True stress(σ) and true strain(ε) can be calculated from engineering stress(S) and engineering strain(e), formulated as follows:
σ=S(1+e)=P/A
ε=ln(1+e)=ln(A0/A)
P is load, A is the instantaneous cross-sectional area, and A0 is the initial cross-sectional area of the gauge section.
Two conversion formulae above are only valid when (1) the volume is constant and (2) the deformation is uniform. Therefore, we cannot use the formulae after necking occurs.
However I often find not a few academic articles that mention "true fracture stress" and "true fracture strain." They explains these two can be calculated by using the instantaneous cross-sectional area at fracture(Af), but we cannot measure Af precisly because the deformation after necking is non-uniform and heterogeneous.
Is it possible to calculate true fracture stress and true fracture strain? If so, how?
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