8.1 Introduction

Mechanical properties of solids define how materials deform, resist, and behave under applied forces, covering characteristics like elasticity, plasticity, strength, and hardness. Key concepts include stress (restoring force per unit area), strain (fractional deformation), and Hooke’s Law, which states stress is proportional to strain within the elastic limit. 

Key Mechanical Properties

Elasticity

The ability of a material to regain its original shape and size after removing deforming forces (e.g., Common examples include stretching a rubber band, compressing a metal spring, or deforming a balloon).

Plasticity

The tendency of a material to undergo permanent deformation without breaking when force is applied (e.g., like putty, clay, or metals). The material does not return to its original shape.

Strength

Resistance to deformation or breaking under load. Strength is the primary material property representing the capacity to resist permanent deformation (yielding) or breaking (fracture) under applied loads. It encompasses various types, including tensile (pulling), compressive (squeezing), shear, and flexural (bending) strength, all measured by the load sustained before failure.

Hardness

Resistance to surface scratching or indentation. It measures a material’s ability to withstand permanent deformation, such as scratching, indentation (denting), or abrasion, caused by a harder, often sharper, object.

Ductility

The capacity to be drawn into wires. Ductility is the physical property of a material, particularly metals, allowing it to undergo significant plastic deformation under tensile stress, such as being stretched or drawn into thin wires without fracturing.

Brittleness

The tendency to break easily with little deformation (e.g., glass).

Toughness

The ability to absorb energy and deform plastically without fracturing. It represents a material’s resistance to breaking under sudden impact or stress.

Some terms related to elasticity are discussed below

• Perfectly elastic body A body is said to be perfectly elastic, if it returns back completely to its original size on removing the external force(s).
• Plastic body If a body remains in the deformed shape and does not even partially regain its original shape, after removal of external force, is called plastic body or perfectly inelastic body.
• Elastic limit It is the upper limit of deforming force up to which if deforming force is removed, the body regains its original form completely and beyond which if deforming force is increased, the body looses its property of elasticity and gets permanently deformed.

Remarks

• Elastic limit is the property of body, whereas elasticity is the property of material of a body.
• All rigid bodies are elastic to some extent which means we can change their dimensions slightly by pulling or pushing them.
• No body is perfectly elastic or perfectly plastic. All the bodies found in nature lie between these two limits. When the elastic behaviour of body decreases, its plastic behaviour increases or vice-versa.