The Difference Between True Stress and Engineering Stress
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:
σ = s (1+e) ε = ln (1+e)
Where "s" and "e" are the engineering stress and strain, respectively, and "" and "" are the true stress and strain, respectively.