The zones which present an important variation of stresses in
the sheet thickness during bending are as follows: zones II and IV
that are subjected to tensile and bending loadings as a consequence
of bending on die and punch profiles and zone III that is
subjected to complex loading due to the fact that the material
that was stretched and bent in zone II, will be stretched and
unbent in zone III. If the radius of curvature due to bending
processr (Fig. 2) is sufficiently greater than the sheet thickness t,
in the case of combined stretching and bending the strains
variation is given by equation: