Deformation of Solids, Stress and Strain, Types of Deformation, Tensile or Compressive Stress, Strain, and Young՚s Modulus (For CBSE, ICSE, IAS, NET, NRA 2022)

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Deformation of Solids

  • The shape of an object is the geometrical description of the part of the space occupied by the object, as determined by its external boundaries.
  • A change in shape due to the application of force is called deformation. Even small forces are known to cause some deformation.
  • Objects under the actions of external forces undergo deformation.
  • That may be: squashing, twisting, ripping, or pulling apart the object.
  • Two terms describe the force on objects undergoing deformation in physics:
    • stress
    • strain

Stress and Strain

  • The quantity that describes the magnitude of forces that cause deformation is known as stress.
  • Stress is defined as force per unit.
  • Types of stress
    • Tensile Stress – When forces pull an object and cause its elongation such as the stretching of a rubber band
    • Compressive Stress – When the forces result in the compression of an object
    • Bulk Stress – When an object is squeezed from all sides like the submarine in the depth of the ocean
    • Shear Stress – A type of stress where the deforming stress acts tangentially to the object՚s surface
  • Strain: The quantity that describes the deformation undergone is known as strain.
  • It is given as a fractional change in either the length, geometry, or volume. It is a dimensionless number.
  • The greater the stress, the greater the strain.
  • The proportionality constant in this relation is known as the elastic modulus. The relation between stress and strain is given as follows:
    • Stress
  • When an object has a large value of elastic modulus, the effect of stress is small.
  • Same as a small elastic modulus means that stress produces noticeable deformation.
  • For example, stress on a rubber band produces a larger strain as compared to a steel band of the same size.
  • The elastic modulus of a rubber band is smaller than the elastic modulus of the steel band.

Types of Deformation

  • It can be of two types as follows:
    • Permanent Deformation (plastic deformation) : it is irreversible. It is a type of deformation that stays even after the removal of applied forces.
    • Temporary Deformation (elastic deformation) : it is reversible. It is a type of deformation that disappears after the removal of applied forces.
  • When a material is subjected to applied forces, the material experiences elastic deformation followed by plastic deformation.
  • The transition from elastic to plastic state is characterized by the yield strength of the material.
  • The plastic deformation mechanism is different for crystalline and amorphous materials.
  • In crystalline materials, deformation is accomplished through a process known as the slip that involves the movement of dislocations.
  • While, in amorphous materials, deformation takes place by the sliding of atoms and ions without any directionality.

Tensile or Compressive Stress, Strain, and Young՚s Modulus

  • Tension or compression occurs when two anti-parallel forces of equal magnitude act on an object along only one of its dimensions, in such a way that the object does not move.
  • A rod segment is either stretched or squeezed by a pair of forces acting along its length and perpendicular to its cross-section.
  • The net effect of such forces is that the rod changes its length from the original length that it had before the forces appeared, to a new length L that it has under the action of the forces.
  • This change in length

  • Tensile stress and strain occur when the forces are stretching an object, causing its elongation, and the length change is positive.
  • Compressive stress and strain occur when the forces are contracting an object, causing its shortening, and the length change is negative.
  • Tensile strain is the measure of the deformation of an object under tensile stress and is defined as the fractional change of the object՚s length when the object experiences tensile stress

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