# Poisson's Ratio Question

1. Aug 26, 2008

### scott_alexsk

Hello,

Is using Poisson's ratio to calculate the cross sectional area accurate after a material has been extensively cold worked and deformed. For example say I have a wire at some nominal length and thickness (which is too small for me to precisely measure, but I know at the start). So can I calculate the cross sectional area from the initial values say after I cold work the wire and get a 20% increase in length, using Poisson's ratio, or perhaps a calculation based on a constant volume assumption. Or is it impossible to calculate it fairly accurately either way?

Thanks,
-Scott

2. Aug 27, 2008

### Mapes

If the 20% elongation was uniform over the sample, you could try a constant-volume (i.e., Poisson's ratio = 0.5) approach to estimate the new cross section. On the other hand, if the elongation results from a localized region of necking, the problem is much harder.

Why not use a microscope or micrometer to track the cross section?

(I assume you know that the typical Poisson's ratio for metals, 0.3, only applies to elastic deformation.)

3. Aug 29, 2008

### scott_alexsk

The deformations I want to give are of the order of 5%, so it does not seem large enough to deem a constant volume correction (to me at least since it's so small and other effects may come into play), and it is plastic so Poisson's ratio does not really apply. I have been considering not bothering.

Thanks,
-Scott