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Elasticity, plasticity, fragility and hardness

Any type of strain in a solid body the displacement of the particle, of which it is composed, relative to each other. This causes the occurrence of material forces, prevent deformation. These forces, called the elastic forces, act as inside the deformed body., between its parts, and other body, which caused it the deformation. They seek to restore the shape and volume of the deformed body.. Property of deformed bodies to adopt its original shape and its volume after cessation of external forces is called elasticity. Deformation of a body, which disappears after removal of the external loads on the body, is called elastic deformation.

Experience shows, the body can be deformed so, it will not restore his old form, when external influence will disappear. Property of bodies to maintain the deformation after removal of external loads is called plasticity. Residual deformation of a body, which remains after removal of external loads on the body, is called plastic deformation.

Elasticity (plasticity) tel is mainly determined by the material, they're made of.. For example, steel and rubber elastic, and copper and plastic wax. The classification of materials into elastic and plastic conditionally, since each material in most cases has both ductility, and elasticity. For example, steel spring can be stretched so, it can't be compressed. On the other hand, copper spiral with a small tension springs (t. e. compressed, if you let her go). Experience shows, usually with the gradual increase of loads on the material in the body first, there is elastic deformation, and then there are plastic deformation.

In addition, material properties are highly dependent on external conditions. For example, usually lead plastic at low temperatures becomes elastic, and elastic steel for high pressure or high temperature plastic becomes.

Important mechanical properties of materials, which must be taken into account in the engineering, are fragility and hardness.

In practice there are materials, which, when relatively small loads elastically deformed, while increasing external load before collapse, than they will have residual deformity. Such materials called fragile (for example, glass, brick). Brittle materials are very sensitive to shock load. With a sharp blow to the fragile body relatively easily destroyed.

The hardness of the material can be defined in various ways. Usually more solid consider the material, which leaves scratches on the surface of another material. Most solid material is diamond. In the technique of a material's hardness is determined by indentation into the surface with a diamond cone or a steel ball (rice. 11.18). Enter the smaller cone into the material at a certain force pressing, the harder the material.

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The hardness of the material significantly affect the magnitude of rolling friction, therefore, ball bearings are made from hard steel. When a small area of contact, the hardness of the material is of great importance for sliding friction, for example, support for the axis in hours is made from solid materials — ruby and agate.

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