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MAT040 not detecting non-linearity

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(@janeshka)
Barista
Joined: 1 year ago
I'm working with a transversely isotropic material with a nonlinear (elastic-plastic) response under compression. I'm looking into the nonlinear buckling of this material, and to do so I'm using MAT040 as the material card in LS-DYNA. However, it fails to recognize the nonlinearity in the stress-strain curve I've introduced under LCDIA. The AOPT I'm using is 2.0, and I've changed the axes respectively. Using an implicit solver.
 
Any help with this is appreciated 🙂
Janeshka
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Negative Volume
Posts: 641
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(@negativevolume)
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Joined: 5 years ago

Hi @janeshka

It's a little difficult to help you figure out the problem without having more details. How do you know your current material is not behaving correctly? What do you expect vs what is happening? Have you read about this material in the LS-Dyna Material Manual? (link below, page 429)

https://www.dynasupport.com/manuals/ls-dyna-manuals/ls-dyna_manual_volume_ii_r12.pdf

I believe this dyna support page will help a lot

https://www.dynasupport.com/howtos/material/orthotropic-materials

My theory is that something is off with how dyna is calculating your axes from AOPT. Try setting ATRACK=1 in *Control_Accuracy.

You can also do a sanity check by creating a single element model with this material and defining your coordinate system on that element, perform your compression, and see if the material is behaving correctly. This is actually what I would do so you can be 100% sure if the material is the problem or not. 

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Posts: 2
Topic starter
(@janeshka)
Barista
Joined: 1 year ago

Hi @negativevolume,

Thank you for your response. I've attached my .k file for further evaluation.
I've set AOPT in the MAT card as 2 and set the A3 vector as 1 as the z-axis is my longitudinal/primary direction.

I know the current model is not behaving correctly as I've created a solid element set and activated that in my CROSS_SECTION_SET in DATABASE to get the cross-sectional force through that section. The force I'm getting is 4000kN, where in actuality it should be around 500-600kN. The stress-strain curve I've defined through my material card also yields a failure strength at 70MPa. As the cross-sectional area is 100mm x 100mm, it is not possible to attain 4000kN.

My observation is that the material card is not reading the non-linearity of the curve and is assuming a linear behaviour. In this case, 4000kN makes sense.

Thank you for sending those links. I've actually gone through them as well.

ATRACK is in the MAT card, however, it's for shell elements only. I'm using solid elements.

The single-element check is a great idea. I'll follow through with that! Any help at this point is great as I've been struggling with this for quite some time.

 

Cheers!

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