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In weld symbols one can specify the finish of the weld. In flush weld, for example, one needs to level the weld into a flat surface. I can imagine when this is needed: if something is required to rest flat on the welded surface, an uneven weld might get in the way.

But why do we have a symbol for a convex finish? When is it required that the weld "sticks out" like that? Similarly, why the concave? If I do a concave fillet weld for example, isn't the resulting weld less strong than intended in the weld size since it is slightly smaller due to the concave shape?

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    $\begingroup$ So how deep is your “concave” ? 1% or 10% or 50%? How much difference to the structural strength if it is 1%? If the joint is already stronger then what is the issue? If the joint must be below the surface then a concave specification seems logical perhaps for clearance... $\endgroup$
    – Solar Mike
    Aug 1, 2021 at 16:44
  • $\begingroup$ A larger concave fillet radius reduces the local stress concentration. There is a trade-off between reducing the peak stress in the weld, and increasing the average stress, because there the weld cross section area is smaller. $\endgroup$
    – alephzero
    Aug 1, 2021 at 20:51

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In general concave welds are avoided. Usually, they are the unintended product of vertical welds, when gravity is affecting the molten mass.

However there are cases, where concave welds are beneficial for the fatigue strength.

For example for bending stresses, AWS recommends the following improved profile for bending loads.

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Figure 1: AWS improved profile (source: SSC-400 WELD DETAIL FATIGUE LIFE IMPROVEMENT TECHNIQUES)

The benefit can be seen in the following image:

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Figure 2: Stress improvement of AWS improved profile (source: SSC-400 WELD DETAIL FATIGUE LIFE IMPROVEMENT TECHNIQUES)

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A colleague of mine from ages past who went on a welding course picked up a tip for welding of joints that required a grind flat finish afterwards, that the weld joint should have a convex finish to ensure there is weld material to grind off. The argument was a flat/standard finish in this scenario meant than in reality, some of the joint would be above the material line, some below; the above material would be ground off ok but the below material wouldnt be touched, leaving small "micro" pits/crevices on the surface that would be potential corrosion points.

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A convex weld finish is required in certain welding scenarios for various reasons. Firstly, it provides increased strength and improved load distribution, making it ideal for structural welding applications. The convex shape helps evenly distribute stress and reduces the risk of stress concentrations. Additionally, it can be used to control weld penetration, ensuring it reaches the desired depth without weakening the joint.enter image description here.

Convex welds also offer additional reinforcement and resistance to fatigue and cracking. In some cases, the aesthetic appeal of a smooth, rounded surface finish makes a convex weld desirable, particularly in decorative or cosmetic welding.

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Okay, so others have answered why you would need a convex weld. That is fine. But the essential question is why do we have a smybol for a weld like this?

The pragmatic answer is for completeness. Its not really the standard makers job to ensure that all of what the designer can define makes sense. There could be all kinds of reasons for a designer to define something that does not seem useful to us now.

Since the set of things is finite in this case it makes sense to define also the case that does not seem to be useful at this moment. After all it is concevable that i want to design the worst thing possible. Alternatively i might not care about the weld. Further future knowledge might change things.

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