Stress-strain are very important terms of solid mechanics. Strength of materials is a branch of engineering which deals with the material's properties to resist its failure. This topic also discusses about the behavior of materials under different level of forces.

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## What is stress and strain

In this topic we will discuss the definition of stress and strain, their formula as well as relationship and units.### What is stress ?

When a body is subjected to external forces, equal and opposite internal forces are developed at the various sections of that body. This resisting force is known as unit stress or stress.

#### Formula of stress :

Stress = Force / Cross sectional are = P/A

Stress is generally denoted by the Greek letter sigma .

IN system international (S.I) the

**unit of stress**is Pascal (Pa) .
1 Pa = 1 Newton /1 square meter.

In practical calculation, bigger units like megapascal (Mpa) or gigapascal (Gpa) is used. I hope that meaning of stress is very much clear now.

### What is Strain ?

When a body experiences a system of force, some deformation (elastic or plastic) takes place. This deformation per unit length of the body is know as the unit strain or simply strain.#### Formula of Strain:

Strain = Change in the length of the body / Original length

Strain is generally expressed by the Greek letter epsilon .

### Stress Strain Relationship

For understanding the stress strain relationship we can explain the

**stress-strain diagram**for mild steel which is under tensile load.
From the stress strain graph we can find the yield stress, ultimate stress and breaking stress. After passing the yield point plastic deformation occurs.

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