Poisson ratios for Orthotropic materials (composites)

AI Thread Summary
Understanding Poisson ratios for orthotropic materials is crucial for accurate modeling in composite applications. The user seeks to confirm the relationships between the Poisson ratios, specifically noting that for orthotropic materials, ν12 is not equal to ν21. They inquire about deriving a complete matrix of Poisson ratios from the known values ν12, ν23, and ν31, and whether additional constraints exist to ensure data consistency. The discussion highlights the importance of verifying material properties to avoid errors in fluid-structure interaction simulations. Overall, ensuring the integrity of the input dataset is essential for reliable results in finite element modeling.
Arjan82
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For an orthotropic material (e.g. carbon reinforced polymer), how to derive the Poisson matrix from 3 values?
I'm new to composite materials. I've studied mechanical engineering but I am actually usually involved in hydrodynamics (in which I've done my masters). However for a project we do fluid structure interaction with composites, and as these things go, you cannot get away with the 'black box' approach (I wish I could sometimes...). So I'm trying to learn the basics.

In the FEM software I use I need to specify three Poisson ratios: ##\nu_{12},\ \nu_{23},\ \nu_{31}## (and also three E and three G moduli). It is orthotropic material (3 symmetry planes). I want to derive the other Poisson ratio's because I want to know if the supplier provided a consistent set of data.

Some questions:
  1. ##\nu_{12}## gives me the strain in 2 direction from the strain in 1 direction, i.e. ##\epsilon_2 = -\nu_{12}\sigma_2/E_2##, correct?
  2. But I believe in general, for orthotropic materials, ##\nu_{12}## is not equal to ##\nu_{21}##. Correct?
  3. If indeed so, can I derive the complete matrix of all Poisson ratios from ##\nu_{12},\ \nu_{23},\ \nu_{31}##? And how?
  4. There are al kinds of constrains that must hold for a real material, e.g. ##\Delta = 1-\nu_{12}\nu_{21}-\nu_{23}\nu_{32}-\nu_{31}\nu_{13}-2\nu_{21}\nu_{32}\nu_{13} > 0##, which is I believe the change in volume...? Are there more of these checks that I can do to find if my input dataset is consistent?
 
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