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Interpretation of results (buckling analysis)

Interpretation of results (buckling analysis)

Interpretation of results (buckling analysis)

(OP)
Hi,
   I have tried to perform a buckling analysis for a simple hollow box (1000*450*300mm)using PATRAN. One end of the box is fixed and other end is loaded giving a transverse load 0f 900 kg (450 each at the ends of top and bottom panel)from down side. I got a value nearer to .06 through eigen value analysis.The buckling mode runs upto a length of 600mm and stops.
                      How can i verify the credibility of my FEA results.Please help me to sort out this problem.

RE: Interpretation of results (buckling analysis)

shear buckling of the flat faces ?

RE: Interpretation of results (buckling analysis)

(OP)
The load is applied in such a way that the hollow box's top panel will go for compression and bottom panel will be in tension.. the top panel is displaying buckling deformation nearly half of the whole span.. The load causing buckling is compressive only...

RE: Interpretation of results (buckling analysis)

compression causes bucking ....yep.... what is the issue/question?

have you calculated by hand the stress in the top panel and calculated the buckling stress using a closed form classical plate buckling equation?

can you post a picture of your part, loading and buckle pattern?
 

RE: Interpretation of results (buckling analysis)

the top panel is in compression, it looks like your load is up.

the sides are shearing load into the top panel, so that the compression is increasing as you get closer to the support.

you can see how the panel is buckling, more than one crest = more than 1 mode.

From this result, i'd consider the top panel as a simple panel in compression, the wide is dfined (yes?), the length would be the length to the undeformed panel between the crest and the trough (yes?), constant compression = the stress at teh crest position (figuring that this is 1/2 the length of the assumed panel), simply supported on 4 sides, = text book solution.

RE: Interpretation of results (buckling analysis)

(OP)
Thanks a lot rb1957.....

I am getting you. But if you have time,please help me to clear the following doubts

1. When one end of the box is fixed how come you can take 4 end simply supported condition?

2.How can i calculate the compression load that should be used for text book formulas.

3.which length i should consider for the panel calculations? the length through out which buckling deformation is found or just the length of first two modes?

RE: Interpretation of results (buckling analysis)

1) if the suppor end is turly fixed, assume 3 sides SS, one loaded side fxd (a not so quite standard problem).  assuming loaded end SS would be slightly conservative.

2) once you determine the length of the panel (the length of the frist buckle), then use the stress at the mid-length position (ie 1/2 a buckle from the end) ... yes?

3) the panel is buckling up then down.  somewhere between it isn't displacing out of plane.  this'll define the length ... yes?

you could investigate how an eigenvalue solution (depending on your FE code) will help you determine an allowable load.  

RE: Interpretation of results (buckling analysis)

(OP)
thanks rb1957. but i want to know whether validation of my model through SOM approach is possible or not?
For that how can i find out the load?

RE: Interpretation of results (buckling analysis)

(OP)
Hi friends,
           I have a new topic to discuss with those who are interested in buckling analysis.
ie. will the eigen factor ever becomes negative?

( I did the hollow box problem which i described above keeping an uniform thickness everywhere. PATRAN shows a buckling at tension side. why it is happening?) :(

Please refer the pic posted with this.

RE: Interpretation of results (buckling analysis)

(OP)
The load application is same as that in the the pic posted first in this thread.

RE: Interpretation of results (buckling analysis)

A negative Eigenvalue indicates that the direction the the load is applied has to be reversed for this buckling mode to occur.
From what I see in you model you should expect the next mode to be approximately the same magnitude but have a positive value and occur on the opposite face.

If you only what to see positive Eigenvalues you need to modify the EIGRL entry in your .bdf file.  for example
EIGRL   10      0.001           15
Where 10 = the value of your METHOD card (look a few lines above) i.e METHOD=10
0.001 is the lowest eigenvalue that will be reported
15 is the largest eignevalue that is reported.
Check the EIGRL in the nastran quick reference quide for more information.

RE: Interpretation of results (buckling analysis)

if your constraint is fully fixed, then the reactions will include moments, the panel will be bending at the support, there'll be moment in the panels, there'll be out-of-plane deflections; try running with XYZ constraints only.

-ve eigen values interpreted above.

"SOM methods" ?

RE: Interpretation of results (buckling analysis)

(OP)
thanks rb1957 & Taz for spending your valuable time here.

I will try the problem with your suggestions and let you know the result.

@ rb1957, By using the term 'SOM',i mean Strength of materials related calculations/the simple 4 side boundary condition buckling calculations

RE: Interpretation of results (buckling analysis)

that's what i was describing (i thought) a flat rectangular panel uner constant compression, with "simple" boundary conditions (4 sides SS or 3 SS/1 Fxd)

RE: Interpretation of results (buckling analysis)

(OP)
hi friends,
              thanking you a lot for your useful suggestions.
Patran is giving negative eigen factor just because of it's coding only(FEA engineer has nothing to do with that).
   Never mind for the sign,just see for the deformation followed by the structure.

I would like to know your opinion about post buckling analysis with nastran. will it give fine results?

Can anyone give some idea about Post buckling analysis in PATRAN 'n' NASTRAN?

RE: Interpretation of results (buckling analysis)

don't know i'd say "never mind the sign" ... the indicates the direction of the critical load.

the question in my mind is "why -ve ?" ... you have a simple looking box beam with one face loaded in tension and the other in compression.  why is the +ve not as critical as the -ve loading ?  assuming the faces are the same thickness, they should be the same criticality ??

