[Mahler9th]

More on testing (2018):

https://www.compositesworld.com/news/titan-reaches-4000-m-depth

"As part of Titan’s extensive testing program, the OceanGate team conducted a series of unmanned dives by lowering the submersible on a monofilament line incrementally to 4,000 m. Onboard the sub, strain gauges, viewport displacement sensors and a custom designed acoustic sensor system measured the health of the hull to provide data that the team analyzed during and between dives. Many of these sensors will remain permanently mounted in the sub and will give the pilot real-time feedback on hull behavior on all future manned dives."

Not sure if that was the same hull that was recently lost. I doubt it.