Seems that the crack follows the line of the two 'rivets' near the top bolt in pic 1. If they have stopped the top layer of steel lying flat on the lower level, tightening the bolt would try to bend the upper layer and create a stress line which ultimately fractures. My guess is that the lower level is not damaged and this should be checked. I would then see if the upper layer could be cut out and replaced, ensuring a proper fit with the lower.