Originally Posted by
Morrie Caudill
All the respondents below are correct to different degrees. You are concerned about a welded, steel, tubular, construction. Fatigue cracks initiate at points where high stress is concentrated and usually that occurs in the welds. To begin, the weld itself has the structure of a casting because it solidified from a molten state. Typically cast structures (welds) are about half as strong as the wrought material they hold together. Most fabrications are designed based on the strength of the welds if they will experience bending or tension loads. The shape of the weld is important. The point where the weld blends into the parent metal, at the edges and where the weld starts and stops, must be smooth. If the "toe of the weld" looks like it laps over and does not blend in smoothly, this is likely an indentation that will be a stress concentrator. Fatigue cracks like to start in these points. Close inspection of the welds for corrosion and stress points is all that's necessary. And if you find any suspect spots, blend them in with a file or grinder.
I haven't run into any welded, aluminum tubing structures in the airplanes I have worked on. They have all been riveted assemblies where the rivets are loaded in shear. Both structures, steel and aluminum, are usually designed to be stiff enough to resist bending, and fatigue cracks are not a problem.