The deformation of the material is divided into elastic deformation (the withdrawal of the external force can restore the original shape) and plastic deformation (the withdrawal of the external force can not be restored to the original shape, the shape changes) When the stress exceeds the elastic limit, the deformation increases faster, at this time, in addition to the elastic deformation, but also produces part of the plastic deformation. When the stress reaches point B, the plastic strain increases sharply, the curve appears a fluctuating small platform, this phenomenon is called yielding. The maximum and minimum stresses at this stage are called the upper and lower yield points, respectively. Because the value of the lower yield point is more stable, so it is used as an indicator of material resistance, known as the yield point or yield strength (σs or σ0.2).
a. Yield point yield point (σs)
Specimen in the test process force does not increase (keep constant) can continue to elongate (deformation) when the stress.
b. Yield point on the upper yield point (σsu)
Specimen yielding and force before the first drop in the maximum stress.
c. Lower yield point lower yield point (σsL)
When the initial transient effect is not counted when the minimum stress in the yielding stage.
(1) for the yield phenomenon is obvious material, yield strength is the yield point of stress (yield value);
(2) For materials in which yielding is not evident, the stress at which the ultimate deviation from a straight-line stress-strain relationship reaches a specified value (usually 0.2% of the original scale). Usually used as an evaluation index of the mechanical-mechanical properties of solid materials, it is the actual limit of use of the material. Because the stress exceeds the yield limit of the material produces necking, strain increases, so that the material damage, can not be used normally.
Some steel (such as high-carbon steel) no obvious yield phenomenon, usually a trace of plastic deformation (0.2%) when the stress as the steel yield strength, known as the conditions of yield strength (yield strength).
