3 ms·

From everything I've read, the entire carbon section of the hull could be considered a weak point.

KineticLensman 3y ago
> the entire carbon section of the hull could be considered a weak point It is possible that the carbon fibre delaminated catastrophically, possibly as a result of stresses accumulated over multiple descents.
- sumo89 3y ago
  The only way the carbon tube hasn't turned to sea dust is if the porthole failed first. When carbon fibre fails it catastrophically fails. I'd put money on the joint between the ti and carbon being the weak point and once there's a chip in the carbon under that much pressure it'll disintigrate.
  - KineticLensman 3y ago
    fair points. Hopefully the investigation will nail the actual cause down.
ortusdux 3y ago
This is my understanding as well. I'm not surprised to see that the titanium piece survived unscathed. IIRC, the companies goal was to provide more cost-effective trips by transitioning from the standard solid titanium hull to a Ti/carbon fiber composite. This decision was controversial. They fired their head of QC after he strongly opposed the change. One key argument he made was that the design prevented ultrasonic inspection, which would mean that they could not monitor the health of the composite over time.
ajross 3y ago
To be clear, way too much of what I've read on this subject has been excessively confident tut-tutting with little to no real engineering expertise. People want to imagine that the simple use of carbon fiber here was the sole mistake, I guess because it's easy to understand and makes us feel smart. But come on, engineering is harder than that. Carbon fiber pressure vessels have a long history. Composite material failure modes are for sure more complicated than metals but they certainly aren't a mystery. We've been using composites for more than half a century, we know how they work. No, my gut says the root cause here (if we ever find one[1]) is going to turn out to be a more routine kind of engineering fuckup. It'll be just some weak point that the analysis failed to turn up. [1] And sure, if we don't then the "carbon fiber death cylinder" theory will just become part of the folk truth, I guess.
- mechhacker 3y ago
  There are non-trivial differences in designing vessels loaded with compression vs. tension, and the most experience we have is with internal pressure (e.g. tension). The analysis is very nonlinear and unfortunately not as widely understood as internally pressurized vessels. It requires a lot more analysis, troubleshooting, and expertise that is short in supply. Add the additional catastrophic failure modes for composites and there's a reason that industry experts warned against this.
  - ajross 3y ago
    With respect: that's exactly the kind of glib, drive-by analysis I'm talking about. What you're saying simply isn't true. People have been testing composites in pressure chambers and every load imaginable for decades. Yes, clearly they don't act like steel. But saying that it's impossible is just too easy. Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. (Edit because this was misunderstood: I'm not saying what the failure is. I'm saying that we DON'T KNOW what the failure is, and that given the choice between makes-me-feel-smart pontification about materials and a routine design flaw in a prototype device, I'm going to bet on the routine failure every single time.)
    - Johnny555 3y ago
      Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. I like how you complain about people making glib drive-by analyses (that are at least based on actual submersible experts saying they shouldn't have used carbon fiber) by making a glib drive-by analysis that's based on pure conjecture.
      - ajross 3y ago
        It was a turn of phrase. I made the same point in a neutral, non-prescriptive manner (c.f "My gut says") upthread. Taking that sentence out of context is pretty poor form, whether you agree with me or not.
TechBro8615 3y ago
Here's a video of hydraulic pressing carbon fiber to its breaking point: https://youtube.com/watch?v=xWTXeGiM8K8 https://youtube.com/watch?v=xWTXeGiM8K8
- mechhacker 3y ago
  Worth noting that because he's using a hydraulic press, the load isn't constant. It deforms, fibers break, and thus the load decreases as the deflection stays relatively constant in that time frame. With external pressure the load doesn't decrease. Fibers break -> deflection increases -> more fibers break. There isn't load relief as long as the vessel is still holding pressure.
  - sojuz151 3y ago
    It is even worse. The load increases with deflation
- N19PEDL2 3y ago
  Imagine how frightening it would have been to hear those crackles inside the submarine. (In reality, however, they had no warning of the impending collapse.)

