Composite Buckling Empirical Knockdown Factor for FEA Results

[Jetgirl8]

Over the course of my career, I have seen vastly different approaches taken to utilizing finite element modeling buckling results on composite structures. I have not seen a recent resource for justification of any of the buckling knockdown factors that are employed (I agree with the last paragraph here

http://composite.about.com/library/weekly/aa122997.htm).

I have looked on the forum and seen SWComposites recommend a knockdown factor of 0.85. I have seen industry use a knockdown factor of 0.6 to 0.75, and most recently I have even seen no knockdown factor at all(!). Does anyone have any reference material they use as a basis for establishing a knockdown factor?

When the future's architectured
By a carnival of idiots on show
You'd better lie low

 

[ESPcomposites]

Part of the reason you see it vary is because it probably should. There was a recent thread on another forum about this, but some things to consider are:

- what is the criticality of the structure? Is it primary or secondary structure?
- Is there a load path that can redistribute the load? In other words, is the buckling local (free frange of stringer), skin buckling, etc.?
- Is the check for limit or ultimate load (in the event of local instability).
- Is there a benefit that is not analytically captured? For example, is there a slight curvature that could increase the buckling load?
- How good are the assumptions about the boundary conditions? Are they conservative or aggressive?
- How well known are the secondary loads (i.e. those that create beam-column effects)?
- What type of structure is it? Missles, unmanned vehicles, etc. may have different criteria than civil aircraft.

I have seen the 0.85 factor in the ATCAS papers - Advanced Composite Fuselage Technology. I believe it was used to cover initial imperfections, which should always be given consideration. It is probably a good starting point, but is not really intended to cover any and every application.

Brian

http://www.espcomposites.com

 

[SWComposites]

Well one needs relevant test data to get "empirical" factors, so you need to find test data that applies to your type of structure. Since you have not defined the specific structure that you are analyzing, then we can't suggest appropriate references.

Also, see: “Buckling of shells – Pitfall for designers”, by David Bushnell, AIAA Journal, Vol. 19, No. 9, September, 1981, Presented as AIAA Paper 80-0665R - which I think can be found here:

http://shellbuckling.com/shellBuckling.php

 

[Hansmeister]

The best source of information about knock-down factors will be from the NASA SP series of reports.

NASA SP 8007 is used for cylindrical shells, isotropic and orthotropic. There a additional SP's for cones, plates, and spherical shells.