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Using an orthotropic material in ansys static structural — Ansys Learning Forum

# Using an orthotropic material in ansys static structural

Member Posts: 6

Hi there wondering if anyone can help.

I am doing a project on ANSYS using wood, an orthotropic material. I have calculated the poissons ratios etc for implementing into a material, but ANSYS comes up with this error;

What I don't understand is how I can reduce the poissons ratio when it is dependent on the Youngs and Shear modulus. How can I fix it and make my material work? Any help is greatly appreciated! I have also attached my material properties as I am trying to set them up below;

I hope this makes sense!

Emily

• Member Posts: 11,369

I found these MIT lecture notes that might be helpful. This section seems to describe your material.

http://web.mit.edu/16.20/homepage/3_Constitutive/Constitutive_files/module_3_with_solutions.pdf

@BenjaminStarling may provide some guidance for you.

• AustraliaMember Posts: 101

That's a great resource you found.The most useful matrix I found is this. You can input this matrix using the compliance matrix type in Engineering Data.

if you follow the above table, remembering each value is divided by E, then you will get the same result as an isotropic material. This image below is two blocks, with two material definitions as I described in your other post.

Hopefully this points you in the right direction. I understand you are trying to model an actual anisotropic material, so there is a bit more work to do than this.

• Member Posts: 11,369

Hi Emily @g_empo

I found a paper that reports the Othotropic elastic properties of many types of wood. That paper uses the terms L, R and T to represent Longitudinal, Radial and Transverse directions in the wood. If you assign the x axis to Longitudinal, which axis is going to be Radial? Aren't most sheets of wood going to have the Radial direction be the thickness direction? Is thickness Z in your definition? I see that the Poisson Ratio in this paper is > 0.5 in many cases. That is because they have measured the shear modulus G. The paper also shows the magnitude of Young's Modulus relative to the Longitudinal axis (grain direction).

Based on this table, I would expect to see values of E.y and E.z to be a small fraction of E.x

The table for Poisson Ratio shows that there are different values depending on which direction is being stressed.

Note that the value for LR is not equal to the value for RL.

The Orthotropic material model in ANSYS can only handle the case where LR = RL.

I'm curious how you got negative values for Poisson's ratio and the large positive value seems wrong.

• Member Posts: 6

Hi Peter,

I am guessing that the way I am trying to calculate poissons ratio is fundamentally wrong I was doing v=E/2G - 1?

For mine the material is the same radially and tangentially, which I have made the x and y axis and the parallel. These are the properties I am trying to model.

I am really confused by the negative and massive poissons ratios as well.

• Member Posts: 6

I have also tried to do it using an anisotropic table which it seems to accept but I have no idea if it is correct, I followed this post here https://forum.ansys.com/discussion/12110/silicon-anisotropic-elasticity

This is what I created

• Member Posts: 11,369

I found these MIT lecture notes that might be helpful. This section seems to describe your material.

http://web.mit.edu/16.20/homepage/3_Constitutive/Constitutive_files/module_3_with_solutions.pdf

@BenjaminStarling may provide some guidance for you.

• AustraliaMember Posts: 101