Carbon fiber has been around for decades and is a proven material, in the right application.
The recent tragic loss of all hands aboard the OceanGate Titan submersible has generated a great deal of criticism over the choice of carbon fibre as a structural material for the vehicle. Carbon fiber has a long history in aerospace and high-performance automotive applications, and it is a proven, durable and safe material in everything from racing cars to space satellites. It can be used for submersibles, as well, but composite materials in general behave differently from metals. Both ultimate and cyclical loading is important and like many aerospace structures, a fatigue life based on number of pressurization cycles may be essential, regardless of the design safety factors.
And, access all episodes of End of the Line on Engineering TV along with all of our other series.
* * *
Episode Transcript:
The tragedy of the OceanGate Titan sub disaster has generated a tidal wave of speculation about the cause of that catastrophic implosion. Poor design, inadequate testing and the choice of carbon composites as a structural material are all under debate right now—and to be clear, no definitive cause has been released by any investigator to date.
In any failure like this, causes are usually several, and are often interlinked. The mere fact of a catastrophic failure of any device in normal operation suggests poor design and/or inadequate testing. But the use of composite materials instead of metals in the submarine’s hull has come under particular scrutiny.
Carbon composites are being characterized by many as too weak for this application. Here are the basic factors that everyone should know about composites compared to metals when used in structural applications.
As the name implies, composite materials get their strength by incorporating two or more different substances, usually long fibers, chopped roving or ground functional fillers embedded in a matrix. As far back as the 1940s, chopped glass fibers embedded in a liquid polyester resin which then hardens in a mold, were used to make fiberglass aircraft parts, particularly radomes, with good performance.
In the early 1950s, General Motors introduced the Corvette, another vehicle whose body is made with fibre reinforced plastics. Today, the outstanding Boeing 787 Dreamliner is almost entirely built from composite materials, and it is a reliable, high-performance aircraft.
But engineers working with composites as a structural material have to keep a couple of factors in mind. We all remember those elementary stress-strain curves from first year engineering, and we know that if we cycle most metals inside their Hookean range, we can cycle them essentially indefinitely with no harm done. Cycle it into the plastic range, and it deforms.
Metallurgists dive deeper into dislocation theory, and note that dislocations can tangle and pile up, creating the common work hardening effect that we’ve all seen by flexing a paperclip back and forth until it breaks. For airplanes, this “metal fatigue” is factored into the expected service of the airplane, and a fatigue life is established after which the aircraft must be scrapped or rebuilt.
Every time an airplane takes off and lands, one pressurization cycle balloons the fuselage with considerable hoop stress, then relaxes that strain. A famous example of not fully factoring the cyclical loads into the product life was the famous Hawaiian airliner which came apart midair, a consequence of the unusual service life of that 737, which involved multiple takeoffs and landings every day. Put simply, that particular airplane had an unusual service life not anticipated by engineers used to airliners flying two or three flights a day.
Carbon fibre structures are no different. Depending on the material chosen, enough strain will fatigue composite materials, evident through micro-cracking in the matrix, pullout of reinforcing fibers or delamination of structures built up from multiple layers. And for continuous fiber reinforced composites, it’s common to lay up multiple layers on the bias to ensure omnidirectional strength.
In short, with known service conditions, and an established projected service life either defined by time or cycles, and a well-characterized material and manufacturing process, there is no reason why a composite submarine, or airplane, or teakettle for that matter, should fail in service.
Why did OceanGate use carbon fibre? Well, it’s strong, light and critically, it can be laid up without expensive tooling or gigantic machines. It’s simply cheaper to work with. Regulations will likely emerge from the submarine tragedy, and I hope that they don’t restrict the choice of materials that builders can use for undersea vehicles. Used correctly, composite materials are an outstanding choice for structures.
