General Mechanical

General Mechanical

Rigid Part Contact Not Behaving as Expected

    • patjoy
      Subscriber

      Hi Everyone,


      I have a simple simulation in LS-DYNA of a *MAT_RIGID shell part that is contacting a rigid plate (*RIGID_WALL_PLANER) under the influence of gravity, essentially a drop test. The simulation performs as expected until the contact occurs at which point the shell part rebounds off of the plate at a very high speed and acceleration. If I change the material to *MAT_ELASTIC this behaviour is not seen and the contact continues as expected however run times increase dramatically. This simulation is a subset of a larger simulation where run times will be very long if I use *MAT_ELASTIC. Does anyone have any ideas why this might be happening?


      In order to obtain the contact between a rigid part and a *RIGID_WALL I have to set RWPNAL = 1.0 in the *CONTROL_CONTACT card but everything else was left as the default. My only other thought on how to proceed is to create a physical plate with *MAT_RIGID and hope I don't see this behaviour again but I have a strong suspicion that I will.


      Cheers!

    • Ushnish Basu
      Ansys Employee

      For rigid-only simulations, you should make sure to limit the maximum timestep using LCTM in CONTROL_TIMESTEP


      If you replace the rigidwall with a rigid shell, you can also set the following parameters for better modeling of the contact: VDC on Card 2, SOFT/SBOPT/DEPTH and BSORT on Optional Card A


      In a drop test, it is more efficient to have the body very near the ground, and give it an initial velocity using INITIAL_VELOCITY_GENERATION


      You can also use DEFORMABLE_TO_RIGID_AUTOMATIC to have the part rigid until just before the point of contact. You can choose to turn it back to rigid after the contact if you feel that the subsequent vibration or wave propagation is the body is not significant.


      Alternately, you can use PART_MODES to impose vibration modes on a rigid body. See here for an example:


      http://ftp.lstc.com/anonymous/outgoing/support/EXAMPLES/superelement_and_linearized_flexible_body_example.tar


       

    • patjoy
      Subscriber

      Thank you for the response, presumably we need to limit the maximum timestep to allow for the contact to be accurate? Could this be the reason why I am seeing very high rebound speed and acceleration?


      I will investigate the rigid shell options, this drop simulation is only to see if I can model rigid-rigid part contacts but will be part of a larger model where the rigidwall acts as the domain for SPH particles.


      The DEFORMABLE_TO_RIGID_AUTOMATIC is an interesting card but I am not sure it will work to well in my larger simulation as the SPH particles will always be in contact with the rigid part.

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