Negative eigenvalues
Negative eigenvalues
(OP)
Hey,
I'm doing buckling analysis with 5 eigenvalues. One of the is negative, but my analysis still completes with out errors or warnings. Does anybody know how I can avoid the negative eigenvalues?
Senad
I'm doing buckling analysis with 5 eigenvalues. One of the is negative, but my analysis still completes with out errors or warnings. Does anybody know how I can avoid the negative eigenvalues?
Senad





RE: Negative eigenvalues
Zero values will indicate the body is free to translate in a particular direction.
corus
RE: Negative eigenvalues
Senad
RE: Negative eigenvalues
RE: Negative eigenvalues
You may well get spurious buckling modes with eigenvalue extraction, I know I've seen them. After all, the eigenvalues are just the mathematical solution to the problem, they may not necessarily be physically possible. It's up to you, the analyst, to determine what is significant and what isn't.
Having said that, it's probably worth double-checking your constraints and loads anyway.
Regards
Martin Stokes CEng MIMechE
RE: Negative eigenvalues
Martin Stokes CEng MIMechE
RE: Negative eigenvalues
Sorry for typing faster !
It's possible to set up a buckling analysis without an applied load, I've done this when writing beam analysis software using a "geometric stiffness" matrix (in place of the mass matrix for a natural freqency analysis) in conjunction with the standard stiffness matrix, and using the same eigen extraction routine as used in natural frequencies. This analysis produced probably as many negative eigenvalues as positive ones.
RE: Negative eigenvalues
RE: Negative eigenvalues
RE: Negative eigenvalues
RE: Negative eigenvalues
Regards,
DLT
RE: Negative eigenvalues
RE: Negative eigenvalues
I'm doing a buckling for non linear analysis for two differents shell structures; I have obtained negative values for both cases, and for all the eigen values.
I know this is possible, but how can I to know which structure is stronger?
I mean, the first eige value for one is -1,54 and for the other -2,34.
Thank very much in advance,
cad1
RE: Negative eigenvalues
Section 6.2.3 in the v6.7 Users Manual covers this quite comprehensively.
Martin Stokes CEng MIMechE
RE: Negative eigenvalues
RE: Negative eigenvalues
Regards
Martin Stokes CEng MIMechE
RE: Negative eigenvalues
I am interested in positive eigenvalues only in my analysis. However the analysis hasn't been able to give me the first positive eigenvalue after many negative values. I don't know how long it needs to run to find the positive value. Is there anyway to get positive eigenvalues only?
Thank you,
Arbor
RE: Negative eigenvalues
This doesn't sound right, I've never heard of this happening before! Could you attach a "small" input file that behaves this way?
RE: Negative eigenvalues
RE: Negative eigenvalues
I am trying to model the buckling of a pressure vessel head knuckle under internal pressure (there will be tangential compressive stress in the knuckle due to internal pressure). However, the model kept giving me negative values for the shell and head buckling which would be due to external pressure. I haven't been able to obtain the first positive number due to the real load yet. So it would be nice for me if there is a way that ABAQUS only finds the positive value for the real applied load.
Arbor
RE: Negative eigenvalues
Assuming you have modelled the whole pressure vessel then the vast majority of your structure will be in tension, hence the many negative eigenvalues found by Abaqus. There may be a considerable number of these encountered before Abaqus spits out the positive eigenvalue you seek. I am not aware of any means of suppressing negative eigenvalues. Have you considered running a static large deflection non-linear analysis using NLGEOM instead? This may be more appropriate than a buckling analysis for this type of problem.
RE: Negative eigenvalues
You are right about my model and the eigenvalues. I have several approaches to check different failure modes of a pressure vessel based on ASME code. The approaches include linear elastic and non-linear plastic analysis. The code also requires to check buckling when there is compressive stress. There is no very good analytical solution for head knuckle buckling load. So I am trying to see if ABAQUS can help for this case. Thanks
Arbor