Determination of yield point and yield strength of ASTM A572Gr.50 steel material

In the tensile test process of ASTM A572Gr.50 steel material, the elastic deformation of the specimen is increasing with the increase of the tensile load. When the secondary load is not increased or decreased, while the deformation of specimen increases suddenly, the specimen seemed to yield to the load and self elongation, the phenomenon is called yield phenomenon. The stress caused by the yield phenomenon is called the yield point and can be calculated according to the following formula:
σs=Ps/S0
In the form, σs- yield point, MPa;
Ps - yield load, N;
S0 - the original cross section area of the sample, mm2.

The appearance of the yield point symbolizes the transformation of ASTM A572Gr.50 steel material specimen from the elastic deformation to the plastic deformation. Because when the applied force reaches or exceeds the yield point of ASTM A572Gr.50 metal material, if the external force is eliminated, the length of the specimen is partially recovered, but it can no longer revert to its original length, that is, a part of deformation (elongation) is permanently retained.