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3 minutes read
1 April 2016

Side-project: When shell model is not enough

3 minutes read

I had some interesting experiences with my current side-project. I have decided to use the shell model as usual, but since in my laboratory test rings and stringers were quite small (so their rigidity was in scale with the model) I have decided to model them as solids. Somewhere along the way I also decided to use a different solution than my favorite 106 (Solutions or “SOL” are a way NX Nastran divides its capabilities, for instance, SOL 106 is for nonlinear static while other Solutions do linear buckling, heat transfers, and other things).

After calculations solver gave an error message 255 (NX Nastran error list does not have such a position) and it was not even described (usually you have some sort of text with the error description). So I have spent 3 days searching for this error on the internet and manuals (no results at all) and also doing a simpler model from scratch with only shells (just to see if it would work). The shell model worked so I have spent some additional time comparing those two models and reading manuals what could be the problem with solids… at the end, there is no problem at all! Luckily for me, I remembered that older computer systems did not work with file paths longer than 255 characters… and if you keep something on your desktop in a neatly organized folder with subfolders the path might be that long! The shell-only model was in a folder with a shorter path and as such did not throw out that error. So… if you ever encounter error 255 with NX Nastran this means that you need to lunch your calculations from a folder with a shorter path.

There is however a positive side to this situation. Since I have made shell and shell+solid models of the same problem I can show you why sometimes shells aren’t enough. In the shell model, the ring and stringer have plate thickness of 20mm (so theoretically everything is the same), but the outcomes vary, as you may see below:


The left model has only shell elements (I set the view so that the thickness of the element is being plotted, so it looks similar to the model with real solid elements on the right). The plots show radial deformations. What is more important however is that the load-bearing capacity of the shell model is 52.4 kN while for the shell-solid model it is 39.4kN. That is a 25% difference in capacity due to stability failure, so not a thing easily ignored. This is also why it is hard to read outcomes from pictures about which I posted before. Without knowing the assumptions it is impossible to tell that the left model had simplified geometry. Normally since different results were obtained from 2 models those should be evaluated to find which one is correct… but since I have laboratory test results for those shells I know that the shell-solid model is correct. Lastly an animated collapse mechanism for elastic stability failure and plastic stability failure. Note that at the end symmetry of the model is lost due to calculation accuracy… this is however way after the collapse.

Have a nice day!

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Author: Łukasz Skotny Ph.D.

I have over 10 years of practical FEA experience (I'm running my own Engineering Consultancy), and I've been an academic teacher for a decade. Here, I gladly share my engineering knowledge through courses, and on the blog!

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