# What is Mechanical Stress : Strength of Material

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When a force is applied to a rigid body, It changes its size and shape due to internal mechanical stress acting on it. In this article we will discuss mechanical stress acting on a body when an external force is applied to it. We suggest you also read this article on stress in mechanics.

#### What is Stress ?

When this resisting force becomes equal to the applied forces it is known as elastic limit of the material. Within Elastic Limit external forces on body are equal to internal forces. When a body is subjected to an external force, it produces internal forces in the direction opposite to applied external forces. These internal forces resist the deformation inside the body.

Mechanical Stress is a measure of internal forces acting on a body when an external force is applied to it. In other words, stress is internal resistance due to external forces. it is denoted by sigma (σ). Mathematically stress is equal to internal resisting force per unit area.  Question : If you are holding a rubber band, Is there will be any stress produced inside the rubber band?

Answer is: No Stress, because there is no resistance to external force exhibited by rubber band. This phenomena is known as “Rigid body Motion”

If we try to pull  the rubber band in the opposite direction with a force enough to produce deflection. There will be a stress produced inside rubber band. Therefore we can conclude no stress will be generated without deflection or internal resistance.

##### Types of Engineering Stress ?

Stress acts on the area of the body. According to position and direction of external force applied on a body and internal forces acting on a body. engineering stress can classified in following types. ##### 1) Uniaxial Normal Stress

Stress acting on a body perpendicular to cross section area. Uniaxial normal stress results in either compression or elongation.

1.1) Tensile Stress Uniaxial Stresses acting on a body, when it is subjected to two equal and opposite pull forces is known as Tensile Stress.

It results in an overall increase in length and decrease in the Cross section area of the body.

1.2) Compressive Stress Uniaxial Stress acting on a body, when it is subjected to two equal and opposite push forces is known as compressive stress.

It results in an increase in the cross section area and decrease in length of the body.

##### 2) Shear Stress Stress acting a body when it is subjected to two equal and opposite forces across different line of action, is known as shear stress.

The shear stress acts tangential to the area of the body. It results in angular deformation, measured in terms of angle.

Shear Stress (γ) = θ

##### Conclusion

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.

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