Yield Strength: The yield point corresponds to the point where the material begins to have permanent (unrecoverable) deformation. While some materials have a well-defined yield region (Figure A), others (Figure B) do not. In the absence of a distinct yield point, a 0.2% offset is used to obtain an approximate yield point. Although the yield and the proportional limit points are close to each other, they do not correspond to the same location on the stress-strain curve.
The stress and strain corresponding to the yield point are called the yield stress and yield strain, respectively.
0.2% Offset:
The 0.2% offset point is determined by drawing a line parallel to the linear region of the curve starting from point 0.002 on the strain axis. The intersect of this line and the stress-strain curve defines the 0.2% yield point.
