Page 39 - MetalForming December 2013

The Science of Forming
A
B
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C
C
When the tensile elastic stresses bal- ance the compressive elastic stresses, the cup stops changing shape.
Rule 5: Any subsequent modifica- tions to the stamping will change the residual elastic stress state and there- fore the shape of the stamping.
Redraw the cup, trim the top of the cup, punch a hole, weld on a bracket, heat the cup or any other modification and the cup will change the shape and
residual-stress pattern. That is why stampings matching part print after the first form operation usually do not end up with the specified dimensions in the final product. Some post-forming operations can even create new elastic stresses that interact with the previous stresses for an unpredictable shape change. To compound the problem, we cannot see or measure these stresses— especially in the press shop. MF
Fig. 4—While still in the die (A), the cup has complete circumferential compres- sive stresses. When released from the die (B), the upper portion of the cup expands to release the compressive stresses. The upper part then goes into tension to balance the residual compres- sive stresses still trapped in the cup.
different forming process, a higher n- value, and a stronger material exiting the forming process. The residual stress balance of channel A is lost as channel B seeks its own minimized residual stress forming mode.
Rule 4: Within the stamping, posi- tive residual stresses must be balanced by negative residual stresses.
Schematic A of Fig. 4 shows the elas- tic stresses in a deep-drawn cup while still in the forming die. The wall is cre- ated from sheetmetal originally in the blank. This material must compress as the circumference of the blank becomes smaller in its journey to the die radius. The material at the top of the cup must travel the furthest, and so must compress the most to create the largest compressive elastic stresses.
When we remove the cup from the die, these elastic stresses attempt to reach zero. However, the geometry of the cup only allows the cup circum- ference to increase (springback). The upper sections of the cup height are less constrained, allowing the elastic stresses to reach zero at the upper rim first. However, while the lower sections of the cup still have compressive stress- es attempting to reach zero, they are more geometrically constrained. These lower zones force the upper zones to continue expanding, generating tensile elastic stresses (Fig. 4 schematic B).

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