Page 197 - Materials Science and Engineering An Introduction

WHY STUDY The Mechanical Properties of Metals?

It is incumbent on engineers to understand how the materials such that unacceptable levels of deformation
various mechanical properties are measured and what and/or failure will not occur. We demonstrate this
these properties represent; they may be called upon procedure with respect to the design of a tensile-
to design structures/components using predetermined testing apparatus in Design Example 6.1.

Learning Objectives
After studying this chapter, you should be able to do the following:
1. Define engineering stress and engineering 7. Give brief definitions of and the units for
strain. modulus of resilience and toughness (static).
2. State Hooke’s law and note the conditions 8. For a specimen being loaded in tension, given
under which it is valid. the applied load, the instantaneous cross-
3. Define Poisson’s ratio. sectional dimensions, and original and instanta-
4. Given an engineering stress–strain diagram, neous lengths, be able to compute true stress
determine (a) the modulus of elasticity, and true strain values.
(b) the yield strength (0.002 strain offset), 9. Name the two most common hardness-testing
and (c) the tensile strength and (d) estimate techniques; note two differences between
the percentage elongation. them.
5. For the tensile deformation of a ductile cylindrical 10. (a) Name and briefly describe the two different
specimen, describe changes in specimen profile to microindentation hardness testing techniques,
the point of fracture. and (b) cite situations for which these tech-
6. Compute ductility in terms of both percentage niques are generally used.
elongation and percentage reduction of area for 11. Compute the working stress for a ductile
a material that is loaded in tension to fracture. material.

6.1 INTRODUCTION

Many materials are subjected to forces or loads when in service; examples include
the aluminum alloy from which an airplane wing is constructed and the steel in an
automobile axle. In such situations it is necessary to know the characteristics of the
material and to design the member from which it is made such that any resulting de-
formation will not be excessive and fracture will not occur. The mechanical behavior
of a material reflects its response or deformation in relation to an applied load or
force. Key mechanical design properties are stiffness, strength, hardness, ductility,
and toughness.
The mechanical properties of materials are ascertained by performing carefully de-
signed laboratory experiments that replicate as nearly as possible the service conditions.
Factors to be considered include the nature of the applied load and its duration, as well
as the environmental conditions. It is possible for the load to be tensile, compressive, or
shear, and its magnitude may be constant with time, or it may fluctuate continuously.
Application time may be only a fraction of a second, or it may extend over a period of
many years. Service temperature may be an important factor.
Mechanical properties are of concern to a variety of parties (e.g., producers and
consumers of materials, research organizations, government agencies) that have dif-
fering interests. Consequently, it is imperative that there be some consistency in the
manner in which tests are conducted and in the interpretation of their results. This
consistency is accomplished by using standardized testing techniques. Establishment

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