Stress and Strain
Deformation is the change in shape of the body when a force is applied on it. Even a very small magnitude of force is enough to cause some deformation. Undergoing deformation following two terms Stress and Strain describe the forces on objects.
Stress
When an external force is acting on an elastic body, it causes deformation that means change in shape or in size or both.
Stress = Force/Area = F/A
The stress is the restoring force acting on a unit area.
Unit: The unit for stress is Newton Metre-2 or Nm-2 or Pascal written as Pa.
Restoring Force: Due to elastic property, a force develop within the material, which is equal and opposite to the applied force. This force is responsible to bring the body to its original shape and size. This force is the Restoring Force. Since both the applied force and the restoring force are equal in magnitude.
Types of Stress
- When the applied force tends to compress the body, the stress is compressive stress.
- This is the case when applied force tends to increase the length in the direction of the force, it is tensile stress.
- When it acts parallel to the surface of a body, the stress is tangential stress or Shear stress.
- When an object is being squeezed from all sides like a submarine in the depths of an ocean. We call this bulk stress or volume stress.
Strain
Strain is the ratio of change in dimensions to original dimensions. Thus the strain produced in a body is directly proportional to the stress which causes it.
i.e. strain ∝ stress or stress ∝ strain
Types of Strain
1. Linear Strain
When two equal and opposite forces are applied to a wire or bar at its ends. As a result there is an increase in the length. The body is elongated if the forces are tensile. The length is shortened in the direction of the forces if the forces are compressive. This is called the ‘linear strain’. The linear strain is the ratio of change in length to the original length. As the linear strain is the ratio of lengths, it has no unit.
2. Bulk (or) Volume Strain
When a uniform force is applied normally to the entire surface of the body. There is a change in volume of the body without changing its shape. This strain is called ‘bulk or volume strain. Volume strain is the ratio of change in volume to the original volume. It has also no unit.
3. Shearing (or) Rigidity Strain
When a force is applied parallel to one face of a body such that the opposite side is fixed. As a result there is a change in shape but not in size of the body. This strain called as the shearing strain. Solids alone can have a shearing strain. It is measured by the angle of the shear ‘θ’ in radian.
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