this post was submitted on 26 Jun 2023
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Fiber composites are complex. Carbon fibers can be made to withstand compressive loads in that you make composites with the CF and other materials. Even so carbon fibers are about half as strong in compression as tension. Even so, carbon fiber might have a specific strength of around 3000 kN-m/kg, steels might be around 63 kN-m/kg. So it's not as simple as "carbon fiber isn't as good in compression as tension, never use it in compression." A lot of current research in aerospace is to produce better manufacturing methods and resins for carbon fibers to phase out aluminum and steel parts. Mostly in tension yes, but compression too, all parts without preloading generally are some degree in compression during their stress lives.
Should he have bought expired carbon fiber for a submarine? No. Is carbon fiber completely absurd in submarine usage? I don't imagine so. Though steel is plenty fine for a submarine, normally the hard part is the sinking by design, not the floating, so weight savings aren't super important.
I have absolutely zero knowledge on deep sea submersibles but every bit of reporting I've read or listened to over the past week has said that carbon fiber is a very poor choice for a deep sea vehicle. Given its propensity to eventually delaminate, it is much more likely to fail over repeated uses than titanium or the other materials the industry uses (I've primarily heard comparisons to titanium).
Delamination is just carbon fiber composites main mode of failure. As long as the material you use is rated for its stress life it's fine. Buying expired material means the resin was decomposing, making the carbon fiber composite more prone to failure.
All materials eventually fail, some have endurance limits with infinite stress lives, like most steels. Some don't, like aluminum. Without knowing the details of their design there's not an easy way to say whether carbon fiber was even a bad choice at all. But buying expired carbon fiber absolutely is a bad idea for anything critical.
The stress life curves for carbon fibers vary a lot, they certainly won't be as nice or long lasting as most metal alternatives, but that just means more replacement or maintenance, something a luxury submarine doesn't necessarily care about as much.