[Mahler9th]

Excerpts from paper about the Nereus design (author Glenn McDonald):

"Underwater vehicles require pressure-resistant housings for components such as computers, cameras, batteries, and electronics for communications and navigation. Because housings must be strong and corrosion resistant, titanium has become the material of choice for most deep ocean work. As titanium pressure vessels (PVs) are sized for depth, wall thickness increases and weight-to-displacement (W/D) ratios increase. Thick-walled titanium housings provide no floatation at full ocean depth.

As housings get heavier, greater floatation and power requirements become necessary; everything gets bigger—more foam, larger frame, bigger vehicle, and larger ship for deployment. For most science autonomous underwater vehicles (AUVs), weight is critical, and keeping it low is an aim throughout the design. Most science AUVs rely on low-weight, high-range geometries that have a good top speed and payload capability. These factors are directly affected by the housing used, and thus the housing material selection [1].

Traditionally, ceramics have been a marginalized material for underwater pressure housings due to their susceptibility to catastrophic failure. Housing materials that yield before they fail—metals such as steel, aluminum, and titanium—have been preferred because they are much more forgiving. Metals have the ability to plastically flow to relieve the effects of stress concentrations created by features such as transitions, penetrations, and O-ring grooves. This simple ability to plastically flow can relieve point loading though local yielding. Ceramics do not share that characteristic. They are brittle in nature and will crack or chip when forces are applied to an improperly designed feature."

"In spite of the potential difficulties, the Nereus housings’ trench-depth capabilities and floatation requirements drove material selection to 96% alumina ceramic [2]. Alumina ceramic provides excellent corrosion resistance and high compressive strength in a light material that can be used to build thin-walled, buoyant housings. 6Al-4V Grade 5 titanium (Gr5 Ti) was the material choice for housing end caps and connectors, as well as joint rings and housing couplings (Fig. 2 and Fig. 3). The Gr5 Ti provides a high strength-to-weight ratio, excellent machining capability, and very good corrosion resistance.

Woods Hole Oceanographic Institution (WHOI) brought in Dr. Jerry Stachiw, noted in the field of underwater housing design, to lead the Nereus ceramic housing development. Stachiw’s years of work resulted in significant advances in many areas of ceramic housing design—strength and cycle limits, methods for joining ceramic sections, end cap shape, ceramic boss requirements. The Naval Command, Control and Ocean Surveillance Center (NCCOSC) technical reports developed through this work clearly document materials, geometries, testing, and lifespan. These reports provided a valuable path for design, construction, and testing. For further reading, please see [2]"

One can download the paper and there are references folks might find interesting.

Again, Nereus was an unmanned vessel designed to go much deeper than Titan.

And it imploded.

And I suspect that it was designed for a fixed number of dive cycles.