[1]All products that come out of industry consist of at least one-and often many-types of materials. The most obvious example is the automobile. A car contains a wide variety of materials, ranging from glass to steel to rubber, plus numerous other metals and plastics.

[2]The number of materials which are available to the engineer in industry is almost infinite. The various compositions of steel almost run into the thousands. It has been said that there are more than 10,000 varieties of glass, and the numbers of plastics are equally great. In addition, several hundred new varieties of materials appear on the market each month. This means that individual engineers and technicians cannot hope to be familiar which all the properties of all types of materials in their numerous forms. All he can do is try to learn some principles to guide him in the selection and processing of materials.

[3]The properties of a material originates from the internal structure of that material. This is analogous to saying that the operation of a TV set depends on the components and circuits within that set. The internal structures of materials involve atoms, and the way atoms are associated with their neighbors into crystals, molecules and microstructures.

[4]It is convenient to divide materials into three main types: (1)metals, (2)plastics or polymers and (3)ceramics.

[5]Characteristically, metals are opaque, ductile and good conductors of heat and electricity. Plastics (or polymers), which usually contain light elements, and therefore have relatively low density, are generally insulators, and are flexible and formable at relatively low temperatures. Ceramics, which contain compounds of both metallic and nonmetallic elements, are usually relatively resistant to severe mechanical, thermal, and chemical conditions.

[6]Metals are divided into ferrous and non-ferrous metals. The former contain iron and the latter do not contain iron. Certain elements can improve the properties of steel and are therefore added to it. For example, chromium any be included to resist corrosion and tungsten to increase hardness. Aluminum, copper, and the alloys, bronze and brass, are common non-ferrous metals.

[7]Plastics and ceramics are non-metals; however, plastics may be machined like metals. Plastics are classified into two types-thermoplastics and thermosets. Thermoplastics can be shaper and reshaped by heat and pressure but thermosets cannot be reshaped because they undergo chemical changes as they harden. Ceramics are often employed by engineers when materials which can with stand high temperatures are needed.