[elp]

If the stories about some parts only being certified to 1.3km instead of 4km are true then it was probably operating closer to the yield point than ideal. My guess is that metal fatigue started to become an issue and it failed too quickly for anyone to react.

If the were too cheap to design and build it properly then they were too cheap to check for wear and tear properly.

[j-a-a-p]

Fatigue occurs after oscillating stress levels (far) below the strength of the material. So - stuff will break under low stress, if you apply it often enough. It must oscillate, otherwise it will never break. Some material, like aluminium, have very low minimum thresholds.

A sub is subjected to static stress mostly. Quite a lot actually at these depths. And having it certified for 1.3 km hints to over stressing.