High Stresses In Computer FEA Models – Can They Be Ignored?

So you’ve run a finite element analysis of a mild steel “thing” and have a lovely looking stress plot, but wait! Some stresses are 800 MPa and glowing red!

What should you do?

It’s extremely disappointing to know that some engineers – so-called experts in their field – would increase member and plate sizes to reduce these high stresses. Yet, in most cases, they represent singularities in the FEA model that can be ignored.

Think about it.

Mild steel yields around 250 MPa, and has an ultimate strength of 410 MPa. 800 MPa is a fairytale stress – it CAN’T happen.
What the software is telling the engineer with a trained eye is that small areas of high stress can largely be ignored – at least in regard to static strength.

When loaded statically (ie not repeated fatigue loading) ductile materials like steel yield prior to failure. Where small portions of the structure see stresses beyond the yield stress they will in actual fact locally yield, and in the process, redistribute this load to adjacent structure elements. As the bits experiencing the high stress hit yield, the stiffness of the material changes and load is picked up by the structure around it (which is still operating below yield, with a higher stiffness than the yielded structure).

If you’re not happy to make a judgement call, there’s a feature in most good FEA software that actually models this behaviour, and loads up adjacent elements. In short, if the model runs successfully, the structure is sound under strength limit state loads (i.e. much higher than expected). If the model doesn’t run, well, the loads are too high and the section too weak. Only now is it time to review the member sizes and plate thicknesses.

Stress plot on steel

Fatigue is a bit of a different story. We’re now talking expected loads being repeatedly loaded and unloaded, not those factored ones above which we accept failure. If these repeated loads produce yield in some elements, that’s not so good. Because when the load is removed the yielded elements get compressed by the stiffer elements of steel around them, and if repeated long enough will lead to fatigue failure

FEA- Stress Plot

The moral of this story? Just because you buy the licence to use FEA software, doesn’t imply you know how to use it!

We’ve been using FEA for years. Used properly, it’s an amazing method that proves complex structures are structurally sound in cases when there’s no equivalent simple code formula

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Author: Dave Meney