*Imperfection: scale factors 1

[mambo5]

Hi,

how to define scale factors of various modes in *imperfection? the scale factor is base on what?

please advise

 

[bassmanjax]

The scale factor is based on the eigenvalue mode you choose from the *BUCKLE analysis. If the card is;

*IMPERFECTION
1,0.1

Then I am scaling mode 1 by a factor of 0.1, or 10%. The eigenvalue buckling modes are normalised to give a maximum deflection of 1 unit.

Regards

Martin Stokes CEng MIMechE

 

[mambo5]

Thanks bass,

so I can choose any scala factor (10%, 15%, 20%)? But how do I know what scala factor value is the one i am 'looking for'?

If I have negative eigenvalues, should I ignored them, and 'call' only the positive eigenvalues from the *buckle analysis to *imperfection?

please advise.

 

[bassmanjax]

I usually do a range of scale factors, say 10%, 25%, 50% and so on, and then plot the buckling load vs the scale factor. You'll usually see a big drop in the buckle load at first, but then any increase in the scale factor gives a progressivelly smaller reduction in the buckle load. It's more or less the same as doing a convergence study.

As for which buckling modes to use, it's down to your judgement and experience to decide which ones are the most physically significant. If possible, try and find a hand calc that will give you the approximate load, or even better, do some actual experiments if you can.

Regards

Martin Stokes CEng MIMechE

 

[mambo5]

Thanks bassmanjax,

I got the eigenvalue from *buckle, I carry on with the load-displacement analysis, by using *Static, *Dsload. I apply *Imperfection on the model, but nothing happen.
The results (displacement) with and without the *imperfection are the same. Part of my input file as follow, was that anything wrong? please advise.

** BOUNDARY CONDITIONS
**
** Name: BCL Type: Displacement/Rotation
*Boundary
_PickedSet162, 1, 1
_PickedSet162, 2, 2
_PickedSet162, 3, 3
_PickedSet162, 5, 5
_PickedSet162, 6, 6
** Name: BCR Type: Displacement/Rotation
*Boundary
_PickedSet163, 1, 1
_PickedSet163, 2, 2
_PickedSet163, 5, 5
_PickedSet163, 6, 6
** ----------------------------------------------------------------
**
*IMPERFECTION, STEP=1, FILE=ipb
1, 0.001
** STEP: load
**
*Step, name=load, nlgeom=YES
*Static
1., 1., 1e-05, 1.
**
** Name: Load-1 Type: Pressure
*Dsload
_PickedSurf181, P, 1.
**
** OUTPUT REQUESTS
*Restart, write, frequency=0
** FIELD OUTPUT: F-Output-1
*Output, field, variable=PRESELECT
** HISTORY OUTPUT: H-Output-1
*Output, history, variable=PRESELECT
*NODEOUTPUT, NSET=SUPPORT
RF,
*NODEOUTPUT, NSET=RESULT
U,
*End Step

 

[bassmanjax]

That's a tiny imperfection - if your units are mm, then you have applied an imperfection of 1 micron to your model at most. That's likely nowhere near enough to trigger the buckle.

Try using values more like 0.1 (10%), 0.2 (20%) etc...

Regards

Martin Stokes CEng MIMechE