Web under local loads – Nonlinear FEA
The more complex a problem is, and the higher the accuracy needed, the more it makes sense to employ Nonlinear FEA. Will it make sense to use it in solving local web loads? Let’s find out!8 February 2021
In this post, I will show you how to define and run a materially nonlinear analysis in a video tutorial. This is the type of analysis that will allow you to calculate a plastic collapse load of the model. This is one of the important parameters in the numerical design.
Plastic collapse is one of the possible ways the structure we are analyzing can fail. Sometimes this criterion is simply met by limiting the stress to the yield stress in the entire model – this way a linear analysis (LA) is sufficient in design since when everywhere stress is below yield there is no need for materially nonlinear design (of course this not include hyperelastic materials etc.). However, such an approach is quite limiting, since in most cases there will be stress concentrations in various places, and to reduce the stress in so-called “hot spots” to values under yield stress requires a usually great thickness of elements. Also, the fact that some parts of the model yielded does not mean that the capacity is exceeded. After all Eurocodes (European norms for civil engineering) simply allows for plastic design, which in certain situations leads to yielding of an entire cross-section of the analyzed beam.
With this in mind, it is actually beneficial to perform a Materially Nonlinear Analysis (MNA) to verify what is the actual capacity of the model due to plastic failure. One should be aware however that in certain situations using plastic capacity might not be the best idea. This is especially true when fatigue is an issue (with periodically changing loads) since yielding leads to such problems as low-cycle fatigue, characterized by much smaller fatigue resistance. Also if the plastic capacity of the model is close to the elastic capacity due to instability then there is an interaction between those 2 effects further reducing the global capacity of the model. This is why not only do you have to analyze plastic and elastic capacity (elastic capacity needs to be verified with proper imperfections), but also you wish to verify both nonlinearities at the same time to check, whether they interact with each other.
Verifying plastic capacity without a stability path is actually quite difficult because looking at the yield zones in the model under various loads it is hard to decide how big a yield zone we can allow. I described this more here. In today’s example yielded zones in the model under increasing loads would look like this:
As you can see it is not easy to decide at which stage the yielded zone is too big and when the model lost capacity. This is, however, a problem for the next post – for now, let’s focus on how to actually perform an analysis that allows us to obtain such results.
In the video tutorial below I explained the most important aspects of MNA calculations on the shell example using Femap with NX Nastran. Please note, that the solver parameters were discussed previously here.
I hope you enjoy this tutorial and find it useful. If you have any questions post them in the comments below.
Have a good one
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