For the same material, the forming-limit curve of soft (O) sheet is in a higher
position than that of hard (H) sheet. Referring to Table 1, it seems difficult to
relate the height or shape of the FLC simply with any property of the sheet. In
general, the limit strains in the region near to plane-strain deformation are
remarkably low compared to the other part of the FLC. However, it may be
difficult to increase limit strains in this region by improving the properties of
the material. A possible way of improvement is by the selection of the strain
path.
It has been shown that the primary straining has an influence on the
mechanical behavior of materials in the succeeding secondary straining in
multiple-stage deformation [4 6]. This may result in more hardening or softening
according to the combination of the deformation mode in the multiple
stage. The work hardening due to uniaxial stretching will be partially relieved,
if the stretched material is compressed in the same direction. In strain path l,
blanks are stretched in all directions in the primary stage and then stretched in
one direction in the secondary stage. In uniaxial stretching, a specimen is
elongated in one direction, but simultaneously contracted in the transverse
direction. So, a part of strain due to the primary straining may be relaxed by
the secondary straining. In the region near to equi-biaxial tensile deformation,
the primary strain reaches almost the maximum amount before failure, so that
any additional straining results in the decrease in the limit strains. This is the
lowering of the FLC with two-stage strain path ! in the region near to equibiaxial
tensile deformation. By decreasing the amount of primary straining,
the secondary straining can be accommodated and the relaxation of primary
strain by the secondary straining results in softening. This gives larger limit
strains in the region of the middle to the left side of the FLC. Thus the increase
of limit strains by strain path I is effective, especially in the region near to
plane-strain deformation.
For strain path II, blanks are stretched in one direction in the primary
straining, which means that blanks are forced to contract in the transverse
direction. Then blanks are stretched in all directions in the secondary straining.
A part of strain due to the primary straining may be relaxed by the
secondary straining. In the secondary deformation using elliptical dies, the
amount of strain differs with direction; maximum in the direction of the minor
axis and minimum in the direction of the major axis. Therefore, the relaxation
effect due to the secondary straining should depend on the direction. The
reason why increased limit strains are obtained when the minor axis of an
elliptical die is parallel to the primary tension axis is not clear at this time. It