Carbon fiber materials and weak design are under scrutiny in the accident investigation.
The catastrophic loss of OceanGate’s Titan submersible with all hands aboard has triggered widespread speculation regarding the cause of the accident. Inadequate design, insufficient testing and the use of carbon fiber as a structural material have all been suggested as contributing factors. Carbon fiber is a well-characterized material that is strong and light, making it a favourite for weight-critical applications in aerospace. Its use in the much less weight-sensitive application of subsea exploration may have more to do with the way composite pressure vessels are built, rather than weight or performance.
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Episode Transcript:
The tragic loss of OceanGate’s Titan submersible with all hands aboard during a dive to the wreck of the Titanic has generated widespread speculation about the cause of the accident.
As an experimental vessel, the submarine was not subject to conventional regulations on ship design and used an advanced structural material for a deep diving submersible: carbon fiber.
Carbon fiber has long been popular in the aerospace industry as it combines the two most important attributes for any material used to build airplanes or spacecraft: light weight with strength. Unlike many advanced materials, however, carbon fiber is also a favourite of experimenters and startup companies because of the way that large carbon fibre structures are built up.
In aerospace, large assemblies such as wings and fuselages are assembled using fasteners combining parts that must be forged, machined, rolled, formed and bent using expensive tooling and sometimes large machines. Carbon fiber can be built up on a mandrel or laid up by hand in relatively simple, open molds, a technique used commonly in the high-performance sailboat and yacht industry.
It’s critical in composite structures to ensure that multiple layers are well bonded, with no trapped air, something accomplished by careful layup which is usually followed by a vacuum impregnation process. It’s difficult to get all the air out, and for safety-critical structures, an appropriate engineering safety factor is commonly built into the design.
But in aerospace structures, the pressure is on the inside pushing outward, a classic hoop stress problem, and any crack or void in a carbon fiber fuselage barrel reduces the effective thickness of the structure locally and creates a site for crack propagation.
While there is no definitive explanation yet for the implosion of the Titan submersible, the same basic principle applies. Excluding crushing pressure—which at the depth of the wreck of the Titanic is some 380 times surface pressure—is a different problem from aircraft, which hold much lower pressure in. It’s likely that investigations will include examination of the interface between the domes and the cylindrical main body structure.
Deep diving research submersibles are commonly engineered as spheres, the most efficient form factor for pressure vessels. The use of carbon fibre for the Titan sub may have had more to do with the cost of manufacture compared to the very expensive and difficult process of machining or roll forming large, thick metal structures, but it will be some time before a definitive cause is determined.
Carbon fiber is a viable material for pressure applications, and it’s likely that in the wake of this tragedy, a standardized, globally accepted testing methodology will be developed for undersea use.
