Theoretical Problem:

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?

Ed Vojcak