Page 41 - MetalForming March 2015

Fig. 3—Any circle grid with a combination major/minor strain above the FLC is a potential failure. Just over the limit is a five- percent chance of failure. Moving up the major strain direction increases the percent failure.
moves up and down the vertical axis depending on the value of FLC0. For low-carbon steel, we obtain FLC0 from an
equation containing the sheet thickness and n-value from the tensile test. Why the n-value? Increasing n-value increas- es the amount of workhardening that delays the onset of the local neck. This allows the FLC to be determined for a given steel sample without conducting the hemisphere-dome tests.
Circle-grid analysis still finds use during die tryout and buyoff, troubleshooting and other studies in press shops around the world. FLCs represent the forming lim- its for virtual forming analyses (computerized die design and tryout). Virtual forming is constantly growing as industries move to higher strength steels and advanced high-strength steels. Part consolidation, tighter dimen- sional specs, drastically reduced die-tryout time and cost, loss of tool and die experts, light-weighting and other demands make virtual forming a standard procedure for many stampings. Having FLCs based on an equation with sheet thickness and n-value is the ideal match for virtual forming. MF
The Science of Forming
Forming Limit
Curve (FLC)
Major
Red Edge of Cliff
Major Strain
Failed Sample
FLC0
–0+
Yellow
Green
Safety Barrier
Minor
Minor Strain
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