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Advanced Building Skins
compressive stresses, the grain structure changes during the process [7]. An ultra-fine grained
structure is obtained. The special characteristics of this grain structure are a good ductility and a
concomitant high tensile strength. The tensile strength is raised by the process in comparison to the
base material up to over 60% [8]. Again a better resource efficiency of this process can be claimed.
2.2 Deep Drawing of Blanks with bifurcated Cross-sections
A bend split metal strip can be cut into blanks at the end of a splitting line. These blanks serve as
wrought material for a subsequent deep drawing process in which they are formed to their final
spatially curved shape.
Deep drawing is a wide-spread and well known forming technology. It is used whenever sheet metal
parts with complex geometries (e.g. car body parts) are to be manufactured in large numbers. The
maximum size of deep drawn parts depends on the target geometry and the machinery used and varies
from a few square millimeters to a few square meters.
Figure 3: Deep drawing
A classical deep drawing tool consists of a punch, a die and a blank holder (see figure 3). The blank is
positioned between the die and the punch, whose surfaces correspond to the target surfaces of the
finished part. By closing the gap between punch and die their shape is reproduced in the work piece.
The blank holder holds the blank in position. The adjustment of the blank holder force controls the
material flow into the gap and is a key parameter of the deep drawing process. It influences the stress
and strain distributions in the workpiece, the resulting sheet thickness at the end of the process and is
also used to prevent wrinkling in regions of the work piece which are under compressive stress during
the process.
Since within the context of deep drawing the term ‘flange’ is used for the outer regions of a workpiece
(see figure 4) the flanges that result from the splitting process are referred to as ‘stringers’ from here
on. Sheets with bend split bifurcations will be referred to as ‘stringer sheets’.
Figure 4: Sheet metal hydroforming of a stringer sheet
(section few of a tool)
- 4 -
Figure 5: Demonstrator sheet
structure made of hydroformed
stringer sheets