Mechanical Properties of Solids

Detailed Notes from Toppers

Reference: NCERT Books for 11th and 12th Classes

1. Stress and Strain

Detailed Notes:

• Stress: Force applied per unit area.

  • Tensile stress: Pulling force.
  • Compressive stress: Pushing force.
  • Shear stress: Force applied tangentially.

• Strain: Fractional change in dimension

  • Longitudinal strain: Change in length.
  • Transverse strain: Change in width.
  • Shear strain: Change in angle.

• Stress-strain curve: Represents the relationship between stress and strain.

• Hooke’s law: Stress is proportional to strain within elastic limit.

• Elastic modulus: ratio of stress to strain within elastic limit.

• Young’s modulus: Elastic modulus for tensile or compressive stress.

• Poisson’s ratio: ratio of transverse strain to longitudinal strain.

2. Elasticity and Plasticity

Detailed Notes:

• Elasticity: Ability to regain original shape after deformation.
• Plasticity: Ability to retain deformation after stress removal.
• Yield strength: Stress at which material starts to deform plastically.
• Ultimate tensile strength: Maximum stress a material can withstand.
• Fracture toughness: Resistance to crack propagation.

3. Hardness and Toughness

Detailed Notes:

• Hardness: Resistance to permanent deformation.
• Toughness: Ability to absorb energy before fracture.
• Moh’s scale: Qualitative measure of hardness based on scratch resistance.
• Vickers hardness test: Measures hardness by diamond indentation.
• Brinell hardness test: Measures hardness by ball indentation.
• Rockwell hardness test: Measures hardness by depth of indentation.
• Charpy impact test: Measures toughness by energy absorbed during fracture.
• Izod impact test: Similar to Charpy test, but with different specimen.

4. Creep and Fatigue

Detailed Notes:

• Creep: Gradual deformation under sustained stress.
• Fatigue: Failure under repeated stress below ultimate strength.
• Creep curves: Plots of strain vs. time under constant stress.
• Fatigue curves: Plots of stress vs. number of cycles to failure.
• Endurance limit: Maximum stress that can be applied indefinitely without fatigue failure.

5. Fracture

Detailed Notes:

• Fracture: Separation of material into two or more pieces.
• Types of fracture:
  • Ductile fracture: Gradual separation with significant plastic deformation.
  • Brittle fracture: Sudden separation with little or no plastic deformation.
• Griffith theory of fracture: Explains brittle fracture based on energy release.
• Energy release rate: Energy released per unit area of crack growth.
• Fracture toughness: Resistance to crack propagation.

6. Dislocations and Defects

Detailed Notes:

• Dislocations: Linear defects in crystal structure.
• Types of dislocations:
  • Edge dislocation: Extra half-plane of atoms.
  • Screw dislocation: Spiral arrangement of atoms.
• Point defects: Defects involving single atoms or ions.
• Line defects: Dislocations
• Surface defects: Cracks, pores
• Volume defects: Voids, inclusions

7. Strengthening Mechanisms

Detailed Notes:

• Strengthening mechanisms: Techniques to improve mechanical properties.
• Grain size refinement: Reduces grain size to increase grain boundaries and hinder dislocation movement.
• Solid solution strengthening: Adding alloying elements to distort the crystal lattice and impede dislocation motion.
• Precipitation hardening: Formation of precipitates that act as barriers to dislocation movement.
• Strain hardening: Cold working to increase dislocation density and hinder further dislocation movement.
• Work hardening: Strengthening due to plastic deformation.