True Stress = (Applied Force / Actual Area) = ( F / A _i)
True Strain = S_i (DL/L) _i = Ln (L_F / L ₀) = Ln( 1 + e_N) where e_N = (dL / L₀ )

Plastic deformation takes place due to shear deformation between planes of atoms. If the material is assumed to be an elastic continuum, the maximum shear stress due to a tensile stress occurs on a plane in the material at 45^o to the tensile axis so that the shear stress required for the onset of plastic deformation is: t_max = s_y /2

From: McMahon & Graham, "The Bicycle & the Walkman,"
Merion (1992)

For the multi-axis stresses involved in drawing (see diagram) the shears due to the compressive and tensile forces are added so that: t_max = (s_x + s_y )/2 where x is the tension direction and y the transverse compression direction.