MOHR-COULOMB FAILURE CRITERIA:
In 1900, MOHR-COULOMB states Theory of Rupture in Materials which defines as “A material fails due to because of a critical combination of normal and shear stress, not from maximum normal or shear stress”. Failure Envelope is approached by a linear relationship.
If you can not understand the below symbols see the attachment below
f  f ()
Where: f = Shear Stress on Failure Plane
´= Normal Stress on Failure Plane
See the graph in the attachment
For calculating the shear stress, when Normal stress, cohesion and angle of internal friction are given. Use this formula: shear stress =  f  c   tan 
Where,
• f is Shear Stress on Failure Plane
• c is Cohesion
•  is Normal Total Stress on Failure Plane
•  is Friction Angle
