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Author Topic: Complex Curvature Span Lengths  (Read 5989 times)

dlj2_msfc

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Complex Curvature Span Lengths
« on: September 10, 2008, 01:19:45 PM »
As I understand it the x and y span lengths for simple cylindrical geometry is barrel height and circumference, respectively, for a full cylinder (with the "Full Cylinder" box checked).  What about complex curve geomertries like ellipsoidal dome geometry?  The y span (circumference) is straight forward, but what about the x span?  Would that be the arclength of the ellipse geometry?  Also would the "Full Cylinder" be checked for a full dome?

On a side note, the reason I am asking this is because some of my cylindrical spans did not come in from the FEM properly.  Upon changing them to the correct values, the dome question arose.

Thanks

Phil

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Re: Complex Curvature Span Lengths
« Reply #1 on: September 22, 2008, 11:40:32 AM »
The x and y span lengths for a cylinder (or any panel actually) depend on the material direction of the cylinder. 
 
On the buckling tab, you will see four buttons, indicating whether the curvature is in the X or Y direction and whether the stiffeners are inside or outside that curvature.  If the curvature is in the Y direction (this is the default), then the X span is the barrel height, and the y span is the circumference.  If the curvature is in the X direction, then the x span is the circumference and the y span is the barrel height.
 
If your material direction (i.e. your stiffeners) are running in the circumferential direction, make sure you pick the proper selection on these buttons.
 
For a complex double curvature, such as a dome, HyperSizer will still approximate the buckling solution as a cylinder. In other words, it doesn't really have a buckling method specific to analyzing a dome.  In the past, we have started with buckling length of about 1/4 or 1/8 the circumference of the dome for the buckling length.  Set the X and Y span to be the same.  I don’t think I would recommend using full cylinder for a full dome (sphere).
This will get you in the ball-park, however, in this case we recommend analyzing the final sized dome using an FEA Eigenvalue buckling solution as a check because our method is still treating the dome as a cylinder.
 
Also, in our experience dome buckling requires a relatively large buckling knockdown.  This is not specific to HyperSizer but to buckling methods in general.  We have seen knockdowns for dome buckling, especially for thin shell domes, as low as 0.16.    There is a NASA document that talks about buckling of thin dome shells.  That document is "NASA-SP-8032, Buckling of Thin Walled Doubly Curved Shells".

Finally, we created a PPT presentation that describes how we recently used HyperSizer and HyperFEA to size a spherical dome common bulkhead for the NASA CLV Upper Stage including the effects of spherical panel buckling.  Download the presentation here