Deformation refers to the change in shape or size of a material or structure when subjected to an external force or load. It occurs because real materials are not perfectly rigid and will experience some degree of stretching, compression, bending, twisting, or shearing under applied forces. Deformation can be classified into two main types: elastic deformation and plastic deformation.

- Elastic deformation is temporary and fully reversible. When the applied load is removed, the material returns to its original shape. This behavior is governed by Hooke’s Law, which states that within the elastic limit, stress is proportional to strain (σ = E × ε), where E is the material’s modulus of elasticity (Young’s modulus), and ε is the strain. Most engineering materials, like metals and polymers, exhibit elastic deformation under small loads.

- Plastic deformation is permanent and occurs when the material is stressed beyond its yield point. In this case, the material does not return to its original shape after the load is removed. Plastic deformation is common in metals undergoing forging, rolling, or other shaping processes, as well as in failure conditions when a material is overloaded.

Deformation is a critical factor in mechanical and structural design, as excessive deformation can affect functionality, alignment, and structural integrity. Engineers use material properties such as elastic modulus, yield strength, and ductility to predict and control deformation, ensuring that components perform reliably under expected loads.