**Stress:**

When we apply an external force on a body, it tends to undergo deformation. The body will resist the force. The internal resisting force is equal and opposite of the external applied force. This internal resistance is known as stress. So a stress can be defined as

*The internal resistance offered by a body per unit area of the cross section is known as stress.*

**Stress, σ = P/A**

Where P represents the applied force and A represents the externally applied force.

In SI units the stress is expressed as N/mm^{2} or N/m^{2}.

N/m^{2 }is also called Pascal (pa).

Types of stresses

**Direct stress****Shear stress or Tangential stress****Bending stress****Torsional stress****Thermal stress**

**Direct Stress: **The direct stress acts perpendicular to the cross section of a body. It can be either Tensile or Compressive.

Tensile stress – When two equal and opposite forces are applied to a body and the body tends to increase in length is known as tensile stress.

σ_{tensile} = P/A

Compressive stress – When two equal and opposite forces are applied to a body and the body tends to decrease in length is known as compressive stress.

σ_{compressive }= P/A

**Shear Stress or Tangential stress: **Shear stress is similar to the direct except that the forces are applied tangentially and the body sheared or twisted.

Shear Stress, ζ = Shear stress/Shear area = P/A

**Bending Stress: **It is a compressive or/and tensile stress due to the non-axial forces acting on a beam. It tends to bend or deflect the beam.

**Torsional Stress: **Shear stress produced when we apply the twisting moment to the end of a shaft about its axis is known as Torsional stress.

**Thermal Stress: **Due to the change in the material temperature, the dimensions of the material also changes. But some of the parts are not free to expand. The stress set up when the parts are not free is expand due to the change in body temperature is known as thermal stress.