Avanti doors are notorious for cracking and outright breaks in the fiberglass. Almost every car I've worked on has needed extra reinforcing and fiberglass work on the doors.
In my opinion, Studebaker did not adequately engineer the doors for the torque of the window regulators. Also the door opening stop is abrupt and this creates high stress fractures around the hinges where they bolt to the door.
I believe had Studebaker stayed in business they would have addressed and corrected these issues. Because of the short run of Avanti production, they did not have the time to work out all the bugs in these cars.
Photo Credits - GBZ
In the above picture, you can see where I've repaired and built up layered fiberglass areas on the inner panel.
Doors tend to have weak points, not only on the inner panel area, but around the latch and front hanger hinges. In the above picture, you can see the steel flat straps that I've glued and riveted onto the inner panel.
Many of the earlier Avanti have problems in the surface of the door skins around the handle. In the above picture you can see where I've inserted and attached to the inner door a second metal strap support. This is in addition to the factory installed single support. This strap helps support the handle to prevent cracking around the holes where the handle bolts on.
Above is a repair in the latch area. This area of the fiberglass often breaks in the sharp 90 degree cutout turn directly above the latch. I made a L shape steel insert that is hidden in the inner door and repaired the fiberglass.