Deformation of Metals - Electrically Assisted Forming - page 18

3. Wrinkling —a local area in the sheet suffers a compressive stress which leads to
plastic instability and sheet waviness.
4. Shape change —during unload of the sheet elastic recovery or springback can
occur.
5. Low stretch —low deformation of flat areas of the sheet can have less work
hardening than other areas and suffer in dent resistance.
6. Soft surface —an uneven amount of deformation in a flat area of the sheet can
cause unbalanced residual stresses such that a small force disturbance on the
sheet after forming can cause elastic instability (i.e., oil canning).
The first two of these defect types are hard failures while the remaining defects are
unacceptable during sheet forming. Six formability index terms analogous to the
above failure modes have been defined:
1. Anti-fracturability
2. Anti-edge-fracturability
3. Anti-wrinklability
4. Shape-fixability
5. Stretchability, and
6. Anti-buckability
1.5.2 Additional Forming Metrics
Additionally, other formability parameters can include the following: the normal
anisotropy, the limiting dome height (LDH), the hole expansion ratio, the form-
ing limit diagram (FLD), and the uniaxial tensile test data. These aforementioned
parameters are described below.
1.5.2.1 Normal Anisotropy
The normal anisotropy coefficient (rn) can be used as a measure of formability as
it defines the resistance of the material to thinning. It can be defined by the ratio
of the strain in the width direction to the strain in the thickness direction. Thus, it
is desirable to have a higher r value so that the material will be more resistive to
thinning and delay material failure. The normal anisotropy relates to formability
as the resistance to thinning is critical in sheet forming as the material fails from
local thinning (necking).
1.5.2.2 Limiting Dome Height
The limiting dome height (LDH) is a measure of formability of sheet metal in that
it collectively quantifies parameters such as the n -value, the m -value, and achievable
elongation. This value is limited to the stretching mode and provides more realistic