Mirror Symmetry Questions

I am trying to be sure I have the right boundary conditions (BC) for my model. I am new to Mecway and quite impressed. I made a 2D model and extruded it -90 degrees. Now I think I have 3D solid elements (I don't know how to check what type of elements I have). I thought I could get mirror symmetry on the two faces with frictionless support BCs, but I am worried about the rotational constraints in the model. I tried to add rotational BCs but the model said the elements don't have the necessary degrees of Freedom (DOF) -- the faces don't move out of the plane in the solution. Do I need to constrain rotation to get mirror symmetry? In the manual I see ( at 10.6) that solid elements need a pair of displacement constraints to prevent rotation about the Y axis (etc); I'm just confused about the proper implementation -- first BC:select z, z=1, x=1, d=0; second BC: select z, z=1, x=-1, d=0 ???? I looked up and down the discussions. Sorry to bother you. Thanks for the help. jhc

Comments

  • Oops. here is the file.
    liml
    liml
    M61212 FEA 1_4 mdl v5B thrd_rt_grv P1_-5150 P3_900 26-Oct-17 step 8 with Rot_0.liml
    7M
  • Elements: you have Hex8 and Wedge6 (go to Mesh Tools -> Node/elements list)

    BC are correct but you can change from "Frictionless support" to Z and Y displacements because your model is a quarter of the complete model.
    Is not necessary to apply rotational degreed of freedom because you're working with 3D elements. These are necessary for shell elements.

    I don't know if the pressure loads are correct because on the lower part is applied internal pressure and on the upper part is applied external pressure (seems to be strange)

    2017-10-27_082942.jpg
    251 x 644 - 61K
  • Hi Andrea, thank you for your help. Are you indicating frictionless support for the quarter model is the same as Z=0 and Y=0 at the appropriate sections? And am I to understand 3D elements don't have rotational freedom?

    The two pressures: (1) the outside pressure is from a press fit due to a pipe thread connection (tapered threads cause deformation, I followed Shigley's analysis to estimate a contact pressure); and (2) the internal pressure is the gas pressure. Thus the two pressures.

    Again, I appreciate your help in learning about BCs. jhc
  • Faces create by cutting planes must remain plane.
    Look at the faces without applying rotational BC. Are they plane?
    Obviously there is a reason

    PS Maybe is necessary to apply gas pressure to all inner surfaces
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