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Shell elements (15) - Axisymmetric - Integration point problem with strain - LS-DYNA

Shell elements (15) - Axisymmetric - Integration point problem with strain - LS-DYNA

Shell elements (15) - Axisymmetric - Integration point problem with strain - LS-DYNA

(OP)
Hi!

I have a problem with a simulation of the Hopkinson bar experiment. I am using Ls-Dyna with shel axisymmetric elements (tipe 15).

In the simulation there is a waves that propagates into the incident bar. It's a pressure wave along y-axis. This wave keeps the bar into the linear-elastic field. This bar is made of aluminium (E=71100Mpa), so, for the Hooke's law (I want to get 600 micro strains): s=e*E=600e-6*71100=42.66 MPa. So, the shape of the wave is trapezoidal with a maximum value of 42.66 MPa.

This is not the problem!

In the simulation I am using shell axisymmetric elements (type 15). The problem is this: when I use these elements with a 2 integration point (card SECTION_SHELL --> NIP) I obtain from the simulation, the rights results in terms of stress and strain (as you can see in the firsts 2 pictures, there are stress and strain takes from a node in the middle lenght of the bar) but these elements, have problem of hourglass.

[URL=http://imageshack.us/photo/my-images/854/nip2.jpg/][/URL]

Uploaded with [URL=http://imageshack.us]ImageShack.us[/URL]

[URL=http://imageshack.us/photo/my-images/26/stressnip2...][/URL]

Uploaded with [URL=http://imageshack.us]ImageShack.us[/URL]



So, in order to avoid this problem, I use the same elements, but with 4 integration points (fully integrated). In this way, I avoided the problem of hourglass, but I get wrong results in terms of strain (not for stresses).

[URL=http://imageshack.us/photo/my-images/198/nip4.jpg/][/URL]

Uploaded with [URL=http://imageshack.us]ImageShack.us[/URL]

In your opinion, where is the problem?

I think that from 2 to 4 integrations points, maybe the code could change the way wich it calculate the strains, but I am not sure about this.

If you want, I can upload the simulations files.

Sorry for my bad english.

Thank you.

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