Fixing cracks in sheet metal, tube, et al.
When I first bought my blue buggy I was very much overwhelmed and missed a lot of things. I even passed on a heck of a deal for a good price on a very nice rail mostly because I didn't understand what I was looking at or being talked about. I was also played with: given some really bad information that I think I am still carrying which is another reason I try to give the best info I can.
I ended up buying the blue buggy then, after one or two short trips to the dunes got ready to tear it down to make it more off-road worth based on what I thought was correct at the time. One of the things I had missed when I was looking at it was there was a crack in the tunnel at the rear of the hole for the e-brake (the angled up mounts of the brake had been removed which weakened up the area. Also, the cut for shortening the pan is usually made about 1" behind the e-brake hole.) going down both sides of the tunnel.
I cleaned the area up then had it welded by someone who was supposed to know what he was doing (he did!). Shortly there after the crack came back again, in the same area, so I had it rewelded again. After the third time I had a large boot/doubler made from sheet metal and bent to fit over the area then it too was welded in place; ~20 years later it is still holding.
One of the things that I don't think was done before the welding what to drill the end of the crack (only one end as the crack came in from the side of the hole but if the crack was in the middle of something then both ends would be drilled. After doing a search on the subject today I am attaching only one URL on this but there is a lot of comments on this, some of it being not too good in my opinion as some things stated were absolutely not true. One of the comments below is a truism: the ends of the crack may extend farther than the end showing on the surface shows!
Also on tubing, the recommendation was to drill, weld then add a doubler over the area.
http://www.millerwelds.com/resources/co ... -car-frame
At the same time I looked up stress relieving after welding but, with the exception of two posts in a string the idea of localized stress relieving via a torch was not talked about. The one said it wouldn't work and the other said yes, it does work. I have seen it done several times by professional welder, depending on what they were told the part would be used for but the results I never heard about so I can't say a "yea or nay".
For what its worth.