The tests were performed at a constant velocity. After placing
the blank on the die (under the blank holder in U-die bending),
the upper shoe which was attached to the ram of the machine,
moved against the lower shoe. The bending process was divided
into two stages; in the first stage, called loading, the punch moved
down until its stroke reached to a specific value, 40 mm. In the second
stage, named unloading (spring-back), the punch moved up.
Experiments were repeated for various sheet thicknesses, punch
tip radii and orientations in V-die bending; Thus 36 experiments
were totally performed. In U-die bending, the effect of punch tip
radius on spring-back was studied for the sheet thickness 1 mm
at different orientations. Also, the effect of sheet thickness was
examined for punch tip radius 4 mm at various orientations; thus
15 tests were totally performed for this die set. Therefore, 51 tests
were totally done for the two die sets