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

- posted
16 years ago