Mechanical stress which results in a certain plastic deformation in case of non-proportional elongation (Ep). It is determined in the tensile test.
The yield strength (Rp) is designated by the numerical value of the non-proportional elongation in % (Rp0.2). For technical materials, not the elastic limit but the 0.2% yield strength (Rp0.2) is usually indicated because (in contrast to the elastic limit), the yield strength can always be determined by the stress-elongation curve.
Yield strength ratio