Fatigue Crack Repair

This thread in the Google Welding group is a pretty lively discussion on how to repair a structural crack in a chassis. (In this case for a backhoe).

I have a question of curiousity here. I have a backhoe with a fatigue
crack in the chassis. The chassis is made from rectangular mild steel
box section. The crack is where a piece of 6" x 2" x 1/4" box section
is welded to a piece of 4" x 2" x 1/4" box section at 90 degrees. The
crack is in the butt welded section and is about 1" long.

Someone else has tried to fix the crack once before. They appear to
have welded up the crack, then created a broad "patch" of weld metal
about 1" wide and 1/8" thick across the affected area. Their weld isn't
messy, but there's a lot of weld metal present. I don't know if this
was a sensible way to fix the crack, but it is just starting to
reappear again. I wonder if they didn't get complete penetration and
left a tiny depth of crack which has helped the crack to grow again.

The responses are all good, but the best one seems to capture the issue: drilling out the ends to relieve stress points.

One thing you're not mentioning that I consider a critical part of any
crack repair is to drill out the ends of the crack. You can plug weld
those holes as part of the repair process, but it is essential to get
rid of the stress concentration points at the end of the crack so it
does not continue/come back.