> the entire carbon section of the hull could be considered a weak point It is possible that the carbon fibre delaminated catastrophically, possibly as a result of stresses accumulated over multiple descents.

sumo89 3y ago
The only way the carbon tube hasn't turned to sea dust is if the porthole failed first. When carbon fibre fails it catastrophically fails. I'd put money on the joint between the ti and carbon being the weak point and once there's a chip in the carbon under that much pressure it'll disintigrate.
- KineticLensman 3y ago
  fair points. Hopefully the investigation will nail the actual cause down.

ortusdux 3y ago

This is my understanding as well. I'm not surprised to see that the titanium piece survived unscathed. IIRC, the companies goal was to provide more cost-effective trips by transitioning from the standard solid titanium hull to a Ti/carbon fiber composite. This decision was controversial. They fired their head of QC after he strongly opposed the change. One key argument he made was that the design prevented ultrasonic inspection, which would mean that they could not monitor the health of the composite over time.

ajross 3y ago

To be clear, way too much of what I've read on this subject has been excessively confident tut-tutting with little to no real engineering expertise. People want to imagine that the simple use of carbon fiber here was the sole mistake, I guess because it's easy to understand and makes us feel smart. But come on, engineering is harder than that. Carbon fiber pressure vessels have a long history. Composite material failure modes are for sure more complicated than metals but they certainly aren't a mystery. We've been using composites for more than half a century, we know how they work. No, my gut says the root cause here (if we ever find one[1]) is going to turn out to be a more routine kind of engineering fuckup. It'll be just some weak point that the analysis failed to turn up. [1] And sure, if we don't then the "carbon fiber death cylinder" theory will just become part of the folk truth, I guess.

mechhacker 3y ago
There are non-trivial differences in designing vessels loaded with compression vs. tension, and the most experience we have is with internal pressure (e.g. tension). The analysis is very nonlinear and unfortunately not as widely understood as internally pressurized vessels. It requires a lot more analysis, troubleshooting, and expertise that is short in supply. Add the additional catastrophic failure modes for composites and there's a reason that industry experts warned against this.
- ajross 3y ago
  With respect: that's exactly the kind of glib, drive-by analysis I'm talking about. What you're saying simply isn't true. People have been testing composites in pressure chambers and every load imaginable for decades. Yes, clearly they don't act like steel. But saying that it's impossible is just too easy. Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. (Edit because this was misunderstood: I'm not saying what the failure is. I'm saying that we DON'T KNOW what the failure is, and that given the choice between makes-me-feel-smart pontification about materials and a routine design flaw in a prototype device, I'm going to bet on the routine failure every single time.)
  - Johnny555 3y ago
    Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. I like how you complain about people making glib drive-by analyses (that are at least based on actual submersible experts saying they shouldn't have used carbon fiber) by making a glib drive-by analysis that's based on pure conjecture.
    - ajross 3y ago
      It was a turn of phrase. I made the same point in a neutral, non-prescriptive manner (c.f "My gut says") upthread. Taking that sentence out of context is pretty poor form, whether you agree with me or not.

TechBro8615 3y ago

Here's a video of hydraulic pressing carbon fiber to its breaking point: https://youtube.com/watch?v=xWTXeGiM8K8 https://youtube.com/watch?v=xWTXeGiM8K8

mechhacker 3y ago
Worth noting that because he's using a hydraulic press, the load isn't constant. It deforms, fibers break, and thus the load decreases as the deflection stays relatively constant in that time frame. With external pressure the load doesn't decrease. Fibers break -> deflection increases -> more fibers break. There isn't load relief as long as the vessel is still holding pressure.
- sojuz151 3y ago
  It is even worse. The load increases with deflation
N19PEDL2 3y ago
Imagine how frightening it would have been to hear those crackles inside the submarine. (In reality, however, they had no warning of the impending collapse.)