Rigidity of GAP elements in contact
GAP element rigidity will depend on the material of the parts in contact... and also on the mesh size! Learn how to calculate it!12 December 2022
Nonlinear buckling is a really interesting analysis. It gives you enormous possibilities, at only a few minutes worth of work. At the start, it might be frustrating to set it up (there is a learning curve involved), but in the end, it is a default engineering tool for me right now! Possibilities are limitless!
Nonlinear buckling is a very powerful tool. It’s not as simple as linear buckling I wrote about lately. The nonlinear approach requires some additional settings in the solver. The results you get are well worth the hassle. To start off I will go with the simplest of cases: elastic buckling of a perfect model.
I will start with the sweet stuff, so you will know what is here to gain. The outcomes from nonlinear buckling analysis are far more robust than those from linear buckling. What you get is:
Nonlinear buckling outcomes:
The typical animated outcome from GNA looks like this:
This is a very good question! In essence, GNA will divide the load you have applied in your model in a way that you requested in solver settings (below). There are several possibilities here. For now, let us assume that you have asked the solver to divide the load into 100 equal parts (often called increments).
During calculations, the solver will apply each part of the load one after another. In the first increment, the load will be 1% of the load you applied. In the second increment, the load is increased by an additional 1% for a total of 2% and so on). Such an approach in linear static makes no sense (since all the outcomes are proportional). In nonlinear solutions things are different. Deformation of the model from the previous increment influence the outcome for the next increment. This is why the division of the applied load into smaller pieces is important!
At the end of the analysis, you get something I call a “slideshow”. In our example that would be 100 different outcome sets, one for each increment. Usually in your software that will look just as if you had just made 100 linear analysis sets with gradually increasing load (but of course the outcome is different, especially at higher loads).
From this “slideshow” you can create the failure animation I mentioned above. Most post-processors simply have a button “animate” or “multi-step animate” for this purpose. You can also create an equilibrium path from this “slideshow” (more on this here). Sample outcomes may look like this:
The setting of nonlinear buckling analysis is surprising “code independent”. Sure, depending on the software you are using, menus will look different, but the things you will be setting up are very similar.
Required steps to set up nonlinear buckling:
This is the general outline of what you need to do. At the start setting this up might be frustrating (it was for me for sure!) but after you get some experience all the setting takes around 3 minutes!
Nonlinear buckling positives:
Of course, nothing is great without the drawbacks. Nonlinear buckling has several, mostly concerning the fact… that it is actually more difficult to use than linear buckling:
Nonlinear buckling negatives:
As you could feel while reading this post I really like the nonlinear analysis. Most drawbacks go away with time and with the experience you gain. The only “real” drawback is the time required to do the calculations. It used to bother me much more, but I have learned to organize my work such, that the longest calculations are made when I sleep. Also, bear in mind that in the case of small analysis this “long computing time” can easily be 3 minutes. The average computing time for my problems is around 15min… You will always have time to make another tea!
I honestly believe that you should learn how to do nonlinear design even if you won’t use it often. In the case of most popular solvers the software, you use already has the needed tools. This can be a great stepping stone for your career (as it was for mine).
Knowing how to do such analysis brings your skills (and value as a professional) to a completely new level! I really think it is well worth learning it. I admit it will take time, and sometimes you won’t be able to convince your boss/customer that it makes sense. But when you understand this… it really changed the way I look at entire engineering!
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