Yet it was put into planes. So either it wasn't really understood in the context, or no one that knew about it was advising on the design of aircrafts.

The corners on the windows did have a fairly large radius. Just not large enough (hindsight being 20-20). I think they knew about it and thought they had more than enough margin for safety and then never thought twice about it until aircraft started falling out of the sky.

I oversimplified. It was understood in specific academic circles and to an extent by leading designers, but not widely by others. The cracks actually started at a hatch on the bottom of the plane which was added to the design without going through the full engineering design QA process.

Then it wasn't well enough understood...

Until it was cheap enough to manufacture with, aluminum hadn't been used in large structural components and the crack formation in aluminum is very different from steel.

With steel, if you add a large enough safety margin it will essentially never fail due to repeat stress cycles. With aluminum it doesn't matter how strong you design it, eventually cracks will form and it will fail.

ship don't go trough a thermal and pressurization cycle at every trip however.

it wasn't square window per se as much as metal fatigue