Page 23 - MetalForming January 2010

mation in the blank would be concen- trated in very narrow bands and fail without creating any useable shape.
• Work hardening strengthens the initial areas being deformed and caus- es deformation to be more uniformly distributed throughout the stamping.
• The most common measure of work hardening is the n-value obtained from the tensile test stress-strain curve equation σ=KЄn,whereσisthetruestress,Kisa material constant, Є is the true strain and n is the work hardening exponent.
• Sharper geometrical features in a stamping naturally tend to localize deformation and generate localized thinning. These stamping designs require higher n-values to prevent early forming failures.
More Understanding About Work Hardening—October 2009
• The n-value is the slope of the stress- strain curve between the yield and tensile strength. A steeper slope means more
work hardening and a higher n-value.
• Unfortunately, the mechanisms used to strengthen the material are the
same ones that decrease the n-value.
• Application of higher-strength steels usually require part or tool designs that avoid localization of deformation. • The n-values also control the max- imum allowable amount of stretch defined by the forming limit curve dis-
cussed in the December 2009 column.
Sheetmetal Failures: Fracture or Necking —November/December 2009
• The usual mode of fracture is a ductile fracture—not a brittle fracture. • As the yield strength or the amount of cold work increases, the fracture
remains a ductile fracture.
• The actual fracture of sheetmetal is
not the termination of useful deforma- tion. Instead, a local or thickness neck usually occurs prior to the onset of fracture and terminates all useful defor- mation outside of the neck.
Determining Maximum Allowable Stretch —December 2009 (Available online only) • For steel and many other metal alloys, the maximum allowable stretch is defined by the onset of the local or
thickness neck in the stamping.
• For a sheet of metal (a single set of properties), the maximum allowable stretch relates to the minor stretch value at each location in the stamping. Since minor stretch depends on part and tool- ing design, a large range of minor stretch values can be found in any stamping. This means a large range of maximum
allowable stretch values also exist.
• The forming limit curve translates this multiplicity of stretching limits
into a useful press shop tool. Numerous mechanical properties
can be obtained for each alloy. Under- standing the interaction of these prop- erties and their influence on the trans- formation of a blank into a useful shape is important for any successful press shop—both physical and virtual. MF
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