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