Thomas Edison tried to use carbon fiber as filaments in light bulbs in 1879. It didn’t work. But the strands of carbon, first created in 1860, found use in aerospace where their high strength to weight ratio is highly desirable. Rolls Royce used carbon fiber for the fan blades in the jet engines when it entered the U.S. market. Since then, it has since found use with the military and in racing cars. It has made its way into consumer goods. The lightest racing bikes and the most expensive fly-fishing rods make use of carbon fiber. Carbon fiber now has a such a high-tech appeal that carbon fiber weave is being used for its cosmetic, rather than its engineering, properties such as with aftermarket car parts.
One would not expect an engineer to select carbon fiber for anything but its engineering properties. But the late OceanGate CEO and founder, Stockton Rush (he perished in the Titan submersible on its fateful dive to see the Titanic on June 18, 2023, along with 4 others) was no ordinary engineer. Educated in aerospace (bachelors in aerospace engineering from Princeton) and business (MBA from Berkeley’s Haas School of Business). He chose carbon fiber for the material for his next level of deep-sea submersibles that would take “Titaniacs” (as he called Titanic fans) all the way down to 4,000 meters below the surface to see their beloved shipwreck – and make big bucks doing so. One Titaniac on a previous, successful dive, saved for 30 years to come up with the $250,000 OceanGate charged.
The Case Against Carbon Fiber
OceanGate was the first to use carbon fiber in the hull
of a deep sea submersible.
James Cameron, director of the Titanic and undersea explorer of some renown (taking his one-person submersible, the Deepsea Challenge to the Challenger Deep, guided by McCallum, where the pressure is over a thousand atmospheres. In an interview with the Wall Street Journal on June 23, when the world had given up all hope of finding survivors, Cameron said he “knew” what happened. “The sub has imploded,” he says with finality.
James Cameron, director of the Titanic and undersea explorer of some renown (taking his one-person submersible, the Deepsea Challenge to the Challenger Deep, guided by McCallum, where the pressure is over a thousand atmospheres. In an interview with the Wall Street Journal on June 23, Cameron says he “knew” what happened. “The sub has imploded,” he says with finality.
“I never believed that technology of wound carbon fiber filament on the cylindrical hull would work,” says Cameron “I thought it was a horrible idea.”
Cameron wishes he had spoken up when he first heard of this use of carbon fiber but he “thought someone was smarter than me,” says Cameron.
That unnamed “someone” is most likely Stockton Rush.
Cameron is part owner of Triton Submarines, which builds and sells submersibles, as mentioned in the Wall Street Journal.
The Long and Winding Fiber
How carbon fiber parts can perform admirably above the surface and fail catastrophically at thousands of meters below bears explanation.
Bands of carbon fiber being wound around a metal core for the Cyclops 2 (later renamed the Titan). Video: OceanGate.
Wet winding carbon fiber around a round core is a best practice approach in aerospace to create stiff, strong and lightweight fuselages, such as in the Boeing 787 Dreamliner.
Rush claimed to have a partnership with Boeing, which Boeing denied after the disaster. The relationship may have consisted of no more than buying carbon fiber from the aircraft manufacturer “at a big discount” “because it was past its shelf life for use in airplanes,” according to Arnie Wiseman, travel editor of the Travel Weekly, who recalled a conversation with Rush for the Washington Post.
The main structural difference between a pressure vessel, such as an aircraft fuselage, and a submersible is external versus internal pressure. Internal pressure makes its gas container want to expand. Think of a balloon. The tightly wound carbon fibers around a fuselage are great at resisting expansion. The tensile strength of carbon fiber (as much as 7.06 Gpa) is 4 times better than steel wire (1.77 Gpa). Carbon fiber in tension also offers satisfying simplicity. Like a string being pulled, it keeps its shape in tension. Put carbon fiber in compression, however, and all bets are off.
Boeing 787 fuselage made with similar process: winding carbon fiber around a core. Video: YouTube.
Carbon fiber is also used in pressure vessels. It has a marine application as well, including scuba tanks, which are valued for their lightness.
A design that put carbon fibers in tension and with internal pressure on the order of 1 atmosphere, as occurs with the Dreamliner, is vastly different than putting carbon fibers in compression with external pressures two orders of magnitude higher, as occur at Titanic depth.
The external pressure on the submersible put the carbon fibers wound into the hull in compression. Expecting carbon fibers to remain in the round rather than crumpling is the job of the epoxy matrix that surrounds each fiber. Still, carbon fiber subjected to compression has as little as 30%[i], and the most, 60%[ii], of its strength in tension.
But the cylindrical section of the Titan’s hull is subject also to significant axial force caused by the pressure against the spherical domes. The resulting force compressing the cylinder along its axis at a depth of 4,000 meters is a whopping 20 million pounds.
Can the carbon fiber hull take it?
A 2017 article in Composite Weekly describes the carbon fiber construction of the Cyclops 2 (later renamed as the Titan) as “alternating placement of prepreg carbon fiber/epoxy unidirectional fabrics in the axial direction” with “wet winding of carbon fiber/epoxy in the hoop direction, for a total of 480 plies.”
In the axial direction, the hull gets no help from the wound carbon fiber which lies opposed to the axis. Consequently, the axial force is resisted only by the compressive strength of the epoxy matrix and sheets of CFRP that are laid by hand containing carbon fibers running along the axial direction. OceanGate shows a metal tube around which the carbon fiber filament is wound but it may be a mandril removed after hardening of the composite. There is no evidence that it stays in the hull so we can’t assume any help from it.
Winding filament around prepreg sheets would help keep carbon fiber aligned with the axis. However, the possibility that the filament ribbon would be cut, compromising some strength, has raised at least one eyebrow among the engineers at Eng-Tips.
Unlike homogenous, isotropic material, such as the titanium used in the endcaps, which springs back into shape as good as new, the carbon fiber hull suffers with each dive. The fibers can crack, bend, lose adhesion with the epoxy matrix… and other problems that are peculiar to composites.
The Titan had multiple dives to greater that 3,000 meter depth which and may have accumulated weaknesses.
No hull monitoring system was needed during a April 2019 dive when Karl Stanley, submersible expert, took the Titan to 12,000 ft off the coast of the Bahamas. Stanley heard a cracking noise and urged Rush to cancel that summer’s dives to see the Titanic, reported the New York Times.
Next: Ill-Fated Titan Submersible Was Based on an Unproven Design
Corrections
July 20, 2023: Changed attribution for kink band graphic.
July 5, 2023: James Cameron is part owner of Triton Submarines, according to the WSJ, not OceanGate.
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[i] A Review of Structural Factors Which Control Compression in Carbon Fibres and Their Composites, Srinivas Nunna, et. al., Composite Structures, 2023
[ii] Compressive Strength of a Carbon Fiber in Matrix, M. Ueda, et. al, 18th International Conference on Composite Materials.