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Thick shells in contact


During one of my recent trips, a question was raised about how LS-DYNA treats thick shells in contact in particular if the contact would detect the surface sides. This simulation Thickshells in contact shows that LS-DYNA treats the thickshells as solids in which all external (free) surfaces are included in the contact. ERODING contact would also regenerate the free surfaces upon element deletion.

  • Shadi says:

    Hi. I am a Masters student and have been trying to model a masonry wall subjected to blast load using LS-DYNA. I’m using contacts between the blocks representing mortar joints.
    I have a lot of difficulties with the contacts. I think maybe the problem rises from the most basic definitions. I am using solid elements in my model. The contact I am using is “AUTOMATIC_SURFACE_TO_SURFACE_TIEBREAK” and the slave and master surfaces are segment sets. The segment thickness according to the manual is 1/20 of the shortest diagonal in the element (in case of using solid elements). I am getting confused when I encounter other parameters in my contact. For example SST and MST are the optional thickness for slave surface and master surface which override tru thickness. Does the true thickness in this definition refer to that 1/20 shortest diagonal in the solid element. Also what about the “SLDTHK” which is optional solid element thickness which activates contact thickness offset.
    I would really appreciate it if you can clear the application of these parameters. I try to explain better how my model is. Basically there is a 10 mm gap between each blocks. Assuming two blocks on top of each other, the bottom surface of the top block is slave surface (a segment set) and the top surface of the bottom block is the master surface (again defined as a segment set). And as I said I’m using AUTOMATIC_SURFACE_TO_SURFACE_TIEBREAK contact. Since there is a 10 mm gap between the slave and master surface I think I should be using OFFSET because in this initial condition (when they are 10 mm apart) they are in contact. Knowing all this, the following parameters are what I am not completely sure about:
    1. SST and MST. I think if I leave these as default (0) then the program takes the element thickness as SST and MST, is that correct?
    2. PARAM which represents critical distance since I’m using Option 6. I have set a value of 10 for this parameter but I’m wondering if it should be different depending on what values I use for SST and MST.
    3. SLDTHK. If I want to use offset, I had aasumed that I should have set THKOPT to the value 1. However, it seems like for automatic contacts, this option is greyed out. On the other hand the manual says that giving a nonzero positive value to SLDTHK would activate the offset. What should I use for SLDTHK?
    4. PENMAX. I don’t want any penetration and based on the manual the maximum penetration distance is defined by segment thickness multiplied by PENMAX. I’m not sure if this segment thickness is SLDTHK or SST/MST or element thickness or what. And regardless of what segment thickness is, should I still set PENMAX to zero so that penetration distance would be zero (since I don’t want any penetration) even though from table 7.1 the default for PENMAX for type a3 contact (automatic_surface_to_surface_tiebreak) is 0.5 for solid elements.

    I am so sorry for the long email and probably confusing one but I would greatlt appreciate it if you help me put a little bit. Than you so much in advance.

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