1 Structural materials

1.1. Classification and General Properties of Structural Materials

Structural or engineering materials are materials used to manufacture parts of structures, machines and devices. Structures are the objects without moving parts erected by engineers, such as a concrete dam, a steel melting furnace, a suspension bridge and an oil refinery tower. Machines include lathes, steam and gas turbines, engines, electric motors and generators. Devices are the most recent addition to engineering materials and refer to such innovations as a transistor, a photoelectric cell, piezoelectric pressure gauges, ceramic magnets, lasers, transformers, voltmeters, etc.

All structural materials fall into two large classes: metals and non-metals. In their turn metallic materials are divided into two groups: ferrous and non-ferrous metals or alloys. The examples of ferrous metals are iron, manganese and chrome. Ferrous alloys are the combination of more than one metal, or metals and non-metals. Ferrous metals and alloys are divided into steels and cast irons (Fig. 1.1). The rest of metals (copper, aluminium, titanium, nickel, silver, gold, etc.) are referred to as non-ferrous ones and used both in pure state and as alloys. Non-metallic materials fall in two groups: organic (wood, leather, plastic, rubber, etc.) and non-organic (ceramics, glass, concrete, graphite, etc.) ones. Organic materials are polymers whose molecules consist of many repeating links.

Another way of classifying the engineering materials is based on their nature:

• metals and alloys;

• ceramics and glasses;

• organic polymers.

Metals and alloys possess characteristic appearance. In addition to their capability of changing permanently their shape, metals and alloys have good thermal and electrical conductivity. Ceramics and glasses are nonmetallic inorganic substances, which are brittle and have good thermal and electrical insulating properties. Organic polymers are relatively inert and light and generally have a high degree of plasticity. Fig. 1.2 lists typical examples from each of these three groups of materials. In addition, a number of examples of materials, which are composites, made up of two groups also shown.

To manufacture any machine, device, or part of a machine an engineer must know properties of structural materials. There are several kinds of properties of structural materials such as physical, chemical, mechanical, technological and service ones.

The physical properties include mass density, melting point, heat and electric conductivity, thermal expansion, magnetic saturation and permeability, etc.

Fig. 1.1. Classification of structural materials

The chemical properties of materials determine their ability to resist moist air, acids and other aggressive media. Hence, corrosion and oxidation resistance are one the main chemical properties.

Structural materials must possess sufficiently high technological properties such as machinability, malleability, weldability, foundry properties (fluidity, shrinkage, etc.) Thus, the technological properties indicate how much labor, machine time, tools and materials are needed to produce a machine part.