Many definitions to define the on-set of plastic deformation exist based on the offset yield (i.e 0.2 % , 0.02%, Johnson's Apparent limit etc.) strengthand all of these depend on a non-robust methodology namely: find the Elastic Modulus, construct an off-set line, and were is crosses the load extension curve there is the on-set of (practical) plastic deformation.
I have often wondered why (say by the use of a biaxial extentensometer) the use the change in Possion ratio as a more robust definition for the on-set of plastic deformation was never utilized. In the elasticity realm roughly =B5 =3D 0.20 - 0.35 (material dependant) but at the onset of true plastic deformation =B5 =3D 0.5 due to the constancy of volume (for all metals - at least up to a second order approximation - i.e. density decreases with workhardening so 0.5 will decrease during increasing deformation).
Does anyone know or remember if this was ever considered or see reason why it will not work?