Since the actual cross- sectional area is reduced, use of the initial area gives a lower value than the actual one (the ratio is Ao/Ac)
True stress :
σ = F/Ac
- F: Load
- Ac: Current area
σ = S (1 + e)
True strain:
ε = ln (lc / l0)
-lc: current length
-l0: original length
ε = ln (1 + e)
Even though the true stress-strain curve gives a more accurate picture of the breaking strength of a material, it is difficult to obtain measurements of the actual area in real time.
Usually the reported values are the engineering stress.
True fracture strength > tensile strength (but the engineering s- e diagram does not show this)