Neutral axis and Shear center of composite beam

  1. Thomas Ricard

    Dear all, 

    I have a cylindrical 7 plies tube with the following ply orientation [90,45,-45,0,-45,45,90].

    This beam is modeled with 5 identical sections computed in VABS and analysed in GEBT with the following boundary conditions:

    - Fully fixed at one end.

    - Axial load at the other end. 

    I would expect the displacement of the center of the section to be along the axial direction and the recovered stress to be constant within each of the plies around and along the tube. However, I notice displacement of the tip of the tube in the 3 axis and some non-uniform stresses in the plies.

    Following some investigations I notice in the VABS dat.K file that the section's neutral axes is not at 0,0 and the Generalized Shear center of the cross section is not at 0,0 (the section center is at 0,0)

    I am wondering if this behavior is an artifact due to some problems in my modeling or it is a fact of the physics. For a matter of simplifying of the model I only have 8 elements around the section but the problem was the exact same with more elements.

    Here the link to the file:

    Thanks in advance for your advices. 



  2. Su Tian

    Hello Thomas,

    I am not quite familiar with the old format of VABS. So, I modified your VABS input file to the new format and the result looks good as shown below. The tension center and shear center are all at the origin, and there is no extension-bending coupling in the stiffness matrix. The modified input file is also attached.


     The Neutral Axes (or Tension Center) of the Cross Section

      Xt2 =   -2.4505976695E-14
      Xt3 =   -5.2675109924E-14

     The Generalized Shear Center of the Cross Section in the User Coordinate System

      Xs2 =    9.0066217427E-15
      Xs3 =    6.8327607342E-15


    721153.932 0.000 0.000 4443.355 0.000 0.000
    0.000 187731.255 0.000 0.000 -945.909 0.000
    0.000 0.000 187731.255 0.000 0.000 -945.909
    4443.355 0.000 0.000 5687910.191 0.000 0.000
    0.000 -945.909 0.000 0.000 5311601.660 0.000
    0.000 0.000 -945.909 0.000 0.000 5311601.660