Page 41 - MetalForming May 2011

“You’ll be hard-pressed
to find a more complete
hydroforming partner.”
Craig Zeleji,
Plant Manager, Hydroforming Schuler Incorporated
Setting the standards.
From production dies and presses to design, engineering and manufacturing of parts, we set the standards for hydroforming capabilities and expertise.
Hydroforming Specialists
To find out more, call 734.207.7200 or visit www.schulerhydroforming.com
Tooling Technology
30
25
20
15
10
5
0
0 0.25
0.50 0.75 1.00 1.25 h, in.
Break
Shear band evident
Diffuse necking
h*
Fig. 2—Graph showing biaxial stretching of aluminum sheet does not terminate as a load maximum when uniform elongation is reached. (Trans ASM, Vol. 56, p. 35).
occurs when the specimen tears or fractures. However, a close look at the tensile test (Fig. 1) shows that at the maximum load, all deformation terminates except in the diffuse neck. For practical purposes, when forming a part shaped like a ten- sile strip, all useful deformation throughout the stamping has stopped. Only a highly localized width neck continues deforming, as the load (stresses) in the stamping decreases. Thus, end of uniform elongation means onset of failure in terms of useful deformation. Fracture comes later.
For biaxial stretchforming over a hemispherical punch, a local or thickness neck occurs when certain modes of defor- mation are allowed to form. The thickness neck is a highly localized thinning with no deformation along the neck. Therefore, with biaxial stretching the material on either side of the local neck remains rigid with no further deforma- tion. This local neck usually starts with shear bands along the neck traversing at 45 deg. through the sheet thickness. The onset of the local neck formation (start of shear bands) defines when the sheetmetal fails. Fracture follows quickly (Fig. 2).
Having found the mechanism for the onset of failure, the third research phase studied several metal alloys in dead-soft, half-hard and full-hard condition. For the research to be use- ful, the final step was to determine if the strain at the onset of failure could be predicted. If so, maximum allowable combinations of useful strain would be available for part designers, troubleshooters, virtual press shops and other applications. We’ll present the results of this final research phase next month. MF
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MetalForming/May 2011 39
Load, lbs. x 10–3