buckling factor as low as 0.05

[shearlag]

hi everyone,

I carried out a linear buckling analysis of a skin panel using nastran solver 105. The loads were applied as enforced displacement form a global model. surprisingly first mode buckling factor was 0.06365. Iam quite sure that the panel will not buckle at the applied load times the buckling factor. Its a very less load. A simple hand calculations gives a reasonably justifiable critical buckling stress. What could the possible reasons for a very low buckling factor ? Thanks in advance.

Regards,

 

[rb1957]

modelling ?

another day in paradise, or is paradise one day closer ?

 

[klaus.]

must be modelling error ....

 

[shearlag]

Well trying to figure it out.....the modelling error...
thanks

 

[GregLocock]

And what have you given us to go on? Zip.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[sdm919]

As suggested, these low modes are usually modeling problems (assuming your estimate is right - maybe it really does buckle at the lower load!). You could continue looking at higher modes until you find a "realistic" one (smooth shape), but ideally you should de-bug the problem to be sure.

Look at the mode shape. If it is smooth, it might be realistic. If it is extremely localized, or "spikey" or even hard to see, it is probably a modeling issue. Where are the largest displacements occurring? What is unique about that area? Try looking at each displacement component separately (i.e. plot x-disp alone, then y, etc.). By default Nastran normalizes the mode so that the maximum value of one component of displacement (not the magnitude) is 1.0. Keep looking until you find the node point where this occurs.

In cases with very low modes such as this, the dominant displacement is often a rotation. If so, this indicates inadequate restraints and/or poor connections in the rotational DOFs. For example, do the edges need to have more rotational restraints applied? If the model consists of a combination of element types (i.e. bars, shells, solids) problems sometimes occur at the connections between these because they all cannot transmit rotations in all directions. For example, the lowest mode may simply be bars spinning when attached to something that cannot take torsion. Connection details, such as RBEs, CBUSHs, pin flags, may also have issues with local rotations.