In general, the mechanical and thermophysical properties of a material depend on its phase.
For example, as you know from your everyday experience, the density of ice is different from
liquid water (ice cubes float in liquid water), and the density of liquid water is different from
that of steam. Moreover, the properties of a material in a single phase could depend on its tem-
perature and the surrounding pressure. For example, if you were to look up the density of liq-
uid water in the temperature range of, say, 4to 100C, under standard atmospheric pressure,
you would find that its density decreases with increasing temperature in that range. Therefore,
properties of materials depend not only on their phase but also on their temperature and pres-
sure. This is another important fact to keep in mind when selecting materials.17.2 Electrical, Mechanical, and Thermophysical Properties of Materials
Properties of Materials
As we have been explaining up to this point, when selecting a material for an application, as an
engineer you need to consider a number of material properties. 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 the flow of electricity does the material offer? In many mechanical, civil, and
aerospace engineering applications, the mechanical properties of materials are important. These
properties include modulus of elasticity, modulus of rigidity, tensile strength, compression
strength, the strength-to-weight ratio, modulus of resilience, and modulus of toughness. In
applications dealing with fluids (liquids and gases), thermophysical properties such as ther-
mal conductivity, heat capacity, viscosity, vapor pressure, and compressibility are important
properties. 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 select-
ing materials.
Material properties depend on many factors, including how the material was processed,
its age, its exact chemical composition, and any nonhomogeneity or defect within the material.
Material properties also change with temperature and time as the material ages. Most com-
panies that sell materials will provide upon request information on the important properties
of their manufactured materials. Keep in mind that when practicing as an engineer, you
should use the manufacturer’s material property values in your design calculations. The prop-
erty values given in this and other textbooks should be used as typical values — not as exact
values.
In the previous chapters, we have explained what some properties of materials mean. The
meaning of those properties and other properties that we have not explained already are sum-
marized next.Electrical Resistivity The value of electrical resistivity is a measure of the resistance of mate-
rial to the 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.17.2 Electrical, Mechanical, and Thermophysical Properties of Materials 555
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