Most deep sea submersibles only seat a couple people, which wouldn't work for the 'titanic tourism carnival ride' business model. I would guess the weight savings were intended to enable a sub with more capacity that could still be winched on and off the support ship without more complex/expensive heavy lifting equipment.
That was more marketing; the main biz model was to create enough buzz with the oil/gas industry to get bootstrapped and produce a product fleet of cheap subs for independent contractors to use for oil rigs.
https://www.fastcompany.com/40406673/the-man-who-wants-to-se...
The operators with the kind of risk tolerance to support this kind of, uh, venture can’t afford to do deepwater exploration - that’s solely the domain of the industry mega-giants, which are the most safety conscious and risk averse of all. The odds of getting a contract with, say, Exxon for anything resembling that piece of shit submarine are just vanishingly small. I actually don’t think the accident makes that prospect any less likely - it was already about as close to 0 as you can get.
A modern DSV for Challenger Deep (2x the pressure) used Isofloat to save weight and improve buoyancy.
In parallel do it with another hull at the same time. Or maybe more than one. Compare.
Testing to failure is a nice option because it finds certain types of unwelcome surprises.
But it's not really complete, either -- there are probably a lot of ways that a submersible can be compromised, and there's no way to be sure what caused this particular failure.
It wasn't rated for the depth. In fact, I think it was rated for 1000 meters less than the target depth.
It certainly does for aircraft, and I doubt the material science would be much different for subs (with the added challenge(s) that (salt) water often brings):
* https://www.smithsonianmag.com/air-space-magazine/what-deter...
* https://simpleflying.com/pressurization-cycles-aircraft-life...
Titanic depths are around 380 atmospheres pressure. So equivalent to the difference of about 133,700 feet and sea level.
No, cyclic stresses are a different animal altogether.
But considering it's just a matter of attaching ballast to sink it, dropping ballast to raise it, there's nothing preventing cyclic testing vs. static @ max depth over the same duration.
It seems obvious to me that you'd want to burn through a few test hulls characterizing the fatigue limits and verifying they are at least consistent hull-to-hull with a deterministic failure point you can plan for retiring before approaching.
You just need resources to burn on destroying enough hulls for the data.
And there's a rub there; as you inform the process via destructive iteration, odds are you'll first find the manufacturing process isn't even controlled/consistent enough to make progress on answering the "so how many cycles before go boom boom?" question until you've gone through a good chunk of runway figuring out how to even make it properly multiple times.
I did see that DeepFlight Challenger, Steve Fossett's unused/untested carbon fiber sub which used the same basic design as Titan, was only built to a 1.5x margin of safety. Experimental tests suggested it was only safe for a one time use, and shouldn't be used for multiple dives.
But yeah for stuff like bridges you're probably looking at something closer to 20x
Unmanned submersibles have plenty of market value. You may still be losing money, but you can strap sensors and cameras to the thing / undercut on price for research missions and substantially reduce the cash burn while building investor confidence.
I think submarines use ballast tanks they flood with sea water and empty with pumps to vary their buoyancy. Titan had none of that complexity AIUI, and multiple articles I read mentioned dropping "ascent weights".
So you really only care about their structural integrity at the surface.
Your data will be all over the place from surplus to surplus sale.
[1] https://www.insider.com/oceangate-ceo-said-titan-made-old-ma...