Strength of Materials

Strength of materials is a term used by engineers to describe how much force a material can resist. Engi­neers also use the term to describe how a material's shape and size change as a result of an applied force. In addition, strength of materials is the branch of engineer­ing that deals with the study of various forces and of the properties of materials that enable them to resist such forces. When engineers design a building or a machine, they consult publications that list the strength of various materials. They may also conduct tests to determine the strength of these materials. Strength-of-materials engi­neers attempt to design structural and mechanical parts that resist external forces in a safe and economical man­ner.

How materials react to force. The strength of a ma­terial depends on its mechanical properties, which in­clude elasticity, hardness, and stiffness. Mechanical properties combine differently in every material. As a re­sult, such materials as aluminium, concrete, and steel differ in their ability to resist a particular force. Also, each material differs in its ability to resist various types of force. A cast iron bar, for example, is better able to withstand compression (force that pushes it together) than tension (force that pulls it apart).

When an external force is applied to a material, a force inside the material resists the external force. This internal resistance of a material to such a force is called stress. A material subjected to an external force changes shape and size. When a weight is put on the end of a rope, for example, the rope stretches. The actual change m shape of a material—in the above case, the stretching of the rope—is called deformation. Deformation per unit of length is called strain. The greater the amount of stress in a material is, the greater is the amount of strain will occur in it. For most structures, such as building and bridges, the strain is so small that resulting changes in shape cannot be seen. An object like a rub­ber band, however, shows a significant change in shape even when subjected to fairly small forces. The ratio of stress to strain in a material, called the modulus of elas­ticity, is a measure of the material's ability to stretch when a force is applied to it.

Materials undergo three types of stress: tensile, com­pressive, and shearing. Tensile stress causes a material to stretch, as with the rope. Compressive stress causes a material to push together. The pillars that support a building undergo compressive stress because the weight of the structure pushes down on them. Shearing stress causes a material to separate into layers by a slid­ing action. Such an action resembles that of the cards in a deck, which slide apart when they are tilted so the edges are at an angle.

The stresses in a material may combine to resist force. Combined stresses cause flexure (bending) and torsion (twisting). For example, various stresses unite in a springboard when a person stands on it. The person's weight causes tensile stress in the top section of the board, and the fibres there stretch. At the same time, the weight of the individual causes compressive stress in the bottom section of the board. As a result, the fibres there push closer together. This combination of stresses resists the weight that is applied and makes the board bend.

How strength is determined. Technicians measure the strength of a material by using special machines that apply force to a sample of the material. First, they deter­mine the material's elastic limit, the amount of force it can resist without changing shape permanently. If the applied force is lower than the elastic limit, the material will return to its original shape and size after the force is removed. But if the force exceeds the elastic limit, the material will change permanently.

Technicians also measure the ultimate strength of a material—that is, the maximum force it can resist without breaking. Engineers consider ultimate strength in terms of the number of kilograms of force per square centime­tre that a material can withstand. For example, a bar of cast iron can withstand about 2,110 kilograms per square centimetre of a pulling force without breaking. Laboratory tests for strength are not exact. The results are affected by a material's age, composition, and mois­ture. In constructing a building or a machine, engineers use a material strong and stiff enough to resist a heavier load than the one expected. This policy helps ensure that the material will not fail when it is in actual use in a structure or machine.

Development of new materials. During the late 1960's, scientists began the extensive development of composite materials. Such a material contains two or more materials. Many composite materials contain a large amount of one substance combined with fibres, flakes, or layers of another. Composite materials have greater strength than many single materials. For exam­ple, glass fibres combined with plastics form fibreglass reinforced plastics. This material has greater strength than either the glass or the plastics alone. Fibreglass re­inforced plastics are used to make such products as boat hulls, building panels, and truck parts.

Since the 1970's, scientists have increased the devel­opment of composite materials that contain fibres. Two of the strongest fibres consist of boron, a chemical ele­ment, and graphite, a form of carbon. Boron fibres and graphite fibres can withstand intense force and high temperatures. These lightweight fibres, as well as light­weight metals, such as aluminium, are used in some spacecraft. See also Ductility; Elasticity; Metal fatigue.