Stress Strain Graph for Mild Steel explained

Stress:

When any object of certain material is subjected to tensile or compressive or shear force, there may be a chance that its dimension will be affected.
If the dimension of a material is changed due to application of forces we can say that the material is under stress or experiencing the state of stress.

Stress is defined as the Force per unit cross section area.
Stress = F/A

Stress is mainly of two types normal stress and shear stress.

Normal Stress:

When the force applied is perpendicular to the cross section of the object then the stress is known as normal stress. (The force can be tensile or compressive)

Shear Stress: 

When the force applied is parallel to the material cross section then the stress is known as shear stress.

Stress Strain Graph for Mild Steel (Ductile Material)

If a relatively small force is applied to ductile material steel and it starts to deform (that means that the steel is in stress and we can measure the strain) then we will a curve which is initially a straight line. After plotting stress vs strain we will get the relation. 

In this stress strain graph of mild steel we will find a straight line , that is up to point A from the origin. From that experimental graph we can conclude that stress is proportional to strain. Up to point A is the limit of proportionality. Within the elastic limit the deformation of the steel will be temporary. After the withdrawal of the force the steel bar will return to its original shape. If the force is increased then steel bar will be deformed elastically up to point B. This is the elastic limit (Point B). Beyond that if the force is increased then the plastic deformation will start and we will have upper yield point C and lower yield point D. For further increase in the force material will experience fracture or breaking stress (Max. Ultimate Stress). From this graph we get Young's modulus of elasticity for steel which is 210 GPa. This graph also associates modulus of resilience as well as modulus of toughness. 

Please Read: 

Stress-Strain Relationship 
Connecting Rod Bearing Failure analysis 
Heat Treatment of Steel 
Different Mechanical Properties of Materials