Weld basics

[Gmenman]

I'm an RE on a bridge construction project. The contractor field welded some clips in the tension zone of a girder, which they now have to remove and grind down to the base metal. I know that this is an acceptable method, but I don't know why (my background is in soils). I'd like to understand this better.

Does anyone have a link or a simple explanation as to why this is the case?

Thanks in advance.

 

[metengr]

To remove any stress risers.

 

[metengr]

In addition, fusion welding results in altered mechanical properties that occur in the base metal heat affected zone. So, removing a temporary welded attachment should be done by removing the attachment itself and some heat affected base material beneath the attachment to avoid stress concentration and crack initiation in the remnant heat affected base material.

 

[Gmenman]

Thanks for the responses, Metengr. I looked more into what a stress riser is, and found it compared to scoring glass and snapping it at the score, so I get that. As far as the heat affected zone is concerned, is it safe to say that it wouldn't be a great depth? If the flange is 1" thick, for example, how deep would the heat affected zone be?

 

[metengr]

Gmenman;
The depth of the heat affected zone can extend into the base material for up to 3/16", depending on base material chemical composition and welding process. In some cases, all you need to do is to grind flush the base metal where the attachment was welded. If you want to remove some of the heat affected material from welding, you may need to locally remove an additional 1/8" to 3/16" of base material beneath the welded attachment. If the material is structural steel, I would not remove any heat affected material after removing the welded support. I would perform surface NDT to ensure no cracks in the base material at the location where the attachment was welded.

 

[Guest102023]

Its not exactly the same situation with glass. One prominent fracture mode in metals is fatigue, which requires many (often > 1M) cycles to occur. This fracture mode is (most often) brittle, but can occur in metals normally considered ductile. Geometric stress raisers, especially cracks and sharp corners) have the effect of multiplying the effective stress. This is why such an effort was called for to remove this potential flaw and subsequently verify removal by inspection. Any mechanical design text will have tables of stress concentration factors (SCF) based on geometries and modes of loading (say, a radius in a rotating shaft).


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