The structure is cut out of board material, not fiber material. Special material, the material of the cut welding root sounds like the blade of a sword!The heat from welding the stiffener plates could have contributed to the cracking. It's not a big excavator so it's doubtful if any special welding procedures or higher nickel filler metals would have been used. Likely no preheat either which may have prevented the failure, especially if the weld on the stiffener started at the same place the crack started. Instant quenching effect. Even slight undercut from the weld on the stiffener could be where the crack started. It may even be case where the end of stiffener would have been better left un-welded. The same reason you don't typically weld across the boom on excavators. I don't know if the steel was cut from plate or was made as a long bar. Bar stock typically has the grain structure going lengthwise where plate has the grain structure going in multiple directions. Basically if pieces are cut from plate they will have equal strength no matter what position they are placed in. Pieces cut from flat bar only have max. strength in the direction of the grain structure. For a lot of stuff it won't matter but will on more critical weldments. Preheat before starting the repair.