There are recurrent cracks in the brick wall shown in the image. The cracks have been repaired before but keep reappearing in the same location under the windows.
I suspect this is because the wall requires expansion joints. Is this a valid assumption? If yes where should the joints be placed?
The wall has a length of around 40 feet, is south facing, and has a mortar filled vertical joint on the right hand side corner as seen from image. Would cutting that existing joint open and making it an expansion joint be enough to possibly solve the cracking problem under the windows?
I would say the problem is the lack of a control joint. The stacked windows five the wall a weak point where it can “pull apart”. Cutting an existing joint and the brick in the adjoining course between the windows and the window and the base of the wall should work, then could patch the crack. there is n guarantee as the crack is there and it is a weak spot now.
I would say that the crack is more indicative of a shrinkage crack that a sagging lintel. If the lintel were weak and sagging, you would typically see step cracks going up from each corner to the center of the window, creating a triangle pattern. The single crack as shown in the photo is more of a sign of a missing control / expansion joint.