Originally Posted by Al and Missy
Years ago when I was involved in building some mobile communications centers I was told by a vehicle up-fitter that one should never weld on a frame. The reason was that the frame members were heat treated and welding would anneal the metal.
These were GM cab-body trucks that were to have shelters mounted to them. Are Airstream frames mild steel so that it is OK to weld, or is there risk of damaging the frame by welding to it? Just curious to know as I may want to add a rear hitch or something in the future.
I would be surprised if the Airstream (or any other travel trailer) used anything other than mild steel for the frame members. The nice things about mild steel are that it's easily weldable, and it's somewhat forgiving of repeated stresses.
My guess (as a guy who has done some welding and amateur race car fabrication) is that this particular situation looks like it was initiated from the elevator bolt hole being too close to the open edge of the frame rail. Then the crack just propagated from the bolt hole, around the corner, and down to where it currently is.
Repair: Drill the *very* end of the crack with a 1/4" drill bit to take away the sharp edge of the crack. Grind off all rust around the cracked area, then jack it closed. Weld up the crack, but then add a doubler of equal thickness (the angle doubler suggestions are good) for 4 to 6 inches on each side (8-12" total) and weld only along the top and bottom edges. You want the weld to be in shear, not in tension. And, you don't want to create another vertical weak zone in the frame member.
Book recommendation: "Engineer to Win" by Carroll Smith. Lots of good info in there about materials properties, as well as anecdotes, by a legend in race car fabrication and preparation.