The stress strain curve of a material shows the relationship between its stress and strain. It is
obtained through the tensile test of a material, by applying a tensile force to the material and measuring the
strain as the force increases. The stress strain curve is used to determine the mechanic properties of a
material, such as the Young’s Modulus, yield strength, ultimate tensile strength and ductility.The stress
curve is plotted from the results of a tensile test.
O-A: Proportional Limit.
According to Hooke’s Law,this limit is where the stress is directly proportional to strain. The Stressstrain curve is a straight line from ‘O’ to ‘A’ within the proportional limit. A material young modulus
of elasticity is constant within the proportional limit.
Point ‘B’:Upper Yield Point
Beyond the elastic limit, ductile material exhibits plastic properties. At the upper yield point, the
material required for the maximum stress to initiate plastic deformation inside the material. The
strength of a material corresponding to Point ‘B’ is known as yield strength.
Point ‘C’: Ultimate Tensile Strength
A material has the ultimate tensile strength at point ‘C’ on the stress-strength diagram. The ultimate
tensile strength of a material is the maximum stress a material can withstand before breaking. After
this point, ‘necking’ starts inside the material.
Point ‘D’: Rapture
Point ‘D’ is the point where material fractures or breaks. Stress at this point is also known as the
breaking strength of a material.
Q2. Mild Steel.
Point ‘O’ to ‘A’: Limit of Proportionality
There will be some elongation in a steel bar when it experiences tensile tension. The stress-to-strain
ratio will stay proportionate if the force is small enough. The graph illustrates this as a straight line
connecting point A and zero.
Point ‘A’ to ‘B’: Elastic Limit
Greater force will cause elastic deformation in the material, but the stress-to-strain ratio won't be
proportionate.
Point ‘B’ to ‘C’: Yield Point
Material starts yielding, for example, the material fails to regain its original size and shape after
removal of load. The material is deform permanently. Yield point is the point at which strain
increases without increasing stress.
Point ‘D’: Ultimate strength
Ultimate strength is the point at which material is subjected to maximum stress meaning it is the
maximum value of stress that the material can resist.
Point ‘E’: Fracture
The stress drops sharply after the necking, indicating the onset of material fracture. This is the point
at which the mild steel specimen breaks apart, and the stress decreases rapidly until the material
can no longer support any load.