All the following MOO: though it is the opinion of a PE with experience in pressure vessel design and maritime conditions.
The hull failed. Greater than 99% chance the site of initial failure was in the carbon fiber. The collapse was highly immediate because if water had simply intruded past a joint and the vessel filled over say, a 5-second period, the noise detected would have been of a different nature.
With composite material brought to shape with the methods utilized, there were always defects in the form of voids and interlaminar gaps and inclusions. Tighter control over temperatures, fiber tensions and guidance, fiber diameter variance, bonding medium thickness and chemistry, rotational speed, adhesive promoters and their distribution, and a host of other variables would reduce these effects. To rationalize how strenuously a manufacturer needs to manage such conditions, would require a great deal of testing and analysis. Manufacturing controls also means investment in the equipment and controls as well as higher quality (controlled) raw materials which would be required to reduce those variables to a tolerable level. For a deep submersible, that tolerable level is very high....the risk and consequence has been exemplified by this event.
For the Titan specifically: we know some of the raw fiber had reached its shelf life and been downgraded and sold by Boeing. We know that at least one prior hull made scary noises at depths nowhere near those of the Titanic. We know that the sonic warning system was looking for "pops" from the hull. Those pops were presumably caused when the matrix filling the space between 2 voids cracked. One pop, expected. A hundred? Might be tolerable if the voids were small enough and close together. Those would be low volume pops. If the voids were large and spread out...3 loud pops may have been the limit. Obviously this mechanism is cumulative. How many pops, and at what frequency volume and proximity, would represent time to condemn the hull? Who knows? How much does it cost to find out?
For a homogeneous material, for instance steel plate: the cost-risk-benefit has been done over many years and is engrained into codes and material standards. Similarly for welding and forming where its is applied to join steel components to form a shape. Steel is an elastic, fracture tough material, but it is susceptible to fatigue. Even though steel had been used many hundreds of years before the Liberty ships were manufactured, it was only after the failure of some of those hulls that the subject of fatigue combined with the effects of temperature variances and cyclic immersion was detail studied....in the 1940's. The point is, we continue to learn; and it sometimes takes a disaster to prompt us into paying all the dues that lead to better comprehension.
There is an extensive report about the Challenger disaster; and this is a whole lot of information about the machine and the flight that was not in any report. That time, it wasn't the technical folks who took the chance; they advised against the launch. Their shortcoming was, they could not specifically say that a small deviation from the design parameters could make the launch unsafe. They did say, they could not trust the design to work outside the prescribed limits when all their testing and QC was focused on conditions within the design envelope.
Summarizing: the reality for the investigators in this case is multi-tiered. Should they act like the acrylic port was not the culprit, then they encourage others to apply them in spite of manufacturers recommendations and rating. Should they conclude that one of the joints failed, they would not discourage others from developing an alternate joint design for a CF hull. To definitively blame the CF hull, would require a lot of investigation of the parts scattered on the ocean floor; and a lot of analysis and lab work of the type that Oceangate did not do. The latter effort is very expensive and to what end?
There has been lots of publicity on this disaster and its causes. One would believe that at this stage no tourist is going to get into a CF vessel to explore at depth....The realistic conclusion is, first, don't use CF. Then pay your dues to design, quality control and real world testing....as well as certification. Those were the expenses that were being circumvented by the "alternate approach"; which led to the passengers appelation, "Mission Tech". A conjured bypass to the rules of commerce, compounded by a limited comprehension of the rules of science.
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