Answer :
Think about pulling a bar in tension. Load divided by cross-sectional area is force, or
stress. But what cross section are you considering? Before starting pull, the bar had a known
cross-section of (lets say) 0.5″ wide x metal thickness. It’s easy to measure these, since it is
your starting material. At any load, the engineering stress is the load divided by this initial
cross- area. While you are pulling, the length increases, but the width and thickness shrink.
At any load, the true stress is the load divided by the cross-area at that instant. Unless
thickness and width are being monitored continuously during the test, you cannot calculate true
stress. It is, however, a much better representation of how the material behaves as it is being
deformed, which explains its use in forming simulations. In circle grid analysis, engineering
strain is the % expansion of the circle compared to the initial diameter of the circle. The
relationships between engineering values and true values are: