The recent sub implosion brings up the question of why they would built a cylinder with carbon fiber. Carbon fiber is lightweight and immensely strong when used in tension but an externally pressurized cylinder would place the fibers in compression. I can’t think of any other applications were it is used that way.
Cost seems to be the main reason. The fiber was purchased when it was over its certified shelf life so was probably inexpensive compared to new manufacture. And winding miles and miles of it on a thin shell has to be far cheaper then fabricating a cylinder from solid steel or titanium. It just seems like the wrong material for the job. Is there something I am missing?
The fibers are in tension. As you press on the carbon fiber tube it tries to deform which places fibers in tension. Compression of carbon fibers is what would occur where the endcaps join the tube if they were just stuck on to the ends of the cylinder but the design will redirect that force to the sides of the carbon fiber tube.
I think it was still the wrong material for the job. I doubt steel would cost more.
Now, these are the Hydraulic Press Channel experiments with carbon fiver. The one showing the CF cylinder standing up shows the likely way the titan may have ended looking.
The fibers are in tension until they start to flex and at exactly that moment they also become under compression on the opposite side of the deformation.
I can’t find it now to cite but IIRC the end caps were metal and the in-between parts were carbon fiber. It was speculated it was where these two materials met that was likely a weak point.
Again…I can’t find the cite so big grain of salt and all that.
I did find this:
“The weakest link, if I had to put money down on what the findings will be, the achilles heel of the sub, was the composite cylinder, the main hull that people were inside.
“There were two titanium end caps on each end. They’re relatively intact on the seafloor, but that carbon fiber composite cylinder is now just in very small pieces. You don’t use composites for vessels that are seeing external pressure.” - SOURCE
It appears to have been known for quite some time that vessels of this construction lose structural integrity every time they are used. Making a dozen trips makes the thing less safe than if it was the first trip.
The other thing to consider with composite materials is that they can be strong as a whole, when loaded as they are intended to carry load, but rather weak when subjected to localised forces; the pressure of all that water was not only loading the structure of the composite carbon fibre tube as a whole (which it might resist), it was also at a microscopic level, trying to push the resin through the fibres.
Most things do. But we know that metals can survive many more cycles than composite materials, and metals are consistent and predictable in that number of cycles while composites are less so and generally incorporate greater margins of safety. In this case there doesn’t appear to be a serious attempt to determine the life of a submersible of this design.
Is it possible to make a perfect cylinder out of carbon fiber? I thought they were made by wrapping the carbon fiber around a mandrel, which was very slightly tapered. That way, they can pull the mandrel out after the carbon fiber sets up.
None of those duplicate the conditions of the submersible. There would have been some small breach that allowed water to fill the compartment at supersonic speeds. The nitrogen in the air would liquify as it was compressed, pretty much what was happening to the passenger’s bodies. The composite structure could have been crushed before it filled with water, or exploded from the inrush of water at such great speeds, or torn to shreds in some violent combination effect, and all in a tiny fraction of a second.
Interesting. Thanks. Sounds more expensive, though.
I wonder if that’s what was done in this case. Would a tiny, tiny bit of taper in the cylinder make a difference in how much pressure it could withstand, and would the designers have taken that into account?
I watched the video further up the thread of cylinders being crushed in the hydraulic press. I wonder if those had any taper to them, and were they fracturing from the large or small end.
Remember they also needed a door, needed to attach the motors and other stuff to it, and wanted a window–and it’s possible weaknesses in connections of the carbon fiber with one of those caused the implosion.
I was wondering if a lot of taper would be wanted. Not simply taper, but some kind of prolate spheroid.
Also, I can only recall dissolving aluminum used with very small lightweight and or odd shaped construction. I think it should be possible for something that large to use a form that could be disassembled.
Yes, I was pointing out the irony of the situation. The owner had been getting warnings of trouble for a long time but was able to fend them off because of the successful trips. What the doomed passengers should have known and what they should have been told will be hashed over probably forever in the absence of living witnesses.
Alternatively, it’s possible to create a mandrel that is made of several pieces like a collet - after forming, a central piece of the mandrel is pulled out, releasing the outer segments of the mandrel to be removed one by one.
There are also low temperature melting allots that can be melted out after forming, but I don’t suppose those would be used for such a large item as this
