Elastic and Plastic Behaviour of Solids
Sometimes, a force acting on a body instead of producing a change in its state of rest or of uniform motion, produces a change in the shape of the body. Such a force is called deforming force. A rigid body can be noticeably stretched, compressed, bent or twisted by applying a suitable force. That a body can be deformed by a force can be easily shown by stretching a rubber band or by loading a spring. Delicate measurements indicate that deformations do take place, even when small forces are applied to the rigid bodies.
A body that returns to its original shape and size on the removal of the deforming force (when deformed within elastic limit) is called elastic body.
Actually, this concept of an elastic body is an idealization and no materials behave as perfectly elastic. A quartz fiber is the nearest approach to a perfectly elastic body. Thus, all bodies are elastic; the difference lies only in degree.
The property of matter by virtue of which its regains its original shape and size, when the deforming forces have been removed is called elasticity.
Contrary to the concept of elasticity in daily life; in physics, elasticity stands for opposition to change. Qualitatively, more rigid a body, more elastic it is said to be. For this reason, steel is more elastic than rubber.
Thus a solid can be imagined to have n-number of springs attached to each of the atoms in 3-D and vibrate simple harmonically about the mean position, when a force is applied. This is given as follows:
A body that does not return to its original shape and size on the removal of deforming force, however small the magnitude of deforming force may be, is called a plastic body. Putty, paraffin wax, etc. are the examples of nearly plastic bodies.
It may be pointed out that elastic and plastic natures of materials of different bodies are idealized concepts. No body is perfectly elastic or perfectly plastic. In a body, when elastic behaviour increases, the plastic behaviour decreases and vice-versa. Hooke's law (extension in a wire is directly proportional to the load applied) is the basic law of elasticity and its holds only, when a body is deformed within elastic limit. Beyond elastic limit, the plastic behaviour dominates even in the elastic materials and they undergo a permanent deformation.