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I know nothing, literally nothing, about material engineering, past what I can skim off wikipedia and ingest on cycling websites. But I always thought composit

I know nothing, literally nothing, about material engineering, past what I can skim off wikipedia and ingest on cycling websites. But I always thought composite nonmetal materials are hard to test nondestructively, which is the main limiting factor for their use in aerospace. Metals can be X-rayed to assess their condition, carbon not. Is that wrong? What am I missing? Just curious.

ceejayoz 3y ago
I don't think you're missing anything. Others, including those in-the-know, were left scratching their heads too. https://www.nytimes.com/2023/06/20/us/oceangate-titanic-missing-submersible.html https://www.nytimes.com/2023/06/20/us/oceangate-titanic-miss... > Years before OceanGate’s submersible craft went missing in the Atlantic Ocean with five people onboard, the company faced several warnings as it prepared for its hallmark mission of taking wealthy passengers to tour the Titanic’s wreckage. > OceanGate’s director of marine operations, David Lochridge, started working on a report around that time, according to court documents, ultimately producing a scathing document in which he said the craft needed more testing and stressed “the potential dangers to passengers of the Titan as the submersible reached extreme depths.” > Two months later, OceanGate faced similarly dire calls from more than three dozen people — industry leaders, deep-sea explorers and oceanographers — who warned in a letter to its chief executive, Stockton Rush, that the company’s “experimental” approach and its decision to forgo a traditional assessment could lead to potentially “catastrophic” problems with the Titanic mission.
- ramesh31 3y ago
  My question is just... why? It's not like carbon fiber was any cheaper than just making the thing out of steel or titanium like literally every other submersible. It just doesn't make any sense. You have none of the weight penalties that exist in aviation, so the heavier and the thicker the better.
  - SV_BubbleTime 3y ago
    As I understood it, you do have a weight penalty for a sub. You need to get back up by powered movement, or down by it. You are limiting your dive time either way. They can’t use foam for flotation at that depth. There is a concrete like “synthetic foam” if you want unpowered rise to surface but it’s expensive. I read early on they wanted carbon to be light enough to get back up by releasing weights on powerloss. From what I have seen of the hull, it was a tube. I can’t remeber seeing a view with a complex or even drafted shape to it. I suspect that spinning carbon by fiber would take far too long, and think they probably used narrow weave to do it. It would have been cheaper than the required thickness of titanium, which is the same strength as steel, but 40% the weight and still a little heavier than aluminum. Who knows!? Nothing they did showed that they ever chose the more expensive option. Carbon was likely just the cheapest thing they could use for their goals.
    - hef19898 3y ago
      Especially if you buy your material because it is past shelf lofe, don't use an autoclav and forgoe curing. Fun experiment: Take a roll from some toilet paper, squeeze it somewhat evenly at the ends. It is kond of stable. Then peel the layers apart, ans squeeze again. The same would happen if the composite tube delaminated at one of the end cap rings. This whole thing is beyond infuriating.
      - SV_BubbleTime 3y ago
        > don't use an autoclav and forgoe curing. I haven’t read that. But, that’s not an option. Carbon won’t just cure on its own, ever. It will be a shitty sticky mess forever. It had to be cured somehow. As to the carbon expiration. I don’t know what that was. I suspect it could be epoxy like in prepreg, but I doubt prepreg here. And it could be shelf life of the carbon itself as it can be hydrophilic in some cases.
        buildsjets 3y ago
        Allegedly, they bought expired (out of freezer time) prepreg from Boeing Surplus. Boeing has released a statement that they have no records of this, so it likely passed through a third party before he acquired it.
- rich_sasha 3y ago
  I was more referring to TFA which seemed to assert that you can test carbon fibre, with some kind of EM radiation. That it was all a mess at OceanGate, that bit I got :)
2-718-281-828 3y ago
well, a carbon frame is trash after a single crash even if it seems alright due to microscopic fissures. that's also the point cameron's making. the compound carbon material will progressively deteriorate with every dive due to the immense pressure difference from surface to target depth. extremely small deformations cause tearing and delamination. so small you can't see it. but one dive too many and it just cracks.
icegreentea2 3y ago
You can ultrasound and radiograph CF to do NDT. I think the challenge is just more that we have over half a century headstart working with Al in aircraft to understand their failure modes and how to adequately test for them. Our understanding of CF (+CF's inherent nature of having more sudden failures) makes designing adequate NDT regimes more challenging. That said, we are seeing much more CF (and CF composites) being deployed into commercial aerospace (famously the 787 wings, but also the A350,
- hef19898 3y ago
  The whole fusselage of the 787 and A350 is composite, not just the wings. Composite wings are old news. Strutural repairs are tricky so.
buildsjets 3y ago
The typical method to inspect composite parts is through-transmission ultrasonic. Essentially, two streams of water are aimed at the part from opposite sides, one stream of water has a ultrasonic generator embedded in it, the other side has a transducer, and you scan/raster the stream of water across the part to inspect it. There are limits on how thick of a part you can inspect with this method, and this hull significantly exceeds how thick we would use these materials in aerospace designs.
- buildsjets 3y ago
  Here's an example. Appears to be inspecting a CFRP jet engine fan case, which are some of the thicker composite layups you see in aerospace. The way this method of inspection works, you also have to fabricate a part with known defects of various sizes (simulated with a little piece of Kapton tape in the layup) in order to determine the minimum size defect that the machine can detect for a given type of part. https://tecscan.ca/ultrasonic-testing-defect-sizing-in-composite-structures/ https://tecscan.ca/ultrasonic-testing-defect-sizing-in-compo...

