Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

3.4 Material Selection 57

they consider the properties of material such as density, ultimate strength, flexibility, machin-

ability, durability, thermal expansion, electrical and thermal conductivity, and resistance to cor-

rosion. They also consider the cost of the material and how easily it can be repaired. Engineers

are always searching for ways to use advanced materials to make products lighter and stronger

for different applications.

In Chapter 17, we will look more closely at materials that commonly are used in various

engineering applications. We will also discuss some of the basic physical characteristics of

materials that are considered in design. We will examine the application and properties of

common solid materials; such as metals and their alloys, plastics, glass, and wood and those

that solidify over time; such as concrete. We will also investigate in more detail basic fluids;

such as air and water.

By now, it should be clear that material properties and material cost are important

design factors. In general, the properties of a material may be divided into three groups:

electrical, mechanical, and thermal. In electrical and electronic applications, for example,

the electrical resistivity of materials is important. How much resistance to flow of elec-

tricity does the material offer? In many mechanical, civil, and aerospace engineering appli-

cations, the mechanical properties of materials are important. These properties include the

modulus of elasticity, modulus of rigidity, tensile strength, compression strength, strength-

to-weight ratio, modulus of resilience, and modulus of toughness. In applications dealing

with fluids (liquids and gases), thermophysical properties such as density, thermal con-

ductivity, heat capacity, viscosity, vapor pressure, and compressibility are important prop-

erties. Thermal expansion of a material, whether solid or fluid, is also an important design

factor. Resistance to corrosion is another important factor that must be considered when

selecting materials.

Material properties depend on many factors, including how the material was processed,

its age, its exact chemical composition, and any nonhomogenity or defect within the mate-

rial. Material properties also change with temperature and time as the material ages. Most

companies that sell materials will provide, upon request, information on the important prop-

erties of their manufactured materials. Keep in mind that when practicing as an engineer,

you should use the manufacturers’ material property values in your design calculations. The

property values given in this and other textbooks should be used as typical values — not as

exact values.

In the upcoming chapters, we will explain in detail the properties of materials and what

they mean. For the sake of continuity of presentation, a summary of important material prop-

erties follows.

Electrical Resistivity— The value of electrical resistivity is a measure of resistance of material

to flow of electricity. For example, plastics and ceramics typically have high resistivity,

whereas metals typically have low resistivity, and among the best conductors of electricity

are silver and copper.

Density—Density is defined as mass per unit volume; it is a measure of how compact the

material is for a given volume. For example, the average density of aluminum alloys is

2700 kg /m

3

; when compared to steel density of 7850 kg /m

3

, aluminum has a density that

is approximately one third of the density of steel.

Modulus of Elasticity (Young’s Modulus)—Modulus of elasticity is a measure of how eas-

ily a material will stretch when pulled (subject to a tensile force) or how well the material

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