Page 198 - Materials Science and Engineering An Introduction

170 • Chapter 6 / Mechanical Properties of Metals

and publication of these standards are often coordinated by professional societies. In
the United States the most active organization is the American Society for Testing and
Materials (ASTM). Its Annual Book of ASTM Standards (http://www.astm.org) com-
prises numerous volumes that are issued and updated yearly; a large number of these
standards relate to mechanical testing techniques. Several of these are referenced by
footnote in this and subsequent chapters.
The role of structural engineers is to determine stresses and stress distributions
within members that are subjected to well-defined loads. This may be accomplished
by experimental testing techniques and/or by theoretical and mathematical stress
analyses. These topics are treated in traditional texts on stress analysis and strength
of materials.
Materials and metallurgical engineers, however, are concerned with producing and
fabricating materials to meet service requirements as predicted by these stress analyses.
This necessarily involves an understanding of the relationships between the microstruc-
ture (i.e., internal features) of materials and their mechanical properties.
Materials are frequently chosen for structural applications because they have
desirable combinations of mechanical characteristics. The present discussion is
confined primarily to the mechanical behavior of metals; polymers and ceramics
are treated separately because they are, to a large degree, mechanically different
from metals. This chapter discusses the stress–strain behavior of metals and the
related mechanical properties, and also examines other important mechanical char-
acteristics. Discussions of the microscopic aspects of deformation mechanisms and
methods to strengthen and regulate the mechanical behavior of metals are deferred
to later chapters.

6.2 CONCEPTS OF STRESS AND STRAIN
If a load is static or changes relatively slowly with time and is applied uniformly over
a cross section or surface of a member, the mechanical behavior may be ascertained
by a simple stress–strain test; these are most commonly conducted for metals at
room temperature. There are three principal ways in which a load may be applied:
namely, tension, compression, and shear (Figures 6.1a, b, c). In engineering practice
many loads are torsional rather than pure shear; this type of loading is illustrated in
Figure 6.1d.

Tension Tests 1
One of the most common mechanical stress–strain tests is performed in tension. As
will be seen, the tension test can be used to ascertain several mechanical properties
of materials that are important in design. A specimen is deformed, usually to frac-
ture, with a gradually increasing tensile load that is applied uniaxially along the long
axis of a specimen. A standard tensile specimen is shown in Figure 6.2. Normally,
the cross section is circular, but rectangular specimens are also used. This “dogbone”
specimen configuration was chosen so that, during testing, deformation is confined
to the narrow center region (which has a uniform cross section along its length) and
also to reduce the likelihood of fracture at the ends of the specimen. The standard
diameter is approximately 12.8 mm (0.5 in.), whereas the reduced section length
should be at least four times this diameter; 60 mm (2 in.) is common. Gauge length
1
4
is used in ductility computations, as discussed in Section 6.6; the standard value is
50 mm (2.0 in.). The specimen is mounted by its ends into the holding grips of the

1 ASTM Standards E8 and E8M, “Standard Test Methods for Tension Testing of Metallic Materials.”