Interpreting FE Buckling Results

[jetmaker]

Morning all.

I have a thin plate that is being loaded eccentrically in compression which is resulting in combined axial and bending stresses. Running a SOL105 Linear Buckling analysis in NASTRAN, an eigenvector value of 19.124 is reported.

If this load were strictly axial compression, then the Margin of Safety would simply be 19.124 - 1 = 18.124. However, I am not sure how to incorporate the bending component into this margin.

Looking for some advice on how to interpret the eigenvector value and use it to write a Margin when the part is subjected to combined loading (specifically axial + bending).

Thanks,

jetmaker

 

[johnhors]

multiply the eigenvalue by the applied load to get your critical buckling load.

 

[jetmaker]

johnhor,

Which applied load??? Axial??? Moment??? Combination???

Thanks,

jetmaker

 

[crisb]

I doubt that an eigenvalue analysis is appropriate. It would be much better to do a nonlinear analysis.

thread727-181294

 

[jetmaker]

crisb,

Thanks for the link... it was informative, but not overly conclusive as SWComposites at the end suggests that the eigenvalue is a scalar on the total combined load.

I understand that a non-linear analysis would more than likely solve my dilema; however, are you aware of any way to resolve this using linear methods?

I have an idea of using:

1) the eigenvalue * applied load as the buckling allowable, then applying it against the total combined stress (axial + bending) to yield a margin. This produces a low Margin (approx 1) compared to the 18.124 if it were only linear.
2) The other idea stems from eqn 7 in

http://www.osti.gov/bridge/servlets/purl/71376-ptUFrH/webviewable/71376.pdf

which combines axial and bending effects. However, I am not sure I have calculated things right. I'd be willing to discuss this in more detail with someone more familiar with this qpproach. The margin obtained using this method was also large (approx. 13).

Any help is appreciated.

Thanks.

 

[rb1957]

jetmaker,

i think you mean, in step 2, for interaction between compresion and bending ... RF = 1/(Rc+Rb); ie Rc = applied compression load/allowable compression (= 1/eigenvalue !) and Rb = max. applied bending moment/allowable moment

 

[dollarbulldog]

Hi

The eigenvalue is considered as a multiplication factor to the applied load to cause system instability. For complex, imperfect geometry or complex load set, this method usually overestimate the limit capacity and should be avoided.

 

[cbrn]

Hi,
just to give an idea (most likely, Jetmaker operates in totally other field), EN13445 states that the buckling analysis must be done as non-linear. Eurocodes have recently prohibited to use linear Eulerian buckling.

Regards

 

[SWComposites]

Jetmaker - what you have is the equivalent of a beam-column (relative to a purely axially loaded column). For the latter, an eigenvalue analysis can give a reasonable approximation of the collapse ("buckling") load. For a beam column, an eigenvalue buckling load is genearally meaningless. Collapse of a beam column is either due to excessive deflection or strength failure. I would not bother with writing a "buckling" MS, rather would run a geometrically non-linear analysis and check deflections and stresses.

 

[dollarbulldog]

Hi all,

It should also be reminded that, for non very slender member, inelastic instability (material failure/yielding) usually happens before the geometric one. This can also initiate collapse mechanism. Nonlinear material model esp. plasticity shall be included if necessary. Dynamic effect is impoertant that in many common cases, it adds up some stress other than that static results.