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Have you ever tried to stretch an elastic band? If yes, you must have observed that it deforms from its original shape when you apply some force to the band to stretch it. However, when you stop applying force on the elastic band, it will go back to its original shape. But does this work on solid objects like metal rods, metal wires or metal plates the same way as on elastic bands?
Obviously, no, because if we try to bend a metal rod or plate by applying force on it, it will get deformed. But when we stop applying force on the plate or the rod, it will remain deformed, unlike elastic bands, which recoil into their original shape. This is due to the elastic behaviour of solids. To answer this question, we have to find out about the elastic behaviour of solids.The elastic behaviour of solids depends on two properties known as elasticity and plasticity.
What are elasticity and plasticity?
To understand the elastic behaviour of solids, we first have to understand these two properties, namely elasticity and plasticity.
Elasticity can be defined as the property of a material regaining its original shape and size after getting stretched by an external force.
Plasticity is the property of a material to be moulded in any desired shape easily. Materials with higher plasticity do not regain their original shape when an external force is applied.
Elastic behaviour of solids
The elastic behaviour of solids is the behaviour that helps solid materials regain their original shape and size after the external force is removed. Moreover, the point where the material loses its elasticity and cannot regain its original shape and size or simply the point where the elastic behaviour of solids is permanently distorted, is known as the elastic limit of the material.
The elastic limit is the maximum force or stress applied per unit area on a solid before it permanently loses shape and size.
However, when a material is stretched beyond its elastic limit, it loses its elasticity and starts to deform permanently, also known as plasticity. Moreover, we can also say that the point where a material loses its elasticity to get permanently deformed or the elastic limit is the point where the plasticity of material begins.
Furthermore, when a material is stretched beyond its elastic limit, it will fracture into two or more pieces. Moreover, it is not necessarily important that materials have both elasticity and plasticity. When stretched beyond their elastic limit, most of the brittle materials directly get fractured without showing any plastic behaviour.
Elastic behaviour of solids at the atomic level
We know that all solids are made up of atoms at the atomic level. Similar atoms always surround an atom. These atoms remain in an equilibrium state by interatomic forces. When we apply an external force to the solid, the atoms are displaced from their original position. This external force also causes a disturbance in the equilibrium state of the atoms. This replacement and disturbance lead to the deformation of the solid object, which we can see with our naked eyes.
Moreover, when the externally applied force is removed from the solid, the atoms start to recover their original state of equilibrium, which results in the reformation of the original shape and size of the object. However, not every atom may get back to its original state and equilibrium, so we can say that no material in the world is perfectly elastic.
Take for example a hairband used to tie hair because of its elastic nature. You may have observed that the hairband is getting looser after some use. However, the hairband does not lose its elasticity in one go; it loses a little bit of elasticity every time it gets stretched.
Conclusion
The elastic behaviour of solids helps them regain their original shape after any external deforming force is applied. Deformation occurs when there is any change in the shape and size of a body due to external force.
However, if the deforming force exceeds the elastic limit, it loses its elasticity forever, and after a certain magnitude of force, the substance even tends to break. Hence, the body’s elasticity remains only up to a certain limit. Some examples of elastic materials are rubber bands, modelling clay, and so on. However, contrary to popular belief, even steel is elastic. It is the most flexible material.
