Poissons ratio

1. Oct 24, 2004

clare*

I'm doing A2 course work on properties of materials and have been looking at Youngs modulus, Bulk modulus, Poissons ratio and Shear stress strain etc.:uhh: Was wondering if any kind person would be able to explain why Poissons ratio has theoretical limits of -1 to 0.5??? Have tried to search on the web but am confused by the answers I find! They dont seem very clear!
Clare

2. Oct 24, 2004

Clausius2

Yes. I've noticed it too when I dealt with structural engineering. Some Elasticity equations seem to break up when such poisson ratios are substituted. Surely you have taken a look at some denominators of the Elasticity equations and have realized of that.

Once upon a time.. I ask to my teacher about it. He said to me is impossible to find a material with such poisson ratios (I think a negative poisson ratio has no physical sense in fact), i.e. there is no material of poisson ratio of 0.5 in the Nature. Thus, the Elasticity Equations are given birth by the Nature, so that they cannot be used with any imaginary material.

It's like the adiabatic constant $$\gamma$$ There are a lot of equations inside Fluid Mechanics that are singular for some value of gamma, but that value is not present in our environment.

3. Oct 25, 2004

clare*

mmmm, I have read that there are some materials with negative poissons ratios that are man made-some, weird types of foam for example.....let me try and find the web site that I saw it on. I realise that if poissons ratio=0.5 then K, the Bulk modulus will be infinite meaning the material is incompressible which obviously cannot be true. But its the -1 limit I'm struggling with! I found a site that said "A Possons ratio of greater than -1 must be required to ensure that K is greater than 0 and that the solid contracts under the influence of positive compressive stress" I may be being a bit thick here but could you explain it to me in a different way?? I dont really get it!