Page 47 - MetalForming November 2016

The Science of Forming
area B starts to deform—areas A and C remain constant.
The purpose of the n-value is to increase the strength of the steel as it deforms. Fig. 2 shows that n-values of zero cause deformation to increase without any increase in stress or strength. Zones A and C do nothing because all of the deformation occurs in area B. The high concentration of deformation in area B forms a gradi- ent—a narrow localization of strain that can reach very high peak strains
“The purpose of the n-value is to increase the strength of the steel as it deforms... n-values of zero cause deformation to increase without any increase in stress or strength.”
(Fig. 3). Failure is easily reached.
Now assume that the entire strip has an n-value. The steel will start to workharden and increase the stress required for deformation (Fig. 4). Deformation starts at area B. After a small increase in height in area B, areas A and C start to deform and reach the strength of area B. Deformation then continues as areas A, B and C move to higher levels of strain. The higher the strain of the entire location the less importance the bump has in creating
failure.
Next month, we will discuss how n-
values for AHSS are quite different than those for HSLA steels. MF
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