Yield Point

HOOKE'S LAW states that for a material such as steel, in wire form, the extension is proportional to the tension, up to what is called the elastic limit. An increase in tension beyond this limit takes the material to the yield point, where a sudden increase in elongation occurs with only a small further increase in tension.

Taken from Dictionary of Science

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The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent and non-reversible.

In the three-dimensional space of the principal stresses (σ₁,σ₂,σ₃), an infinite number of yield points form together a yield surface.

Knowledge of the yield point is vital when designing a component since it generally represents an upper limit to the load that can be applied. It is also important for the control of many materials production techniques such as forging, rolling, or pressing. In structural engineering, this is a soft failure mode which does not normally cause catastrophic failure or ultimate failure unless it accelerates buckling.

Taken from Wikipedia