Dear friends in the similar field:
Happy new year and all the best wishes for you!
I am recently doing some analysis about non-elastic buckling, and after the help from some kind people here, I now know how to model the whole process, but there is still a little question about the imperfection introduction.
I feel unclear about the below statement in the documentation 11.3.1 (http://129.97.46.200:2080/v6.14/books/usb/default.htm)
"You must choose the scale factors of the various modes; usually (if the structure is not imperfection sensitive) the lowest buckling mode should have the largest factor.
The magnitudes of the perturbations used are typically a few percent of a relative structural dimension such as a beam cross-section or shell thickness."
For these statement,how I can determine the value of the scale factors? and what does this mean that "The magnitudes of the perturbations used are typically a few percent of a relative structural dimension"? The magnitude of the superposition of all the eigenmodes or the magnitude of each eigenmode after multipling the corresponding scale factor?
THANK YOU VERY MUCH!
Sincerely yours,
Kyle
The scaling of the elastic buckling modes is the most common method for applying an imperfection. It is noted that the first mode not always gives the smallest critical load. A superposition of some modes can result in smaller critical loads. In a tube a single dent as imperfection gives smaller critical moments than the first elastic buckling mode.The size of the imperfection should be based on the manufacturing tolerances. The nonlinear analyses should take into account the plasticity.
for values of the amplitude of the modal shapes you should look into what the codes prescribe as imperfection (see eurocodes for example)
Dear Song Mengli,
You have already got some very good answers....I just want to add a few thing....If I am remembering correct I have seen in some report of NASA about their SLWT...the value of a small imperfection (of the order that we are discussing) was reducing the buckling load drastically (more than stiffened thin cylindrical shell on which we were working).....as I could understand sensitive to imperfections may be function of degree of thinness or type of loading (my guess)...you may also get a sharp reduction in buckling load as you increase the imperfection by a small amount...what I suggest is do a sensitivity analysis and keep on increasing/reducing the imperfection (starting value may be used as suggested by Mr. Saleh Yazdani) and finally use a very low value of imperfection that is necessary to get a buckling phenomenon in simulation...Also the imperfection created should be at a proper location..that you can get by the elastic buckling modes....
hope that will help
Naveen
Thank you all for your elaborated reply, I am so appreciated it! following your suggestion, I tried a amplitude as 1 and 0.1, they all showed very similar result, and the sensitivity of the model could roughly be outlined.
But I got another question, since I generate a node file to store the buckling modes, and use that in Riks for initial imperfection, My question is: how I can open the node file, with ".fil" as its extension (attached)? Can anyone help me? I want to see exactly what is the deformation value of some points.
Thanks very much!
Best Regards and Sincerely yours,
MENGLI
Hi,
I agree completly with Saleh Yazdani . The only thing that I think should be added (mainly because it is not described properly in the Abaqus Manual) is how the scaling of the Eigenmodes, as a function of the Imperfectionsize, works in Abaqus.
A straight forward example would be:
If you are in a µm System and you use:
**----------------------------------------------
*IMPERFECTION, FILE=YOUR_FILENAME_HERE, STEP=1
1,1
**----------------------------------------------
in your input file.
This will result in a 1µm shift at the nodes at the maximum value of the Eigenmode in the directions given by the Eigenmode shown in your Eigenvalueanalysis. The rest of the nodes is shifted according to the smaller values from the used Eigenmode. You can see this shift on the added pictures.
It is important to know that, because you can't get this information directly from the Inputfile.
Best regards,
Claus
Dear all,
If we have a job with 2 steps, the first step is static, general and the second one is static, Riks. so how can we apply the imperfection for only the second job? wish your helping!
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