The Lankford coefficient (also called Lankford value, R-value, or plastic strain ratio) 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.
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 plastic strain in-plane and is the plastic strain through-the-thickness.
More recent studies have shown that the R-value of a material can depend strongly on the strain even at small strains . In practice, the value is usually measured at 20% elongation in a tensile test.
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
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 . In such cases, any press-forming process design on the basis of does not lead to an improvement in deep-drawability.