Nonlinear analysis with orthotropic 3D material doesn't work

[martin.kehr]

Hello,

im trying to run an nonlinear analysis with an orthotropic (3D) material i created (sugarmaple (wood)).

The values (in MPa) i've used can be seen in the following pictures.

But when i try to run the simulation i get the fatal erros T2088 and T2090 as to be seen below. 

I found out that when setting the material to isotropic the nonlinear analysis works. So i assume that the stress/strain input i used is not the problem.

But i'm running out of ideas what else i can manipulate to make it work.

If anyone has an idea of what could cause the issue i'd be very thankful to read it! 🙂

Thanks in advance

Martin

[John_Holtz]

Hi Martin, @martin.kehr

I am far from a materials expert, so the following may be way off .

Does it make sense to have one stress-strain curve for an orthotropic material? This is somehow related to the problem with the software.

1. One stress-strain curve is valid. The first two points on the stress strain curve (0,0) & (yield strain, yield stress) are automatically calculated based on the modulus of elasticity entered for the isotropic material and the initial yield stress. It looks like the interface is ignoring the fact that the material is orthotropic. In order to get the calculated modulus from the two points to work out properly, you can trick the interface by temporarily changing the type from Orthotropic to Isotropic, and enter the correct modulus of elasticity. (The question then becomes, what is the equivalent modulus of elasticity for the isotropic? E1? E2? E3? Some formula that includes all 3, like a von Mises stress formula?)
2. Multiple stress-strain curves needed. If this is the case, then the interface is not designed to handle the case. You can probably edit the Nastran file directly to enter the additional input.
3. Plastic material not valid for orthotropic. If for some reason it is not valid for an orthotropic material to experience permanent deformation (plasticity), then the interface is wrong for letting the user activate the nonlinear materials. (I would think that there is some orthotropic material out there that yields. I would not expect wood that you mentioned to be one of those materials that behaves plastically, but maybe you are using it as an approximation for something else.)

Sorry for rambling

[martin.kehr]

Hi John,

first of all thanks you for your reply.

It’s quite hard for me to reply on you cause im far from being expert in every way (just a student). 😬

But i can tell that wood is reacting in a nonlinear way under stress. The following picture for example shows some stress strain curvatures of laminated timber with different moisture content under compressive load.

To me it sounds very reasonable that there are different stress strain curvatures needed for every direction. But i think i won’t be capable of editing the Nastran file, thats why i’m happy with the in-cad solution. That in a way leads to the question that you mentioned about letting the user activate nonlinear plastic for orthotropic materials. I hoped to only have overseen some detail to make it work…

But what i found out lately is that „Elasto-Plastic (Bi-Linear)“ modus is working with the orthotropic input. That is closer to reality than a linear static analysis and probably close enough for what i want to do. 😊

✌️

[martin.kehr]

Hi John,

first of all thanks you for your reply.

It’s quite hard for me to reply on you cause im far from being expert in every way (just a student). 😬

But i can tell that wood is reacting in a nonlinear way under stress. The following picture for example shows some stress strain curvatures of laminated timber with different moisture content under compressive load.

To me it sounds very reasonable that there are different stress strain curvatures needed for every direction. But i think i won’t be capable of editing the Nastran file, thats why i’m happy with the in-cad solution. That in a way leads to the question that you mentioned about letting the user activate nonlinear plastic for orthotropic materials. I hoped to only have overseen some detail to make it work…

But what i found out lately is that „Elasto-Plastic (Bi-Linear)“ modus is working with the orthotropic input. That is closer to reality than a linear static analysis and probably close enough for what i want to do. 😊

✌️