In case of open die forging if initial disc diameter is dint & final disc diameter is dfinal, the true strain is given by
1
dfinal/dint
2
\(2 \ln \left(\frac{\mathrm{d}_{\text {final }}}{\mathrm{d}_{\text {int }}}\right)\)
3
dint/dfinal
4
d2 int/d2 final