PhilipG,
Sorry, no - the bulkhead that has cracked has always been aluminium. The titanium bits that were replaced were the keel beams, which were that way to meet the C model requirements for catapult launch and recovery. Those were changed out to new aluminium beams on the A and B.
Structures fail for all sorts of reasons, including selection of material, but for most combat aircraft, high strength aluminium is really the best choice. The main reason structures fail is due to failings in design, mainly failing to correctly model and predict load paths and load magnitude. Fatigue also plays a massive part, and here things get really complicated, depending on how you apply fatigue spectra. Clever people all around that stuff.
I don't for one moment want to suggest that 'all is well'. It's not and fixes will have to be found. My own feeling is that a rapidly executed weight reduction programme led to some structural design features that should have been better. But I could very well be wrong.
That said, the sky is not falling in here. It's perhaps best to remember that the F-35 team set themselves a very, very high bar for achieving a complex yet light weight structure. Nobody is perfect. The designers of the Tornado wing box found that out. So did the C-5A wing team. So did the F-22 aft fuselage team. So did the Buccaneer team. So did the Hornet team. So did the Comet team.
It's easy to criticise. It's a bit harder to do.
Best Regards as ever to all those doing the doing.
Engines