It should be noted that the conclusions are drawn from using
the same level of normal stress. In fact, to have a fair comparison,
the FLCs under the same ratio of normal stress to the material's
yield stress should be obtained and compared. To qualify the
sensitivity of FLC to the normal stress for various materials, the
increase of lowest point in the FLCs (FLC0) caused by normal stress
is used to indicate the effect of the normal stress on forming limit,
while the level of normal stress is described by the ratio of the
absolute value of normal stress to the materials’ yield strength
(jσzzj=σy). Fig. 7 shows the increase of FLC0 predicted under
different levels of through-thickness normal stresses for AA5XXX
and AA6011. The figure reveals that the normal stress can significantly
influence the FLCs. For instance, when the normal stress
of 54.55 MPa (jσzzj=σy is 1) is applied to AA6011, the increase of
FLC0 is 64.61%. Meanwhile, the FLC will move right slightly when
through-thickness normal stress increases.