Page 38 - MetalForming July 2016

The Science of Forming
RIGID
A = Start of neck
RIGID
B = Deepening neck C = Fracture
Fig. 3—The start of the local neck (A) represents the start of the red severity zone for that sheetmetal blank.
stampings over several days. Output provides the strain magnitude and direction, information previously obtained by the circle-grid analysis. The missing piece of virtual data was the severity of stamping, able to be obtained using the FLD.
Around the world, FLD research has continued. Among the goals of this research: address complaints that plotting
the strain path on the FLD (Fig. 1) was incorrect for many nonlinear applications. Deeper research has revealed that using stress instead of strain for the Y axis is a more accurate approach. This modification fits well into virtual-forming programs and is now in use.
One challenge not yet solved, even after 50 years, is how to measure the start of the red zone (Fig. 3). Near the end of the stress-strain curve is the start of the local (through- thickness) neck. Once the local neck starts thinning, tensile load drops, deformation on either side of the local neck stops and overall deformation is terminated. The red-zone line represents the onset of the local neck.
The early measurement of the FLC0 was a finger detect- ing the onset of a neck on the hemispherical dome. Later, researchers continuously photographed the circle grid at the neck location and plotted the rate of straining. They deemed that the increase of strain rate represented the onset of failure. Different procedures remain under experimentation. MF
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36 MetalForming/July 2016
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