Elasticity

Elasticity, property of a material that causes it to resume its original size and shape after having been compressed or stretched by an external force. An external force applied to a material creates stress within the material; this stress causes the material to deform. The amount of deformation, as a fraction of the original size, is called strain. For many materials, including metals and minerals, stress is directly proportional to strain over a certain range of these quantities. This relationship is known as Hooke’s Law after the British physicist Robert Hooke, who first expressed it. However, if the external force is too strong, the material can become permanently deformed, and Hooke’s Law no longer applies. The maximum amount of stress a material can withstand before becoming permanently deformed is called the elastic limit.

The ratio of stress to strain, called the elastic modulus, and the elastic limit of a material are determined by the molecular structure of the material. The distance between molecules in a stress-free material depends on a balance between the molecular forces of attraction and repulsion. When an external force is applied, creating stress within the material, the molecular distances change and the material becomes deformed. If the molecules are tightly bound to each other, there will be little strain even for a large amount of stress. If, however, the molecules are loosely bonded to each other, a relatively small amount of stress will cause a large amount of strain. Below the elastic limit, when the applied force is removed, the molecules return to their balanced position, and the elastic material goes back to its original shape. Beyond the elastic limit, the applied force separates the molecules to such an extent that they are unable to return to their original positions, and the material is permanently deformed or broken apart. See also Materials Science and Technology: Mechanical Properties of Materials.