Plastic strain ratio r is a measure of the ability of a sheet metal to resist thinning or thickening when subjected to a tensile or compressive force. It is typically advantageous if the material reduces in area a minimal amount when subject to this force, meaning a good drawing material has a high r value. Once the material is taken beyond its elastic limit, it has deformed. If this flows easily in the plane of the sheet it will result in a high r-value. Alternatively, if it flows from the materials thickness it will thin during drawing, which may result in weakness. To determine this ratio, assuming constant volume, both the axial and transverse strain needs to be obtained during a uni-axial tensile test.