4. Conclusions
Substrate strains which developed during bending and unbend-
ing had significant impact on the texture of aluminum sheet alloy.
The three dimensional surface roughness Sa, and the normalized
surface volume (NVOL) were useful quantifiers of the changes in
surface texture which may affect properties such as friction and
paint finish. For the aluminum alloy studied, Sa and NVOL changes
were approximately linear to substrate strain at the first bend,
and became increasingly non-linear under the medium and large
strains developed by successive bending and unbending.
Texture changes during straining were more significant for MF
samples, which initial roughness Sa was less than one third the EDT
value. Percent changes were smaller for EDT textures, which have
a higher initial NVOL and Sa values. Research toward optimizing
surface textures must take into account the initial values and the
variability of these parameters upon deformation.
The effect on texture by tensile strains correlated well for uni-
axial and plane strain modes. However, the effects on texture under
compressive strains at the concave side of the bend were not gen-
erally equivalent to tensile strains. Compressive strains were more
significant at the initial stages, while changes under tensile strains
were more significant at successive bending. Roughness changes
were larger along the transverse direction. Changes in texture were
initially isotropic for EDT samples, but became larger along the
transverse direction as the deformation became increasingly non-
linear.
Sa roughness and NVOL changed significantly upon unbending
the sample to its flat condition. This operation disturbed any mono-
tonic Sa trend. The Sa values typically decreased as the magnitude
of the net strain became smallest at the flat condition. The rela-
tionship between Sa and strain history became complex, and, as
shown by this study, became unfeasible to determine strain his-
tory effects upon unbending based only on measurements of Sa
roughness values.
Under drawbead flow, the sheet unbends to flat in a very small
area at the point of inflexion between the bent and reverse bend.
This study showed that texture changes at that section are signifi-
cant and must be measured. The understanding of texture changes
under bending strains is a contribution to further research on fric-
tion and contact phenomena in forming areas such as drawbeads
and corner radii.