Yield point

A yield point marks onset of permanent deformation under the stated experimental criterion. A specified offset proof stress is an operational measure when a sharp yield transition is absent.

An offset proof stress is defined by the intersection of a loading curve with a line parallel to its initial linear elasticity response but shifted by a specified engineering strain. A 0.2 percent convention uses the line $\sigma =E(\epsilon -0.002)$, with Young's modulus $E$.