The most appropriate measure of formability for deep drawing is the r factor, which is a measure of the resistance of the material to thinning.
This is of particular importance when drawing sheet to form cans, as the greater the resistance to thinning, the less material is wasted, as the thinnest gauge possible can be used to start with.
The r factor is the ratio of true strain in the width direction, εw to that in the thickness direction εt as measured in a tensile test on a flat specimen:
The strain is related to crystallographic texture in sheet steels and will vary with test direction in anisotropic materials. r values are measured parallel, transverse and at 45° to the rolling direction, with the average value given as:
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