Stress analysis question - negative eigenvalues from buckling analysis?

I am running a linear buckling analysis on a composite graphite epoxy support structure for a suspended liquid oxygen tank for a launch vehicle. I am getting negative eigenvalues. What does this mean?





e1 = -0.0827


e2 = -0.0828


e3 = -0.0845


e4 = -0.0845


e5 = -0.0883





The buckled waveforms (mode shapes) all look normal however.

Stress analysis question - negative eigenvalues from buckling analysis?
err... dunno?
Reply:I can't say I know a lot about this, but I was wondering if this might imply that you have a structure that is made more stable by applying a your load(or if the load you're applying is putting critical parts into tension rather than compression)? This would, I suppose, correspond to buckling occurring at negative loads (or frequencies in the analysis?), giving negative eigenvalues. If your composite members are highly anisotropic, it might be possible for odd things to be going on with the internal stress distributions - perhaps a simple stress analysis might be in order to get a feel for the load paths?





Just a guess - would be interested to know the answer when you find it! Refreshing to get a question on here that isn't some 13 year old's homework - sorry I can't be more help.



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