ceejayoz 3y ago

I don't think you're missing anything. Others, including those in-the-know, were left scratching their heads too. https://www.nytimes.com/2023/06/20/us/oceangate-titanic-missing-submersible.html https://www.nytimes.com/2023/06/20/us/oceangate-titanic-miss... > Years before OceanGate’s submersible craft went missing in the Atlantic Ocean with five people onboard, the company faced several warnings as it prepared for its hallmark mission of taking wealthy passengers to tour the Titanic’s wreckage. > OceanGate’s director of marine operations, David Lochridge, started working on a report around that time, according to court documents, ultimately producing a scathing document in which he said the craft needed more testing and stressed “the potential dangers to passengers of the Titan as the submersible reached extreme depths.” > Two months later, OceanGate faced similarly dire calls from more than three dozen people — industry leaders, deep-sea explorers and oceanographers — who warned in a letter to its chief executive, Stockton Rush, that the company’s “experimental” approach and its decision to forgo a traditional assessment could lead to potentially “catastrophic” problems with the Titanic mission.

ramesh31 3y ago
My question is just... why? It's not like carbon fiber was any cheaper than just making the thing out of steel or titanium like literally every other submersible. It just doesn't make any sense. You have none of the weight penalties that exist in aviation, so the heavier and the thicker the better.
- SV_BubbleTime 3y ago
  As I understood it, you do have a weight penalty for a sub. You need to get back up by powered movement, or down by it. You are limiting your dive time either way. They can’t use foam for flotation at that depth. There is a concrete like “synthetic foam” if you want unpowered rise to surface but it’s expensive. I read early on they wanted carbon to be light enough to get back up by releasing weights on powerloss. From what I have seen of the hull, it was a tube. I can’t remeber seeing a view with a complex or even drafted shape to it. I suspect that spinning carbon by fiber would take far too long, and think they probably used narrow weave to do it. It would have been cheaper than the required thickness of titanium, which is the same strength as steel, but 40% the weight and still a little heavier than aluminum. Who knows!? Nothing they did showed that they ever chose the more expensive option. Carbon was likely just the cheapest thing they could use for their goals.
  - hef19898 3y ago
    Especially if you buy your material because it is past shelf lofe, don't use an autoclav and forgoe curing. Fun experiment: Take a roll from some toilet paper, squeeze it somewhat evenly at the ends. It is kond of stable. Then peel the layers apart, ans squeeze again. The same would happen if the composite tube delaminated at one of the end cap rings. This whole thing is beyond infuriating.
    - SV_BubbleTime 3y ago
      > don't use an autoclav and forgoe curing. I haven’t read that. But, that’s not an option. Carbon won’t just cure on its own, ever. It will be a shitty sticky mess forever. It had to be cured somehow. As to the carbon expiration. I don’t know what that was. I suspect it could be epoxy like in prepreg, but I doubt prepreg here. And it could be shelf life of the carbon itself as it can be hydrophilic in some cases.
      - buildsjets 3y ago
        Allegedly, they bought expired (out of freezer time) prepreg from Boeing Surplus. Boeing has released a statement that they have no records of this, so it likely passed through a third party before he acquired it.
rich_sasha 3y ago
I was more referring to TFA which seemed to assert that you can test carbon fibre, with some kind of EM radiation. That it was all a mess at OceanGate, that bit I got :)

2-718-281-828 3y ago

well, a carbon frame is trash after a single crash even if it seems alright due to microscopic fissures. that's also the point cameron's making. the compound carbon material will progressively deteriorate with every dive due to the immense pressure difference from surface to target depth. extremely small deformations cause tearing and delamination. so small you can't see it. but one dive too many and it just cracks.

icegreentea2 3y ago

You can ultrasound and radiograph CF to do NDT. I think the challenge is just more that we have over half a century headstart working with Al in aircraft to understand their failure modes and how to adequately test for them. Our understanding of CF (+CF's inherent nature of having more sudden failures) makes designing adequate NDT regimes more challenging. That said, we are seeing much more CF (and CF composites) being deployed into commercial aerospace (famously the 787 wings, but also the A350,

hef19898 3y ago
The whole fusselage of the 787 and A350 is composite, not just the wings. Composite wings are old news. Strutural repairs are tricky so.

buildsjets 3y ago

The typical method to inspect composite parts is through-transmission ultrasonic. Essentially, two streams of water are aimed at the part from opposite sides, one stream of water has a ultrasonic generator embedded in it, the other side has a transducer, and you scan/raster the stream of water across the part to inspect it. There are limits on how thick of a part you can inspect with this method, and this hull significantly exceeds how thick we would use these materials in aerospace designs.

buildsjets 3y ago
Here's an example. Appears to be inspecting a CFRP jet engine fan case, which are some of the thicker composite layups you see in aerospace. The way this method of inspection works, you also have to fabricate a part with known defects of various sizes (simulated with a little piece of Kapton tape in the layup) in order to determine the minimum size defect that the machine can detect for a given type of part. https://tecscan.ca/ultrasonic-testing-defect-sizing-in-composite-structures/ https://tecscan.ca/ultrasonic-testing-defect-sizing-in-compo...