I have done some searching and I found this at a sub forum at warships1.com posted by BP

"Crush depth on the diesel boats was very conservative. Probably close to 100% over test. In general they sort of knew which area was prone to be the first failure point, usually something like the torpedo loading hatch. But if you went close to crush and survived, the boat would suffer hull damage. The yield stress on hull materials would not completely recover when the stress was removed. Depending on the amount, the boat was junk from that point on.

Examples, ones like USS Chopper was made into a Reserve Boat and was not even allow to leave the harbor or dive. Basically welded to the pier once she came very close to crush depth in an uncontrolled dive off Cuba. Most USN boats that experienced these evolutions were removed from service and put to other uses, (training, tests or scrapped for parts). The Germans also had some boats like this. There are notes of boats found after WWII that were laid up and not considered sea worthy after exceeding test depth. Even operating such a boat on the surface is risky as they could break up in a storm without much warning."

According to this it seems to be dangerous to repeat a very deep dive.
Also what fist got me thinking was the book Das Boot where they manage to survive a very deep dive. And then they are more or less afraid that the boat will fall apart before they get home.

Any comments?

Leif...