Yield point

A yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. 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.

The yield point determines the limits of performance for mechanical components, since it represents the upper limit to forces that can be applied without permanent deformation. In structural engineering, this is a soft failure mode which does not normally cause catastrophic failure or ultimate failure unless it accelerates buckling.

Yield strength is the critical material property exploited by many fundamental techniques of material-working: to reshape material with pressure (such as forging, rolling, pressing, or hydroforming), to separate material by cutting (such as machining) or shearing, and to join components rigidly with fasteners. Yield load can be taken as the load applied to the centre of a carriage spring to straighten its leaves.

The offset yield point (or proof stress) is the stress at which 0.2% plastic deformation occurs.

In the three-dimensional principal stresses (${\displaystyle \sigma _{1},\sigma _{2},\sigma _{3}}$), an infinite number of yield points form together a yield surface.

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