Thread: What is Areal Poisson's Ratio?

Hello. I found an interesting article about Auxetics and negative Poisson's ratio.
http://msp.org/jomms/2007/2-8/jomms-v2-n8-p08-p.pdf
I know that Poisson Ration is the ratio of transverse contraction to longitudinal extension strain in the direction of stretching force. That is v=dEy/dEx.
But this article is always nagged me with Areal Poisson's Ratio. What is that? Where is a distinction? (Between usual Poisson's Ratio)
And the second question I wanna find the derivation for Poisson's ratio of triclinic and monoclinic crystals, as Google keeps silence in it.
Please give me any article, textbooks and so on where I can find it.
Immense thanks in advance to you!

Hello again. Can anybody tell me what triclinic and monoclinic crystals have negative Poisson's ration. Please, I need in answer promtly. Please, help me.