat arbitrary hardness indexing scheme was devised, termed

the Mohs scale, which ranged from 1 on the soft end for talc to 10 for diamond. Quan-

titative hardness techniques have been developed over the years in which a small

indenter is forced into the surface of a material to be tested, under controlled condi-

tions of load and rate of application. The depth or size of the resulting indentation

is measured, which in turn is related to a hardness number; the softer the material,

(^13) Creep compliance,Jc(t), the reciprocal of the creep modulus, is also sometimes used in this
context.