Definition of Stress

Consider a small area  δA on the surface of a body (Fig. 1.1). The force acting on this area is  δF
This force can be resolved into two perpendicular components

• The component of force acting normal to the area called normal  force and is denoted by  δF_n
  
• The component of force acting along the plane of area is called tangential force and is denoted by δF_t

Fig 1.1 Normal and Tangential Forces on a surface

When they are expressed as force per unit area they are called as normal stress and tangential stress respectively. The tangential stress is also

called shear stress

The normal stress

(1.1)

And shear stress

(1.2)