Yield Point

he yield point is the point on a stress–strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. 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.

Advances in measurement techniques allow higher precision mapping of the yield point which, as Marcus Reiner stated, showed "there was no yield point"[1]

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, bending, extruding, 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.
When certain materials such as mild steel are deformed in tension, it is found that the stress-strain curve is not smooth, but shows marked irregularities, with negative slopes occurring at or near the initial yield on the curve. The actual shape of the stress-strain curve is dependent, to some extent, on the type and characteristics of the tensile testing machine used; nevertheless one may include all cases where ??/?? is negative as examples of yield point effects deserving attention.