[eutectic]

So is mine, and they made several successful trips which (among other things) makes me think that carbon fiber could be an appropriate material for a submarine. Metallic hulls would also implode catastrophically if not well designed.

[protastus]

The reasonable and ethical engineering approach is to do one's best with models, simulations, first principles analysis and design something that works on paper. And then built prototypes, test the hell out of them. Without people inside. Standardize everything they can about manufacturing. If all looks good, they can operate. Never stop measuring and testing.

The operators decided this was too much work. They gambled and they lost.

[Neekerer]

They did try a model but it failed at the euqivalent of 2000m I believe

[HelloNurse]

Move fast and break people. How many entrepreneurs learned from the Titan disaster that they should really hire test pilots?

[SoftTalker]

The problem with carbon fiber is that it's fiber. Think of a rope. It's very strong in tension; you can hang from it, climb it, hoist things with it, etc. Carbon fiber is useful in an aircraft fuselage because the inside pressure is a tension load on the fuselage.

Rope is useless under compression. You can't push a rope, or climb a rope that's only attached to the floor. So under compressive loads, you're relying on the glue mainly. The carbon fiber gives the glue something to stick to. A submarine is the opposite of an aircraft: the high pressure is on the outside, so the hull is under compression not tension.

It's a terrible choice for a submarine.

[Gravityloss]

Some good points from first principles. But there are ways. If you manage to make the composite with a very good process so the fibers are straight and closely packed, then it can have good compression strength as well.

This can be achieved for example in pultruded carbon rods, where the carbon fiber is under tension when it passes through an epoxy bath. They have been used in aircraft wing top spars which receive compression loads (when taking positive gees).

How to make a cylindrical vessel that can take compressive loads? The creation process certainly needs some thinking and attention.

[Gravityloss]

Thinking about this more from first principles, if I got it right, the cylinder hoop stress is twice the longitudinal stress. So you could pultrude a unidirectional layer of hoops, or very shallow angle spiral to handle half of the the hoop stress only on the outside. Have normal cross wrapping layer on the inside probably.

[83]

This is misleading. Only raw CF is like a rope, once made into a composite with resin it becomes a stiff member with entirely different properties. It's more analogous to a wood beam - very stiff and resistant to bending but once you get past it's limit it cracks.

I suspect the failure mode in a submersible like this isn't so much the carbon fibers themselves but either the carbon > metal interface at the ends or gradual delamination between layers of fibers due to the cyclical pressure loads.

[hef19898]

Under outside pressure, the fibers in the matrix take close to no load, while under tension they take a lot. That alone is the reason why carbon fibre composites are the least suited materials for submarines and submersibles.

[hadlock]

You should watch the initial interviews with James Cameron. He explains 1) composite hulls have a finite number of compression cycles before they fail, typically less than 10 (this was dive #7 give or take, I forget the exact numbers) and 2) no composite material is perfect, the water pressure will find imperfections and work it's way into the material, causing weak points

Steel is a significantly better material for this task. I'm stunned they even considered carbon fiber.

[thsksbd]

Composite failure is very hard to predict and hard to evaluate. By comparison metal fatigue is very well understood.

Compression loads on submarines are extreme and cyclical. This causes the fibers to delaminate loosing strength. Furthermore, De-lamination is difficult to measure non-destructively (I think only xrays work?)

Manned conposite subs should always have their chief designer or financier aboard

[avs733]

ultrasound works quite well as well - but no NRE technique is perfect

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7503374/

[eternityforest]

Didn't they specifically refuse to do any testing because they trusted that proprietary acoustic system?

[thsksbd]

The paper is from 2020. My understanding is that composite testing is still in its infancy.

Also note that the hull was 5 inches thick (safety factor > 2.5). Depending on the dispersion properties of the medium ultrasound might not penetrate and get a good result. In Ti (some alloys at least), for example, you cant get much signal past a couple of inches due to the scattering (from the grains?)

[namibj]

That was supposed to hear the hill cracking/creaking.

Ultrasound for delamination is like medical ultrasound and looks for a discontinuity in the sheet from the delamination.

[eternityforest]

I think someone wanted to do the delamination hull scan and they skipped it presumably because they trusted the creak listener

[fecal_henge]

Just to add, isnt the reason why metal fatigue is well understood because it has killed a lot of people?

[adrian_b]

No, it is well understood because the failures of metals are much more reproducible than of other materials.

Both for ceramics and for composites the failures are caused by microscopic defects that are never the same for different samples, so the conditions for failure are very variable from sample to sample. Therefore the results of experiments with such materials have lower predictive ability for the behavior in a real application.

[thsksbd]

To some extent. "Mistakes were made" as they say. But its a lot more nuanced than that.

Metal fatigue is understood because we have a mechanistic understanding of what it is, how fatigue accumulates, and how different metal crystal strictures respond to fatigue.

We know, for example, that metals like aluminum will always fail from fatigue given enough loading cycles, no matter how small the applied stress.

We also know that other metals, like iron and titanium, have a "fatigue limit" below which fatigue doesn't accumulate and these metals can endure infinite loading cycles.

We have, to some extent, the ability to repair metal fatigue.

We build airplanes from aluminum knowing their aluminum hulls and wings will fail (whereas if it were built with Ti fatigue failure could be eliminated) because metal fatigue is very predictable and we can withdraw a hull from service after a regulatory determined number of landings.

So yes, until we developed our current understanding of fatigue, people died. But, often, this was from a callous disregard to traditionally accepted safety factors by cowboy "innovators".

(Im a materials eng. PhD in polymers w/ background in Eng. Phys. Im not a metallurgist for what its worth)

[jacquesm]

Metal hulls do not degrade on every dive. If you start off with a safety factor of 2.5 (according to the design specs as they are known today) then after a number of dives you'll be below '1' and failure is a given. Without the ability to test the degradation it's a roulette.

[midoridensha]

I'm just an armchair observer here, but from what I can tell from the discussions, carbon fiber certainly can be an appropriate material for a submarine, as long as it's only used for one dive.

[fargle]

The reason it's a useless terrible material for a sub is that until it survives the first dive, you don't know if it has a hidden flaw.

And after the first dive, even if it survives, you know that 100% that it does.

Weight is not nearly the problem it is for a sub as it is for spacecraft or aircraft. The problem wasn't safety protocols or XBox controllers or any of that. It was conflation of "big idea people"[1] with real engineers.

[1] see Jobs, Musk, etc.

[midoridensha]

As I said, I'm an armchair observer here, not a materials scientist.

However, there are a lot of things that are absolutely designed and built to be single-use, especially in fields like space, where rocket engines have been single-use until very recently.

As for weight, I think the reason that was important was because they didn't have a big enough ship that could operate a crane and haul the submersible out of the water. Really, the whole operation was a bad idea: this kind of exploration is expensive as hell, and the only way to cut costs (with current tech) is to do really dangerous stuff.