I see a shiny spot in the photo, on the right hand part of the broken cap, near the outside edge. The fatigue striations seem to point to this spot as being the origin of a fatigue failure. It's tough to say from just a photo like the one shown, but with fatigue fractures, you should be able to trace the striations (ridges) back to the point where they began. The striations are like a road map of a fatigue failure. There is also a shear lip on the surface of the cap, immediately adjacent to where the bearing rests. That is the last part of the cap that fractured.
I'm a Metallurgical Engineer by day. My only recommendation would be to make sure that all machining marks, sharp corners, steps, etc. are smoothed out before putting the new main caps in service.
Steve M.