Notes on Elasticity, Plasticity, Stress, and Strain

Key Concepts

Elasticity

• Definition: The ability of a material to return to its original shape after being deformed.
• Example: Kids using a catapult to hit a mango. The catapult changes position when force is applied but regains its shape when the force is released.

Plasticity

• Definition: The ability of a material to undergo permanent deformation when a force is applied.
• Example: A paper clip bending and not regaining its original shape after force is applied.

Stress and Strain

Stress

• Definition: The internal resistance offered by a material to deformation, caused by an applied force.
• Example: A boss applying pressure on an employee, leading to stress.
• Types of Stress:
  • Longitudinal Stress: Force applied along the length of an object (e.g., iron pipe).
  • Hydrostatic Stress: Uniform pressure applied on all sides (e.g., diving into a pool).
  • Shear Stress: Force acting perpendicular to the object (e.g., chopping a watermelon with a knife).

Strain

• Definition: The measure of deformation resulting from applied stress.
• Example: An employee getting depressed and changing their work attitude due to stress.
• Types of Strain:
  • Linear Strain: Change in length of an object (e.g., iron pipe).
  • Volumetric Strain: Change in volume of an object.
  • Shearing Strain: Related to the amount of force applied on a material.

Hooke's Law

• Principle: Stress is directly proportional to strain within the elastic limit of a material.

Factors Affecting Physical Properties of Materials

• Temperature: Changes in temperature can affect elasticity (e.g., t-shirts that shrink or elongate).
• Impurities: The presence of alloys can affect the purity and elasticity of materials (e.g., gold).

Ductility and Brittleness

• Ductile Materials: Can be deformed significantly before breaking (e.g., metals, plastics).
  • Example: Chewing gum at room temperature stretches before breaking.
• Brittle Materials: Break without significant deformation (e.g., glass, ceramics).
  • Example: Chewing gum in a freezer breaks without stretching.

Tensile Strength

• Different substances exhibit varying tensile strengths based on conditions such as temperature and pressure.

Summary

• Understanding the concepts of elasticity, plasticity, stress, and strain is crucial in material science and engineering.
• The mechanical properties of materials can greatly vary based on internal and external conditions.