Calculation of strain prior to necking
Calculation of true strain prior to necking is very simple. You will need to determine the length of the material that will be placed under stress or load.
Consider an example of a rubber band 1 inch long and use a rule to measure its length in an unstretched position.
As you apply tension to the rubber band, its length will increase. Tension can be applied by holding one end of the rubber band while pulling the other in opposite direction. Again, use a ruler to measure the length of the stretched rubber band.
For example, the length of the rubber band was increased to 2 inch after tension was applied.
Next, you will be required to divide the extended length of the rubber band by the actual length of the material, which will be 2/1 = 2.
Now use a scientific calculator to take the natural logarithm of the length ratio. The natural logarithm function is usually represented by LN or ln abbreviation on the calculator. Taking log of the 2, the answer will be 0.693, which is the strain prior to necking or deformation of the material.
Calculation of strain after necking
True strain after necking can be calculated using the areas of the material before and after applying tension. Divide the actual area by the necking area and then take the natural logarithm to determine the true strain after the necking process.