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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 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 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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