Modify Hex8 Element Coordinate System

[Lyster]

I'm creating a sandwich panel by using shells for the outer surface laminates and hex solid elements for the core.
I'm creating the core elements by extruding the shell elements in the z direction. However when I do so, the resulting hex element coordinate systems are rotated by 180 degrees about the z axis so the element normals are the same for the shells and hex's but the x & y axes are 180 degrees out.
I'm doing a buckling analysis and am concerned that this may effect the results however, given that Patran is so user friendly (NOT) I haven't been able to find a way to rotate the hex element coordinate systems to align with the shells.
Firstly, does this matter, will the results be effected?
Secondly, does anyone know how to modify the hex element coordinate systems to align with the shells or to create the hex's so that the element coords do align with the base elements?
Thanks.

 

[40818]

The CHexa element derives its coordinate system based upon the relation between the faces of the element, and is defined in terms of vectors R,S and T, which join the centroids of opposite faces. The origin of the coordinate system is located at the intersection of these vectors. The X,Y and Z axes of the element coordinate system are chosen as close as possible to eh R,S and T vectors and point in the same general direction.
Now, the grid points G1 to G4 (say your base Quad4) are extruded and the opposite face has grid points on the opposite face G5 to G8 (with G5 opposite G1 etc). I think that the face of the solid must have an opposite normal (imagine a solid you would want all the normals to be pointing outwards). Therefore, as the face normal of the extruded side is opposite to the basic quad starting point, the orientation is reversed (i think). As your using Chexa's a material coordinate system must be entered which is used to derive stress components. You Mat card must have defined properties for each core ribbon direction), and your Psolid card must use this.

You could modify the quad coordinate systems to match the solids, but i wouldn't have thought that necessary.

You say your running a buckling analysis, may i ask a bit more about the structure,loading etc?

 

[Lyster]

Thanks for your response.
I've just done a Show-Element-Coord System to check what you've said above and all elements (solid and shell) have the same normal direction (z) but as I said above, the x & y for the shells (which align with CID) = -x & -y for the hex's (not sure how to find out the orientation of the face but I would expect that would follow the same as the hex orientation.
Since these solids are are extrude from the shells I'm just not sure why it would align the axes with the parent.

I'm trying at the moment to run a 'simple' bucking analysis for a simply supported sandwich panel (foam core) under a edge applied uniformly distributed load. Since there are 2 facing sheets (shell elements) and 3 solid elements through thickness, I've created a node line remote to the panel and alligned with the midsurface of the panel and connected that node line to the panel edge with MPC's. I've then redistributed the UDL as a nodal load o the remote node line.

 

[40818]

If you were to extrude the solids the other orientation , you would find that the normals is reversed also. As i said, the coordinate system of the Chexa is defined by the solid face relationships. This might be worth carrying out to help see how the solid faces work; take a single quad element, extrude it, then create solid skins on each face and see what the coordinate system of each of the face skin's is.

Not sure on the 3 elements through the thickness for the core, how are you going to recover the correct shear stresses in the core? Is your model non-linear?