RE: Interpretation of results (buckling analysis)

(OP)
Even i dont have much idea.... but i feel like the length to width ratio is having some role on making the eigen factor negative.

RE: Interpretation of results (buckling analysis)

Jeram you should get matching positive and negative eigenvalues when the model and loading has a plane of symmetry, as your model does.
I would guess that the negative eigenvalue is slightly more critical than the positive one and that is why it is listed as the first eigenvalue.

Your statement "the length to width ratio is having some role on making the eigen factor negative" - is incorrect, this is not what is happening please understand that a negative eigenvalue mean that the direction of the applied load must be reversed for this buckling mode to occur in reality.

RE: Interpretation of results (buckling analysis)

(OP)
yea...you are right.  But still more clarity is needed. I think i should work more to see what is the real reason. May be some highly experienced people from industries can help to connect the result with the practical case

RE: Interpretation of results (buckling analysis)

- The eigen solution algorithms can find both positive and negative eigenvalues if you do not specify that you only want positive values. For example, if you had a uniaxial tension load on a beam, the eigenvalues would be negative. In your case, the applied load creates both tension and compression stresses on the box, so you have positive and negative eigen solutions. As previously mentioned, they occur at about the same values if the box is symmetric.

- Yes, Nastran can do a post buckling solution, but you will need to use a nonlinear solution (as opposed to the SOL105 eigen solution). SOL106 is the basic one, but you can also use SOL600 (Nastran has another one as well...400 I believe, but have not used it)

Brian
www.espcomposites.com

RE: Interpretation of results (buckling analysis)

(OP)
Thanks rb,esp comp & taz...

Did anyone tried some postbuckling analysis? Is that possible with a simple euler beam?  

RE: Interpretation of results (buckling analysis)

post-buckling analysis is long but not stunningly Hard.

i don't think you'll get much mileage out of a post-buckled comlun ... the point of post-buckled analysis is that one element (typically a thin skin) can buckle (typically in shear) but the structure doesn't have to collapse ... it can redistribute the loads and carry (limp) on.

RE: Interpretation of results (buckling analysis)

From my previous post:

"- Yes, Nastran can do a post buckling solution, but you will need to use a nonlinear solution (as opposed to the SOL105 eigen solution). SOL106 is the basic one, but you can also use SOL600 (Nastran has another one as well...400 I believe, but have not used it)"

As far as "how" to do perform the analysis, you have not yet defined what type of post buckling you are interested in. A few forms are:

- Local instability (such as a cap or web of a section)
- Skin buckling via compression (and redistribution to stringers)
- Shear buckling that results in diagonal tension (the load redistributes as a bending).

But in general, you would apply your "load" and perform a geometric nonlinear analysis. I say "load" because you may be more interested in a fixed displacement. For example, take bullet point (2) from above. You may want to apply a fixed displacement and let the load redistribute (i.e. the skin load will not be linear). You must perturb all of the modes in general.

But first, familiarize yourself with the classical approaches for:

- local buckling
- crippling
- the concept of effective
- diagonal tension

See the books by Bruhn, Niu, or Flabel. You probably don't want to perform a nonlinear FEM until you understand the physical behavior.

Brian
www.espcomposites.com

RE: Interpretation of results (buckling analysis)

NACA 2661 and 2662 are the basic references, if you're interested in post shear buckled strength.  Bruhn and Niu, of course, cover it too.  i'd start there, as they explain briefly (readably) what's happening.

RE: Interpretation of results (buckling analysis)

(OP)
gr8 work rb & ESP...

I will try to make my question clear..

When talking about a simple euler column, can we say that there will  be entirely no post buckling capacity?  

RE: Interpretation of results (buckling analysis)

as in everything ... that depends !

if this is truly single loadpath structure then i'd say yes there is no post-buckling strength.

but if there is an alternate loadpath that would pick up more load once the primary loadpath became soft (ie yielded/buckled) then there is post-buckling strength in the structure (though not in the column).

RE: Interpretation of results (buckling analysis)

(OP)
well said  rb1957!
  But in my problem the buckled part have it's unloaded edges SS; one loaded edge Clamped and other SS.

Can i find solutions for this BC's from any books or through any other means?

RE: Interpretation of results (buckling analysis)

?, how's that relate to an euler column ?  sounds like you're back talking about the original problem ?

my 2c, FWIW, ...
1) use 4 sides SS (as being slightly conservative), or
2) an FE model of the panel, or
3) test something.

personally, i'd expect 3) to be impractical, but either 1) or 2) should get you an answer.

RE: Interpretation of results (buckling analysis)

(OP)
i did (1) & (2), But both are giving different answers...
   i think the problem is with b.c's (4 side ss)
    
     So i need to know if we can solve 1 side ss & other clamped loading case through any theory         

RE: Interpretation of results (buckling analysis)

i'd expect 1) and 2) would give different answers.

if you have to solve this from 1st principles, i'd suggest timoshenko "plates and shells"; also a thick pad of paper, lots of pencils, and lots (and lots) of coffee.

RE: Interpretation of results (buckling analysis)

agree - 1) and 2) should give different results

but start with Timoshenko and Gere, Theory of Elastic Stability ...

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