Deformation: If an applied force causes a change in the distance
between the particles of a material body, it results in a change in size or
shape or both. Such a change in size or shape is called deformation.
Deforming Force: A force causing the deformation of a
material body is called a deforming force.
Elasticity: The property possessed by a material body
due to which it offers a resistance to the deforming forces and recovers its
original size and shape after the deforming forces are removed is called
Perfectly Elastic Body: A material body,
which completely recovers its original size and shape upon removal of
deforming forces, is called a perfectly elastic body. Example: Quartz fibre (almost perfectly elastic).
Plasticity: The property possessed by a material body
due to which it offers no resistance to the deforming forces and remains in
the deformed state even after removal of the deforming forces is called
Perfectly Plastic Body: A material body that
does not recover its original shape or size at all even after removal of the
deforming forces is called a perfectly plastic body. Example: Clay.
Internal Restoring Force: When deforming
forces are applied to an elastic body, internal opposing forces are set up
which tend to restore the body to its original size and shape. These forces
are called internal restoring forces.
Generally, Internal restoring force = Applied force.
Stress: The internal restoring force per unit area
of the body is called stress.
Longitudinal Stress or Tensile Stress: If the applied force
causes a change in the length of a body, then the corresponding stress is
called longitudinal or tensile stress.
Volume Stress or Bulk Stress: If the applied force
causes a change in the volume of a body, then the corresponding stress is
called volume stress or bulk stress.
Shearing Stress: If the applied force produces a change in
the shape of the body, then the corresponding stress is called shearing stress.
Strain: The ratio of the change in dimensions (i.e.
length, volume, etc.) of the body to the original dimensions (length, volume)
is called the strain.
Longitudinal or Tensile Strain: It is defined as the
ratio of the change in length to the original length.
Volume Strain or Bulk Strain: It is defined as the
ratio of the change in volume to the original volume.
Shearing Strain or Shear: When a tangential
force is applied to a body, there is a lateral displacement of the different
layers of the body. The displacement between any two parallel layers,
separated by a unit distance in a direction at right angles to the direction
of displacement is called shear strain.
Elastic Limit: The maximum value of stress up to which the
body shows elastic behaviour is called the elastic
Hooke's Law of Elasticity: Within the elastic
limit, the stress developed in a body is directly proportional to the strain
produced in it.
Modulus of Elasticity: The modulus of elasticity is defined
as the ratio of the stress to the corresponding strain within the elastic
Young's Modulus (Y): It is defined as the ratio of
longitudinal stress to longitudinal strain within elastic limit.
Bulk Modulus (K): It is defined as the ratio of the volume
stress to the volume strain within elastic limit.
Reciprocal of Bulk modulus is called compressibility.
Shear Modulus or Modulus of Rigidity (h): It is defined as the
ratio of the shearing stress to the shearing strain within elastic limit.
Lateral Strain: The change in the dimension per unit
original dimension perpendicular to the direction of the deforming force is
called the lateral strain.
Poisson's Ratio (s): Poisson's ratio is defined as the
ratio of the lateral strain to the corresponding longitudinal strain.
Yield Point: The point on the stress-strain curve,
beyond which the strain increases without any increase in the stress and at
which the wire begins to extend of its own accord is called the yield point.
Breaking Stress (or Ultimate Strength): The maximum stress,
which can be applied to a wire, is called the breaking stress.
Breaking Point: The point on the stress-strain curve at
which the wire breaks is called the breaking point.
Strain Energy: The work done by the applied force in deforming
the body is stored in it in the form of potential energy. This energy is
called the strain energy.