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Material keyword for PMMA

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(@sarvin)
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Hi, I am currently doing my project on high speed impact on PMMA or Acrylic glass and I haven't found the correct material that could show the same results with the paper that I am validating, I need help in choosing the correct material keyword for PMMA or acrylic. If anyone could help me please comment down below. Your help is very highly appreciated. Thanks in advance

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Negative Volume
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(@negativevolume)
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Hi @sarvin

Have you checked out either of these links? The first is a link to a material selector where you input the element formulation of the part that you will assign the material and then the general type of material you want. It'll also give some high level description of each one. 

http://www.lstc.com/dynamat/

https://www.dynasupport.com/howtos/material/material-models-for-polymers

https://www.researchgate.net/publication/312040006_Material_Models_of_Plastics_for_Crash_Simulation_in_LS-DYNA

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(@sarvin)
Joined: 3 years ago

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@negativevolume Yes I have already tried the options from the first link and the second link, but still did not manage to get the simulation done.

It is a impact test where a small projectile hits the PMMA plate and the residual velocity must be recorded. 

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(@sheida)
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Posted by: @sarvin

@negativevolume Yes I have already tried the options from the first link and the second link, but still did not manage to get the simulation done.

It is a impact test where a small projectile hits the PMMA plate and the residual velocity must be recorded. 

Hi Sarvin, 

I am modeling a high velocity impact between a bullet and a plate as well. Can I ask how we can obtain the residual velocity after running our model?

Thank you,

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(@icebreakersours)
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How do you know its the material that is the source of the discrepancy? Could it not be something else in the model set-up? Or, could it not be that the paper itself has a mistake?

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(@sarvin)
Joined: 3 years ago

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@icebreakersours I don't think the paper has a mistake, but there might be other sources that made my simulation wrong. I am still looking out for possibilities of error but couldn't figure it out. The reason I posted this question is that I want to be sure that the MAT keyword that I am using is accurate so that I can focus on getting the simulation to run by fine tuning other parameters

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Negative Volume
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(@negativevolume)
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@sarvin keep in mind too that a lot of LS-Dyna materials require an analysis to fit the parameters within the *material to match real-world experimental data. Not sure if this is the case or not but just something to keep in mind. 

If you are just plugging in values from a paper then I'd try to use something as simple as possible so that you know that you aren't missing any parameters in more complex material models. 

What are you using to fit your model? (ie. elastic modulus, yield stress etc.)

You can also try more traditional materials like *Mat_piecewise_linear_plasticity or *Mat_plastic_kinematic. Polymers can vary so much in terms of material response so I'd give those a shot. 

Do you mind sharing what paper you are referencing?

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(@sarvin)
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@negativevolume https://doi.org/10.1016/j.ijimpeng.2020.103597  
this is the link to the paper that I am modelling. By the way i have managed to get the results which is almost similar but just that the damage on the plate is not visible. I used mat 029 piecewise linear plasticity. thank you very very much for your help

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Negative Volume
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(@negativevolume)
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@sarvin great to hear! Regarding the damage on the plate, is that part of the PMMA or the testing apparatus?

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(@sarvin)
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@negativevolume The PMMA, it should have a hole when the bullet pass through it , but it is not visible in my case

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Negative Volume
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(@negativevolume)
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@sarvin do you have any information on failure strain of the PMMA? Because you can add *Mat_add_erosion to that part and elements will delete when they reach the defined threshold.

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(@sarvin)
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@negativevolume Yes,  I have added mat_add_erosion, but the elements did not delete, I could not figure out why

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Negative Volume
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(@negativevolume)
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@sarvin did you use strain as the threshold or something else? And a way to check that it’s working would be to plot the fringe of that threshold (ie. first principal strain) and see what strain values you are hitting in that part. If they are passing your threshold and not deleting then there is an issue with the add_erosion card. 

You might need to add *Database_extent_binary and change the strain flag (strflg) to be 1 in order to visualize the strain in the fringe. That would require you to rerun the simulation after changing that.

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(@sarvin)
Joined: 3 years ago

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@negativevolume  I used the Define_curve and defined a curve of stress vs strain and assigned it to the mat piecewise plasticity, I also added the database_extent _binary as you said but could not figure out how to plot the fringe. 

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