Yield Point, Offset Yield Point

The stress/strain point at which material behavior
changes from elastic to plastic is known as the
Yield Point. Because of the practical difficulty of
locating this point, the Offset Yield Point is used
in tabulations of material properties.

At the Offset Yield Point the sample has deformed
plastically to an extent that will leave the material
with a permanent strain of 0.002 (0.2%) when it is
at zero stress. The stress, s_y , at which this occurs
is the Offset Yield Stress.

The diagram opposite illustrates this deformation
condition, and locates the 0.002 Offset Yield Point
on the stress/strain curve.