
Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Bill McEachern Mar 11, 2014 3:15 PM (in response to Philip Eddy Eriksen)I might have posted that at some other point  it was the AMSE pressure vessel code FOS. How it was determined  don't have a clue. Might be experience.
You can do a post buckling analysis and it will be a much closer estimate.
If the structure doesn't move much in a linear buckling estimate  small displacements such that the eccentricities of the loads on the deformed shape are small then it might be close. Best to be cautious if it looks like it moves such that the eccentricities grow by a large amount.

Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Philip Eddy Eriksen Mar 11, 2014 4:00 PM (in response to Bill McEachern)Okey, thanks!
I'm sorry, but I'm pretty new to SolidWorks and FEA. What is AMSE pressure vessel code FOS? The displacements of the local buckling are large, and therefore a linear buckling analysis will not be valid. The results are way bigger then what i get in the lab on my practical experiments. For example i get a buckling load of 280 kN in SW compared with 7578 kN in the experiments. For my results to be reliable a nonlinear analysis is required i think. How do i simulate a post buckling analysis? Is it complicated?

Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Bill McEachern Mar 11, 2014 4:32 PM (in response to Philip Eddy Eriksen)is the structure free to move axially so that is can buckle the way it is suppose to? Maybe post a picture of your set up. The displacements might just look large as it is an eigen value extraction procedure  it didn't do a displacement analysis. The solidworks linear estimate is about 4 times bigger  might be interesting and it might not be. Restraints require some thought so it might be an issue with thtem or it may require a nonlinear post buckling analysis. Post a picture.


Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Bill McEachern Mar 11, 2014 5:26 PM (in response to Philip Eddy Eriksen)you should try a shell model for the large flat things  just model it as surfaces then add thicknesses in the analysis.

Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Philip Eddy Eriksen Mar 11, 2014 7:20 PM (in response to Bill McEachern)The large flat thing are forkesupports and just independently transversal stiffeners (part of the IPEsection) near the forksupport.
The safety factor of 4 is just a factor by experience? I should not use it?
Thanks for the help, Bill!
Philip

Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Bill McEachern Mar 12, 2014 12:00 PM (in response to Philip Eddy Eriksen)Chances are is that the descretization of your structure has resulted in an overly stiff structure. YOu need to model it with structural elements (ie shells) and do a mesh convergence study or add a lot of elements to your current study  23 thru the thickness and the buckling estimate of the critical load will drop.







Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Shaun Densberger Mar 11, 2014 9:12 PM (in response to Philip Eddy Eriksen)The main question:
The critical buckling load is based on Euler`s theory and elastic behavior, right?
How do I decide the factor of safety in witch im searching for the design buckling load?
A linear buckling analysis is solving an eigenvalue problem to determine the modes of buckling, where your eigenvalues are the buckling load factors and the eigenvectors are the buckling mode shapes. This is a linearized perturbation of the initial solution (hence the name, "linear buckling") and is sometimes called, "bifurcation" buckling due to the loaddeflection curve. There are two very important points that you must keep in mind when doing a linear buckling analysis:
 There is a unknown amount of error between the linear and nonlinear buckling solution.
 The linear buckling solution is unconservative (i.e. the calculated buckling load is higher than the true buckling load).
The implication of these two points (particularly the last one) means that you need to be very careful how you use the results from a linear buckling analysis. Realistically, a linear buckling analysis should only be used for preliminary comparisons; if the results are used for anything else, then a high factor of safety is needed. More accurate results can be obtained by running a nonlinear analysis with largedisplacement formulation. If you do this, be sure to include some eccentricity in your model (whether load or geometric).

Re: Buckling analysis: Buckling Load Factor, factor of safety and design buckling load?
Jared Conway Mar 12, 2014 11:09 AM (in response to Philip Eddy Eriksen)my first recommendation would be to go through the tutorials on buckling, the validation example and the help.
then come back and explain your application, what your expected results are and where you're stuck. i think you're trying to do everything at one time which is going to get you in a tight spot and circles of questions.