# What is Mechanical Stress : Strength of Material

When an external force is applied to a rigid body. Internal resistive forces acting in the direction opposite to applied force resist any change in size and shape of the body. Due to mechanical stress or external forces, a body changes its shape that is denoted by strain in mechanics. This phenomenon of Mechanical Stress and Strain can be best understood by understanding Stress-Strain Curve.

## What is Mechanical Stress ?

Mechanical Stress is a measure of internal resistance exhibited by a body or material when an external force is applied to it. it is denoted by sigma (σ).

When this resisting force becomes equal to the applied forces it is known as the elastic limit of the material. Within Elastic Limit, external forces acting on the body are equal to the internal forces.

##### Mechanical Stress Formula

Mathematically mechanical stress is equal to the internal resisting force acting on a body per unit area.

Stress is an area or surface-based property. Its value at any point can be determined by considering A→0

##### Unit of Stress

SI unit of mechanical stress N/m². But in the material datasheet mostly it is written as N/m².

Unit Conversion
1 MPa = 1 N / mm²
1 ksi (killo pound per square inch) = 6.895 N / mm²
1000 psi (pound per square inch) = 6.89 N / mm²
1 bar = 0.1 N / mm²

## Example of Stress acting on a Body

Consider a case when a rubber band is placed on a table and you pick it up softly. How much mechanical stress will be produced inside the rubber band?

While holding the rubber band No Stress will be produced inside the rubber band. Because there is no resistance to external force exhibited by the rubber band. This phenomenon is known as “Rigid body Motion”.

In a Rigid Body, moving object is transferred from its original position without any physical deformation.

But if the rubber band is pulled in the opposite direction with a force enough to produce deflection. Mechanical stress will be produced inside the rubber band. Therefore we can conclude that stress is not generated without deflection or internal resistance.

## Types of Mechanical Stress ?

Mechanical Stress acts on the cross-sectional area of a body. According to the position and direction of applied external force. Engineering stress can be classified into the following types.

##### 1) Uniaxial Normal Stress

Stress acting on a body perpendicular to its cross-section area is known as uniaxial normal stress. It results in either compression or elongation in the rigid body. It can be classified into two types:

1. Tensile Stress
2. Compressive Stress
###### Tensile Stress

Stress acting on a body, when two equal and opposite pull forces are applied is known as Tensile Stress. Tensile Stress results in an overall increase in length and decrease in the cross-section area of the body.

###### Compressive Stress

Mechanical Stress acting on a body, when two equal and opposite push forces are applied is known as compressive stress. Compressive stress results in an increase in the cross-section area and a decrease in the length of the body.

##### 2) Shear Stress

Stress acting on a body when two equal and opposite forces across different lines of action are applied is known as shear stress. The shear stress acts tangential to the area of the body and results in angular deformation that is measured as an angle.

##### Shear Stress (γ) = θ

To sum up, mechanical stress is the internal resistance exhibited by a body when an external force is applied to it. There is no stress produced inside a material without strain.