Yield Strength

Summary

Yield strength or yield point is an important indicator for most of the engineering design and material science as the stress at which a materials which begins to deform plastically. Before the yield strength, the material will deform elastically and will return to its predefined shape when the applied load is taken out of it. But whenever the yield point is passed some fraction of the deformation will be permanent and non- reversible. In the 3 dimensional space of the principle stresses, an infinite number of yield strength from together a yield surface. Acquaintance of the yield strength is very important in designing component, since it usually represent the upper limit to the load that can be applied. For controlling many materials production techniques such as forging, rolling, or pressing yield strength is very important.

Yield strength

Yield strength is the amount of stress, at which the plastic deformation becomes noticeable and noteworthy. In the Fig.1 represents the engineering stress strain diagram in tensile test. In the curve where elastic strain ends and plastic strain begins, has no definite point, the yield strength chosen to be that strength when a definite amount of plastic strain has occurred. The yield strength is chosen when 0.2 % plastic strain has taken place, for the general engineering structural design. Then the 0.2 % yield strength is calculated at 0.2 % offset from the original cross- sectional area of the samples.

Fig.stress – strain diagram

The materials will deforms without an increase in applied load, during yielding stage. But during the strain hardening stage, the materials will undergo changes in its atomic and crystalline structure and resulting in the increased resistance of the materials to further deformation. In the engineering structural design, yield strength is very important. For example, if we are designing the component, it must support the force during use and we must confirm that the component does not deform plastically.  Therefore the material that has high yield strength should be selected. The tensile strength is relatively unimportant for the ductile materials selection and application, since too much plastic deformation takes place before it is reached. Nevertheless the tensile strength can produce some indications of material, such as hardness and material defects.

Yield strength Vs tensile strength

The yield strength for the low alloy steel is usually about 75-90% of the tensile strength, and for the mild steel is about 65-75%. The tensile strength increases more than three times the yield strength with the higher carbon content. Nevertheless the yield to tensile strength ratio is not a value suitable for characterizing the safety requirements.

Questions:

• What is yield strength?
• The knowledge of yield strength is more important in designing component or not?