Question

Why for practical use should the poisson' s ratio be positive lie between 0.2 - 0.4?

Answer 1

Actually more accepted values of poison's ratio is 0-0.5 rather than just being 0.2-0.4.

Now, When we stretch a material from one side the other dimensions of the material decrease to keep the volume in check ,due in some materials volume increases, but it never decreases it is not possible. When Poisson's ratio becomes more than 0.5 then when we stretch the material then the volume tend to decrease from the original, which is not possible.

When volume increases then Poisson's ratio becomes NEGATIVE and there are some exceptional materials which exhibit that kind of behaviour. But Poisson's ratio less than -1 is not possible.