zone BC is the one which has the most complex deformation history,
since it is stretched and curled and then stretched again to become
the side wall of the U-piece, whereas zones AB and CD are
only stretched and then curled.
The analysis of sheet thinning that we did, as represented in the
curves of Fig. 4, shows clearly that the stretching effort on the sheet
in the zones curled by the stretching rod and by the die (zones AB
and CD) decreases from B towards A and from C towards D. The decrease
in this effort is due to the effect of friction. Through their
mathematical deformation, Zhang et al. [31] have well explained
the decrease in the stretching effort in the bended zones, pointing
out that the maximum stretching effort is at B and C. We can also
observe that the ratio r = Rd/Rp (curvature radius of the die/radius
of the stretching rod) can make the difference in deformation between
zones AB and CD. Indeed, for r = 1.25, thinning is important
at section C which is nearly equal to that at B (etC etB), whereas
for a greater value of Rd leading to a greater value of r (r = 2), thinning
at section C is less than that at B (etB < etC). Fig. 4a and b also
shows that thinning at sections B and C is greater than that at A
and D. This is in agreement with the work of Zhang et al. [31]. In
their numerical simulation, Samuel [2] and Kim and Koç [13] have
observed that sections B and C are the regions where internal stresses
are considerable. This is due to the concomitant effect of the
bending moment and the maximum stretching effort.