What is the definition of yield point of the steel material

Given the steel material or sample is in tension, when the stress exceeds the elastic limit, even if the stress no longer increases, and the steel or sample still continues to have obvious plastic deformation, and the minimum stress value when the deformation phenomenon occurs is the yield point.

Suppose Ps is the external force at the yield point s, Fo is the cross-sectional area of the sample, then the yield point σs=Ps/Fo (MPa), MPa is called MPa equal to N (Newton)/mm2, (MPa=106Pa, Pa: Pascal =N/m2).

Yield strength (σ0.2): The yield point of some metal materials is extremely inconspicuous, and it is difficult to measure. Therefore, in order to measure the yield characteristics of steel materials, it is stipulated that permanent residual plastic deformation is equal to a certain value (generally the stress at 0.2% of the original length) is called the conditional yield strength or yield strength σ0.2 for short.