And in the corners of the Liberty ship deck hatches!
Figured they'd get torpedoed by Jerry before that became a problem. Or that big riveted plate amidships to keep the boat from breaking apart. Actually, deck hatches could have square corners, it was just that the steels and welding of the time were not of such a nature as to allow this. Greater problems, as with the RMS Titanic, were those related to steel ductility, and the temperature of water in the North Sea and North Atlantic, which turned the steel from a ductile to a brittle state. But they were useful, and only took 24 days to build. The first American Ship to sink a German Ship, the Commerce Raider
Trier. But yes, any square corner on a ships deck hatch would be a stress concentrator.