How would a carbon fibre hull collapse?
From the engineer who sued them after being fired:
Given the prevalent flaws in the previously tested 1/3 scale model, and the visible flaws in the carbon end samples for the Titan, Lochridge again stressed the potential danger to passengers of the Titan as the submersible reached extreme depths. The constant pressure cycling weakens existing flaws resulting in large tears of the carbon. Non-destructive testing was critical to detect such potentially existing flaws in order to ensure a solid and safe product for the safety of the passengers and crew.
So that’s one possible reason why it ‘might’ fail not on the first outing, but after a number of them, with the repeated (and massive) changes in pressure. I suppose they might retrieve a bit of the carbon from the bit that breached, if it was that that failed, rather than the titanium bits, or the window, but it’s equally possible that we’ll never find out.