Wednesday, 18 September 2019

# Stress-Strain Curve-True Stress V/s True Strain and Engg Stress V/s Engg Strain

Stress-Strain Curve: This is one of the important question asked in most of the interviews and when I had gone to an interview in Hyundai, they asked the same question to me and I had explained as below. In order to understand the topic, you should have to follow the below figure.

The figure represents the relationship between the load and elongation I.e. on the X-axis, elongation is plotted and on the Y-axis, load(F) is plotted.

When the sample is fixed between the grips of Ultimate Tensile Machine(UTM) and the top plate is moved upwards with a certain speed, implies elongation is produced in the sample, implies strain is produced. But the sample will resist the strain with a downward force(F) which is measured by the load cell connected in the UTM.

Therefore, the Load v/s Elongation curve (or) Stress-Strain Curve of Mild Steel is obtained as shown in fig.

## Stress-Strain Curve for Mild Steel:

*If the sample is loaded up to the point A, Stress is directly proportional to Strain called as Proportional region ‘OA’.

*If the sample is loaded up to the point B, stress is directly proportional to strain but by removing the elongation load on the sample, it will gain its original shape called as Elastic region ‘OB’.

*The corresponding stress value at the point of B is known as Yield Stress.

*If the sample is loaded beyond the point B, the displacement of atomic planes is slow, which means the plastic strain is produced in the sample but it will bear the maximum load up to the point C.

*The corresponding stress value at the point C is known as Ultimate Tensile Stress and that equals to the tensile strength of the material.

*If the sample is loaded beyond the point C, the displacement of atomic planes is fast and which means the severe plastic strain is produced in the sample ==>  a severe increase in length and decrease in cross-section area of the sample takes place and finally it will fail at the point D.

Below figure represents the change in cross-sectional area of a specimen when a force F0, F1 F2 etc.are applied on a specimen.

Difference between True Stress V/s True strain and Engineering Stress V/s Engineering Strain: Difference between True Stress Vs True strain and Engineering Stress Vs Engineering Strain

True Stress = F0/A0,F1/A1,F2/A2 …….

But during testing time, it is difficult to measure the reduction in the cross-sectional area of the sample(which is very small value).

Therefore,A1,A2,A3…….are not available.

Hence A1=A0,A2=A0,A3=A0…….is substituted.

Therefore,

Engineering Stress = F0/A0,F1/A0,F2/A0,.……..

Actually,A1<A0,A2<A0,A3<A0 implies curve will fall down.

Therefore Engineering Stress V/s Engineering Strain will follow the path of OABCD. whereas, True stress V/s True strain curve will follow the path of OABCD’ (here ‘ =dash).

*The curves will deviate from elastic point Onwards because above the yield stress value, the change in cross-sectional area of the sample is significant.

Note:

*If the material is tested at high temperature, the distance between the atoms increases implies binding energy decreases, implies strength decreases.

*Therefore it is always suggested to operate a component at low surrounding temperatures to increase the load bearing capability of the material.

*If the material is tested at high temperature, the displacement of atomic planes is easy implies plastic strain can be produced easily implies yield stress decreases.

*It is always suggested that heating is one of the processes to shape change the component because at high temperature the yield stress of the material is low, implies shape change will be easy.