ELASTIC AND PLASTIC DEFORMATION

ELASTIC AND PLASTIC DEFORMATION

Figure 1. A spring wire is an example of elasticity, since it returns to its original shape, after being pulled and pushed on.

Objects deform when pushed, pulled, and twisted. Elasticity is the measure of the amount that the object can return to its original shape after these external forces and pressures stop. This is what allows springs to store elastic potential energy.

The opposite of elasticity is plasticity; when something is stretched, and it stays stretched, the material is said to be plastic. When energy goes into changing the shape of some material and it stays changed, that is said to be plastic deformation. When the material goes back to its original form, that's elastic deformation. Mechanical energy is lost whenever an object undergoes plastic deformation. Manufacturing goods from raw materials involves a great deal of plastic deformation. For example, rolling steel into a particular shape (like rebar for construction) involves plastic deformation, since a new shape is created.

Most materials have an amount of force or pressure for which they deform elastically. If more force or pressure is applied, then they have plastic deformation. Materials that have a fair amount of plastic deformation before breaking are said to be ductile. Materials that can't stretch or bend much without breaking are said to be brittle. Copper is quite ductile, which is part of why it is used for wires (most metals are ductile (but copper especially so). Glass and ceramics are often brittle; they will break rather than bend!

Figure 2. Plastic wrap is an example of plasticity. After stretched—it stays stretched.

Main Difference – Elastic vs Plastic Deformation

Deformation is the action or process of deforming or distorting. When a force is applied to an object, the object will either compress or stretch as a response to the force. In mechanics, the force applied to a unit area is called stress. The extent of stretching or compressing (as a response to the stress) is called strain. Every material responds differently to stress. The response is highly dependent on the chemical bond type of the substance. Deformations can be elastic or plastic based on what happens after the stress is released. Elastic deformation is the deformation that disappears upon removal of the external forces causing the alteration and the stress associated with it. Plastic deformation is a permanent deformation or change in shape of a solid body without fracture under the action of a sustained force. The main difference between elastic deformation and plastic deformation is that elastic deformation is reversible whereas plastic deformation is irreversible.

Similarities between elastic and plastic deformations

• Any type of loading (normal, shear or mixed) may result both types of deformations.

• Plastic deformation can occur only after the material is elastically deformed. So without elastic deformation, plastic deformation is not possible.

• Both elastic and plastic deformations are useful; however, based on the application.

Differences between elastic and plastic deformations

Elastic Deformation	Plastic Deformation
Elastic deformation is a temporary deformation under the action of external loading.	Plastic deformation is the permanent deformation.
Once the external load is removed from an elastically deformed body, it regains its original shape.	When a body is plastically deformed, it retains its deformed shape even after the removal of external load.
In elastic deformation, atoms of the material are displaced temporarily from their original lattice site. They return back to their original position after the removal of external load.	In plastic deformation, atoms of the solid are displaced permanently from their original lattice site. They don’t return back to the original position even after the removal of external load.
Elastic deformation is characterized by the property Elasticity. By definition, elasticity is the property of the solid material by virtue of which it tends to regain its shape after the removal of external load.	Plastic deformation is characterized by the property Plasticity. By definition, plasticity is the property of the solid material by virtue of which it tends to retain its deformed shape even after the removal of external load.
Amount of elastic deformation is very small.	Amount of plastic deformation is quite large.
External force required for elastic deformation of solid is quite small.	Force required for plastic deformation is also higher.
Energy absorbed by the material during elastic deformation is called module of resilience.	Total energy absorbed by the material during elastic and plastic deformation region is called module of toughness.
Hooke’s Law of elasticity is applicable within this elastic region.	Hooke’s Law is not applicable if the material is plastically deformed.
Most solid materials display a linear stress-strain behavior within this elastic region.	Stress-strain curve is non-linear in plastic region.
Material first undergo elastic deformation under the application of external loading.	Plastic deformation occurs after it is elastically deformed due to the application of external loading.
Mechanical and metallurgical properties of the solid material remain unaltered when it is elastically deformed.	Many properties of the solid material change considerably for plastic deformation.