Did Carbon-Fiber Submersibles Just Have Their Hindenburg Moment?
Carbon fiber has been in the news a lot lately, and not in a good way. The OceanGate Titan submersible, made with a carbon-fiber hull, went missing in the vicinity of the Titanic’s graveyard. The hull imploded and doomed the five passengers aboard.
Information about the company, its CEO (and his reckless bravado regarding safety), the design of the ship, and more changes daily. Many of the updates have been surprising, and there are likely more surprising updates in the future (meaning that this write-up may not age well).
I suspect that finding the exact root cause of the implosion will be impossible. What I’ve seen of the recovered wreckage so far doesn’t include any carbon fiber, but instead a tangled mess of metallic parts and wiring. This implies that there isn’t much recoverable carbon fiber left.
Finding initial failure point is unlikely
Implosions are frightening. They always happen extremely quickly, and they squish “everything, everywhere, all at once” (sorry, couldn’t resist). One part of a hull buckles from the external pressure, and then neighboring areas are dragged along so that the implosion propagates without stopping. With a large majority of the ship missing, finding that initial failure point is unlikely.
Carbon fiber is actually a composite material that is more than just carbon fibers. Building anything from just carbon fibers will leave air gaps between the fibers. Water would easily pass through, so something needs to fill the air gaps — liquid epoxy resin. This is then heated to cure the epoxy, with the epoxy supporting the fibers in compression and bending.
The role of sizing
Carbon fibers are usually coated with a chemical (sizing) to help adhere the epoxy to the fibers. This chemical is somewhat reactive, so carbon fiber usually has a shelf-life. To hear from the CEO that he used “expired carbon fiber” is not a good sign. It means that the sizing was probably becoming less effective in helping the epoxy adhere. If the epoxy didn’t adhere well to the carbon fibers, this would create a weakness that could have led to failure.
Cycle fatigue is also particularly relevant here. Every time the hull is exposed to high pressures, it can weaken. The pressure does shrink the hull, and since it is not flat, the shrinkage can create shear stresses since everything will not shrink the same amount. If the stress is high enough, it can cause the fiber and/or epoxy to move around permanently, creating weak spots. Poor adhesion between the fiber and epoxy will compound this; hence, the importance of having good sizing. Cycle fatigue would explain how it is possible that the same sub made multiple trips without issues, only to fail later.
We don’t know how the hull was built, nor by who — I imagine they are “lawyering up.” I’m sure that will be insightful. But without large amounts of the hull to examine, investigators will only be able to come up with multiple likely scenarios rather than a specific one.
Limitations of carbon fiber
Engineers in the submersible arena have previously been aware of the limitations of carbon fiber, but now so is the general public. The equation — (carbon fiber) + (ocean pressure) = (sudden implosion) — is now permanently etched in our heads. So much so that I don’t think anyone will be building another submersible from carbon fiber for a very long time, regardless of their skill or processing breakthroughs. No sane person would risk his or her life.
It’s analogous to how the fire on the Hindenburg put an end to airships. It’s not widely known, but prior to the Hindenburg fire, airships had been safe for quite a long time, logging millions of miles around the world without incident. All it took was one very public mishap and that perception was permanently changed. It’s now going to be the same for carbon-fiber submersibles.
About the author
Born and raised in Minnesota, John Spevacek earned a B.ChE. from the University of Minnesota (Twin Cities) and a PhD in chemical engineering from the University of Illinois (Urbana). He worked in the plastics industry for 25 years for several companies, large and small, in the Minneapolis-St. Paul area.
He began teaching so that he could share his experiences and knowledge with others. He and his wife became fed up with Minnesota winters and moved south shortly after this career change. Spevacek currently is an assistant professor of engineering at Wake Tech Community College in Raleigh, NC.