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Force-Displacement Curve

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Posts: 5
Topic starter
(@kemoo)
Unpaid Intern
Joined: 2 months ago

Hey guys, I have been running crash analysis of lattice. I managed to run model with roughly 3 days run-time. However, When it comes to extract force datas from ASCII by using RCFORC, I got issue with the results. Cause I cant coincedence my ls dyna force-displacement curve with reference study. Here what can I do ? Am I extracting data by using wrong ASCII flag (RCFORC) ? As far as I am concerned , RCFORC is used for geting surface contact force. So how I am gonna see force-displacement datas from my solid's top surface ?

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Negative Volume
Posts: 577
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(@negativevolume)
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Joined: 4 years ago

Hi @kemoo

What type of contact is defined where you are trying to extract force? rcforc will not plot single surface contacts which might be your issue. 

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Posts: 5
Topic starter
(@kemoo)
Unpaid Intern
Joined: 2 months ago

Hey There,

Thank you in advance. The model consists of 3 different parts which are top rigid platen, bottom rigid platen and deformable lattice structure. *SURFACE_TO_SURFACE contact was utilised between rigid platens and lattice. *SINGLE_SURFACE contact was assigned to lattice itself. In such case How to extract force correctly? Cause ı am able to see forces using rcforc however it is not reliable and almost doubled when comparing experimental results.

Best Regards.

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Negative Volume
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(@negativevolume)
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CEO
Posts: 577

@kemoo do you have 2 separate surface to surface contacts for the top and bottom rigid plates?

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(@kemoo)
Joined: 2 months ago

Unpaid Intern
Posts: 5

@negativevolume Thats right. I have one surface to surface contact between top platen and lattice , second one is between bottom platen and lattice. Total i have two surface to surface contact that i used in my model.

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Negative Volume
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(@negativevolume)
Joined: 4 years ago

CEO
Posts: 577

@kemoo and your problem is that both of those contact forces are higher than the experimental data? Is it really noisy / are you filtering? If it’s double the force of the experimental data then it sounds like you might just have some issues with your lattice being too stiff in your model.

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(@kemoo)
Joined: 2 months ago

Unpaid Intern
Posts: 5

@negativevolume whenever i try to read forces using rcforc-Scontact or Mcontact i get same results with high values. Even if i use filtering , nothing seems right and still higher compared to experiment. How to reduce stiffness of lattice structure ?

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Negative Volume
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(@negativevolume)
Joined: 4 years ago

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Posts: 577

@kemoo it's difficult to tell whether the increased force is coming from the actual lattice structure being too stiff or from a factor in the boundary conditions/force output. Something else you can do to check that your forces are being output correctly is to use a cross-section set on one of the impactors instead of looking at contact force. This is the most full-proof way to check because contact forces can be influenced by all kinds of things ranging from contact parameters to mesh size. 

I made a diagram below but the gist is to change one of the impactors to a stiff deformable material so that you can get forces from a layer of solid elements within that impactor. You will then need to either make a top layer of rigid shell elements attached to this in order to drive your motion or create a node set if you are using boundary prescribed motion. I've also included a link describing various *database cards and how to set them up in a youtube video. 

https://feassistant.com/forums/ls-dyna/database-functionality/#post-115

https://www.youtube.com/watch?v=5YmL3Jj5Ihc&t=1254s

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