Stress-Strain diagram for Ductile materials

Stress-Strain diagram shows the relationship between a load applied to a material and the deformation of the material, caused by the load . 

 

Stress-Strain diagram can be determined by the tensile test of a specimen.

The Stress-Strain diagram for a ductile (Mild Steel) are shown below.

• The initial straight line (OP) of the curve characterizes the proportional relationship between the stress and the deformation (strain). The stress value at the point P is called the Proportional Limit. 
• The line OE in the Stress-Strain curve indicates the range of elastic deformation. It means that removal of the load at any point of this part of the curve results in return of the specimen length to its original value. The elastic behavior are characterized by the elasticity limit (stress value at the point E). 

       For the most materials the points P and E coincide and therefore σ_el=σ_p. 

• A Point where the stress causes sudden deformation without any increase of the force is called yield limit (yield stress, yield strength)_.
    >>The highest stress occurring before the sudden deformation, which is called upper yield limit .
    >>The lower stress value, which causing the sudden deformation is called lower yield limit.
  The commonly used parameter of yield limit is actually lower yield limit.
• As the load increase, the specimen continues to undergo plastic deformation and at a certain stress value its cross-section decreases due to “necking” (point U in the Stress-Strain Diagram). At this point the stress reaches the maximum value, which is called ultimate tensile strength (tensile strength)
• Continuation of the deformation results in breaking the specimen - the point R in the diagram. 

Tensile Test of a Specimen: