Page 104 - Mechanical Behavior of Materials

Section 3.8 Materials Selection for Engineering Components 105

2500

(a) 100 (b)
2000 Composites
Tensile Strength, MPa 1500 Steels Al alloys Composites Strength/Weight, km 75

50
1000

500 Ti alloys Mg alloys 25 Mg
Steels Ti Al

0 0

Figure 3.28 Comparison of strength for various classes of structural metals and polymer
matrix composites, showing ranges for (a) tensile strength, and (b) tensile strength per
unit weight. (Data from [Farag 89] pp. 174–176.)

laminate (ARALL) has layers of an aluminum alloy and a composite with unidirectional Kevlar
fibers in an epoxy matrix. See Fig. 3.26(b).
Where stiffness in bending is needed along with light weight, layers of a strong and stiff material
may be placed on either side of a lightweight core. Such sandwich materials include aluminum
or fibrous composite sheets bonded on each side of a core that is made of a stiff foam. Another
possibility is a core made of a honeycomb of aluminum or other material.

3.8 MATERIALS SELECTION FOR ENGINEERING COMPONENTS

An engineering component, such as a beam, shaft, tension member, column, or machine part, must
not deform excessively or fail by fracture or collapse. At the same time, the cost and often the weight
must not be excessive. The most basic consideration in avoiding excessive deformation is to limit
the deflection due to elastic strain. For a given component geometry and applied load, the resistance
to elastic deflection—that is, the stiffness—is determined by the elastic modulus E of the material.
As to strength, the most basic requirement is to avoid having the stress exceed the failure strength
of the material, such as the yield strength σ o from a tension test.
Consider the general situation in which an engineering component must meet one or more
requirements related to its performance, such as a maximum permissible deflection and/or a
given safety factor against yielding in the material. Further, assume that any of several candidate
materials may be chosen. It is often possible in such situations to perform a systematic analysis
that will provide a ranking of materials for each performance requirement, thus providing an
organized framework for making the final choice. Such methodology will be introduced in
this section.
Before we proceed, note that materials properties such as the elastic modulus and yield strength
will be considered in detail in the next chapter from the viewpoint of obtaining their values from