STRESS

          Consider some external force acting on a body, the force of resistance per unit area offered by a body against deformation is known as Stress. This external force which is acting on body is called the Load (or) Force. When the load is applied on the body while the stress is induced in the material of the body. The loaded body remains in equilibrium when the resistance offered by the body against deformation and the applied load are equal. 

Stress is denoted by a letter "σ" (SIGMA)

Mathematically stress is written as σ = P/A

          Where σ = Stress (or  Intensity of stress)

                       P = External force (or) Load

                       A = Cross sectional area.

Units of Stress:

          Units of stress depends upon two factors, they are units of Load and Units of area. 

In MKS units, the force is expressed in kgf and area in square meters, so stress units are kgf/m^2.
In SI units, the force is expressed in Newtons and area in meter squares, hence units of stress becomes N/m^2.

SIMPLE STRESSES AND STRAINS

INTRODUCTION:

          When an external force acts on a body, the body tends to undergo some deformation. The body resists the deformation due to cohesion between the molecules. The resistance by which the body tends to oppose the deformation is termed as "Strength of Materials". The resistance that the material offered within certain limit is proportional to the deformation of the material by the external force. Also within this limit resistance is equal to the deformation. But beyond the elastic limit, the resistance offered by the material is less than the applied load. In this case, the deformation increases until the failure takes place.

          Within the elastic stage, the resisting force equals to the external force. This resisting force per unit area is called Stress (or) Intensity of Stress. 

Credits: All mechanical engineers

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