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I have a structure made out of steel consisting of many thin rods and some of them are welded to each other. I conducted a simple FEA static structural analysis to determine if the structure would fail under the critical loading conditions or not. For simplicity of the analysis, I did not model the welds between the rods and treated the whole as a single structure (which means that the connection between the rods were sharp with no welds and no fillets whatsoever). While examining the results, it was observed that the structure overall was safe from undergoing any plastic failure but the sharp connections between the rod (which are supposed to be welded) were experiencing plastic failure.

Now my question is, what should I do? Should I just believe that the weld will be strong enough in reality so that we can safely ignore and disregard the stress accumulation at the rod connections, or should I be concerned of that highly stressed region and do some geometric modifications to overcome that apparent local plastic deformation?

P.S. : If I increase the thickness and external diameter then these stress concentrations decreases and the structure is no longer failing at these connections, but I could not tolerate more mass into my structure. Plus, in FEA results, it didn't come out as a singularity either according to my interpretation.

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  • $\begingroup$ Additionally, it should be noted that not all kinds of steel can be welded without losing their properties. Nowadays that's not a problem with most grades of modern steel, but when dealing with older structures it should be a major concern. $\endgroup$
    – Pere
    Commented Sep 24, 2021 at 8:22

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Regarding the problem you have, without reviewing the results its difficult to make a definitive answer. However, if you are observing high loads near the weld, I think you should try to make geometric modifications, to decrease any stress concentrations or loads.

The reason is that welds are generally very "temperamental". What I mean by that is that, compared to other fasteners, the skill, ability and technical expertise of the welder is very important.

If the welder is not skilled enough then the overall structure (not necessarily the weld) can have significant variations in strength.

In some cases the heat from process, will create a very strong weld seam. However the Heat Affected Zone (HAZ), might degrade the material near the seam, making it brittle. As a result, in a poor weld, you might have a strong weld seam but a weak surrounding area.

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Yes, it increases and decreases strength in traditionally steels. Depends on what steel you have and what you mean by "thin rods", and the weld process , and the specific weld parameters. Properly selected filler metal will be close to the base metal strength. Common stick electrodes are "60" which means the weld deposit will be over 60,000 psi tensile. The base metal in the HAZ next to the weld may be high strength. the base metal at the farthest point in the HAZ may be lower strength than the base metal. As an example ,when evaluating a weld in an above ground storage tank, a weld cross-section is cut and ground and 18 hardness tests are made ( 30 kg Vickers is popular). This evaluates hardness/strength near the outer surface , inner surface and center , close to , and at two other distances from the fusion line.

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