It had already been down quite a few times. Do you mean 100's of drops? Although, couldn't they just leave it at the bottom for a month and see if it implodes?
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...
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.
It wasn't rated for the depth. In fact, I think it was rated for 1000 meters less than the target depth.