Every engineering structure is subjected to external loading of various kinds (normal load, shear load or mixed load such as bending, torsion, etc.). When substantial load is applied on the material, it automatically tends to deform. The very basic mechanism of deformation is the shifting of layers of atoms from their original lattice position under the influence of external force.

When such shifting or deformation is very small, the structure can recover its original shape and size once the external load is removed. Such a deformation, which is temporary and easily recoverable, is called elastic deformation. Corresponding property of the material by virtue of which the structure tends to regain its shape after the removal of external load is called elasticity. Most engineering materials display linear behavior (load-deformation or stress-strain curve) within this region.

However, when intensity of external load exceeds the capability to elastically sustainable limit, the material deforms permanently, which means the structure loses its original shape and size and thus retains its deformed shape and size even after the removal of load. The property of the solid material by virtue of which the structure tends to retain its deformed shape even after the removal of external load is called plasticity. Stress-strain curve also becomes non-linear in this region. Various similarities and differences between elasticity and plasticity are given below in table form.

Similarities between elasticity and plasticity

• Both are properties of solid.
• 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 elasticity and plasticity

Elasticity	Plasticity
Elasticity is the property of the solid material by virtue of which it tends to regain its shape after the removal of external load.	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.
In elastic deformation, although atoms of the solid are displaced from their original lattice site, they return back to their original position once external loading is removed. So atoms are temporarily displaced.	In plastic deformation, atoms of the solid are permanently displaced from their original lattice site. They retain the new position even after the removal of external loading.
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.
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.
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.
Elastic deformation occurs prior to plastic deformation.	Plastic deformation occurs only after it is elastically deformed.
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.
Elasticity is an important consideration for: Machine tool structures Bridges, other civil frames Equipment body Many household structures and frames that should retain their shape	Plasticity is an important consideration for: Sheet metal working Various forming operations such as rolling, forging, extrusion, etc. Rivet joining.

 

References

• Book: Callister’s Materials Science and Engineering by R. Balasubramaniam (Wiley India). Buy this book
• Book: Introduction to Machine Design by V. B. Bhandari (McGraw Hill Education India Private Limited). Buy this book
• Book: A Textbook of Strength of Materials by R. K. Bansal (Laxmi Publications Private Limited). Buy this book
• Book: Strength of Materials by S. S. Rattan (Tata McGraw Hill Education Private Limited). Buy this book