Learning Forum

[Adrian Herranz Mollejo]

Hi everyone, i have an issue with my model on LS-Dyna. I´m developing a model of a ball impact on ballistic gelatin as consecuence of free fall. Everything run OK, but the box of balistic gelatin suffer a plastic deformation wrongly. I attach three images, what has to be and 2 of what is happening(first the impact and then after relaxing time). I have a Material 10+eos as gelatin and Material 20 as the steel ball. Also have an automatic contact surface to surface, so i dont know what is happening. Anyone can help me?? thank you so much ! <3

[Loic Ivaldi]

Hello,

In th real test, it seems pretty elastic. Check carefully the units : https://www.dynasupport.com/howtos/general/consistent-units

also did you try another set of data like : https://www.dynalook.com/conferences/12th-european-ls-dyna-conference-2019/high-speed-impact/zochowski_military_institute_of_armament_technology.pdf

Best Regards,

Loïc

[Adrian Herranz Mollejo]

Hello, first all thank you for helping me :)

Yeah with lower impact speeds its behaviur is elastic. But i had review the units, and having defined : m, secs, kg ; the pressure unit is Pa (everything in IS). That´s why i have opened the issue. To validate the units are in shape, kinetic impact energy is the same as the theorical calculation. Don´t know if the problem is the propierties value , but dont think so because it has no sense. If you or someone has another POV of what can be happening, eternally gratefull! :)

[Loic Ivaldi]

In the paper I sent you,

 

You can see a shear modulus of 0.85 MPa and a SIGY of 0.22 MPa. SIGY is relativley high compared to the shear modulus allowing probably in this case some elastic deformation. If you have a shear modulus really high compared to SIGY you probably have less elastic deformation and quickly go to plastic deformation.

 

How do the values you are using compare to the ones from the paper ?

[Adrian Herranz Mollejo]

Yeah they are copied exactly from that paper and other ones with the same values. The difference is the units, because in that it is used mm and tonnes to density and i am using m and kg. So doing the conversion we have the same values but different unit.

[Loic Ivaldi]

I have no more idea for now. I hope someone else could help you

[Peter Yip]

What is the length of the simulation and time step? If you ran it out for longer times, are you sure you are not getting elastic recovery?

[Adrian Herranz Mollejo]

4 secs, but the relaxing time in physical test is lower than 0.2 secs. So it doesnt matter but thank you! It seems its because the sigma0 is too much lower than shear modulus and it has no elastic phase so im fixing that. Thank you for your help!