The Lankford coefficient (also called Lankford value, R-value, or plastic strain ratio)[1] is a measure of the plastic anisotropy of a rolled sheet metal. This scalar quantity is used extensively as an indicator of the formability of recrystallized low-carbon steel sheets.[2]

Definition

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If   and   are the coordinate directions in the plane of rolling and   is the thickness direction, then the R-value is given by

 

where   is the in-plane plastic strain, transverse to the loading direction, and   is the plastic strain through-the-thickness.[3]

More recent studies have shown that the R-value of a material can depend strongly on the strain even at small strains [citation needed] . In practice, the   value is usually measured at 20% elongation in a tensile test.

For sheet metals, the   values are usually determined for three different directions of loading in-plane (  to the rolling direction) and the normal R-value is taken to be the average

 

The planar anisotropy coefficient or planar R-value is a measure of the variation of   with angle from the rolling direction. This quantity is defined as

 

Anisotropy of steel sheets

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Generally, the Lankford value of cold rolled steel sheet acting for deep-drawability shows heavy orientation, and such deep-drawability is characterized by  . However, in the actual press-working, the deep-drawability of steel sheets cannot be determined only by the value of   and the measure of planar anisotropy,   is more appropriate.

In an ordinary cold rolled steel,   is the highest, and   is the lowest. Experience shows that even if   is close to 1,   and   can be quite high leading to a high average value of  .[2] In such cases, any press-forming process design on the basis of   does not lead to an improvement in deep-drawability.

See also

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References

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  1. ^ Lankford, W. T., Snyder, S. C., Bausher, J. A.: New criteria for predicting the press performance of deep drawing sheets. Trans. ASM, 42, 1197–1205 (1950).
  2. ^ a b Ken-ichiro Mori, Simulation of Materials Processing: Theory, Methods and Applications, (ISBN 9026518226), p. 436
  3. ^ ISO 10113:2020 [1]