Composite Buckling Empirical Knockdown Factor for FEA Results
Composite Buckling Empirical Knockdown Factor for FEA Results
(OP)
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 ://composi te.about.c om/library /weekly/aa 122997.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





RE: Composite Buckling Empirical Knockdown Factor for FEA Results
- 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
www.espcomposites.com
RE: Composite Buckling Empirical Knockdown Factor for FEA Results
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
RE: Composite Buckling Empirical Knockdown Factor for FEA Results
NASA SP 8007 is used for cylindrical shells, isotropic and orthotropic. There a additional SP's for cones, plates, and spherical shells.