Page 40 - MetalForming June 2011

The Science of Forming By Stuart Keeler
Tooling Technology
Is Metalforming Failure Predictable?
Forming Limit
Curve (FLC)
Major True Strain
Failed Samples
Red Yellow
Safety Barrier Green
Major
Edge of Cliff
Major Safe Sample
Minor
Minor
–0+ Minor True Strain
A 1957 research program on metalforming tackled two important questions:
• When, or more accurately how, does sheetmetal fail during forming?
• Can the onset of failure be predicted?
We discussed the first question in last month’s column. Failure occurs when useful forming throughout a part ter- minates, and all subsequent deformation localizes in a very narrow band—a through-thickness local neck. The maximum useful stretchability or tensile deformation in the stamping terminates when the local neck begins to appear. Not only does initiation of the local neck determine maximum punch travel and stamping configuration, but the visibility of the neck spoils any class-A surface. Deformation in the local neck continues until the material in the neck tears and fractures.
Stampers often overlook the importance of the local neck as they attempt to correlate its onset with properties from the tensile test—onset of the diffuse (width) neck at the load max- imum, uniform elongation, ultimate tensile strength or total elongation. Instead, the onset of the local neck is a unique event and a complex material property.
Fig. 2—Forming Limit Diagram displays Forming Limit Curve (FLC), safety margin (yellow zone), safe and failed samples, and different deformation paths within a stamping.
Defining how sheetmetal fails has limited practical press- shop application unless one can predict the onset of failure, by the amount of strain and the location in the stamping. This situation is akin to walking near the edge of the Grand Canyon in the dark, wondering exactly where the edge is and knowing that going too far in the wrong direction will take you over the edge. One wrong step leads to falling off the cliff— failure, followed by a severe crash—fracture.
Knowing what occurs after falling is not useful informa- tion. What is useful? Knowing the exact distance from the starting location to the edge of the cliff, for the chosen path (Fig. 1), which proves helpful in establishing a safety zone and barrier to warn of an impending fall.
Phase II
During the second phase of the research program, scien- tists sought to define the edge of the deformation cliff for a variety of alloys formed into different stampings. For this they developed a circle grid, which measures the strain accu- mulated by the stamping (Fig. 2). Like a map of the area around the Grand Canyon, the circle grid provides the start- ing point (zero-by-zero strain). Instead of north/south and east/west tracking of movement, the circle grid uses major strain (largest positive strain shown as the long axis of the resulting ellipse) and minor strain (perpendicular to the major strain).
The circle grids allow tracking the deformation at every location in the stamping.
Research included samples of steel, aluminum, copper and brass, although the most extensive tests were on steel. After 10 years, the research produced the first map (Fig. 2) defining
North
Grand Canyon
Edge of Cliff
Safety Barrier
West
Start
Direction East
Fig. 1—Walking toward the edge of the cliff along different paths results in different travel distances.
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementa- tion of other press shop analysis tools. Stuart’s sheet- metal forming experience includes 24 years at National Steel Corporation and 12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and the many seminars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
38 MetalForming/June 2011
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