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Last paragraph of TFA: > Some expeditions were delayed after OceanGate was forced to rebuild > the Titan’s hull because it showed “cyclic fatigue” and wou

Last paragraph of TFA: > Some expeditions were delayed after OceanGate was forced to rebuild > the Titan’s hull because it showed “cyclic fatigue” and wouldn’t be > able to travel deep enough to reach the Titanic’s wreckage, > according to a 2020 article by GeekWire, which interviewed the > company’s CEO. If you simply rebuild it, but don't redesign and/or change its material composition, won't the rebuilt hull possess similar cyclic fatigue limits to what it replaced?

3327 3y ago
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ender341341 3y ago
yes. Sometimes that can be OK, if the cyclic fatigue was from high levels of testing. If on the other hand it happens quickly and you haven't even tested it at anywhere near the depth you expect to take it and don't have tons of testing yeah, it's really bad idea. (which seems to be the case here)
- tappaseater 3y ago
  It reminds of the story of the de Havilland Comet, which had a bad habit of breaking up in mid-air. Engineers submerged the fuselage of the plane in and out of deep water to simulate pressure changes and it finally failed. leading to the diagnosis of metal fatigue. I mean that was 70 years ago. You'd think it might have crossed his mind.
  - pengaru 3y ago
    I think aluminum is much better understood as having effectively no endurance fatigue limit [0] in that its threshold keeps dropping as the cycles progress. It's expected for cracks to form in aluminum parts of such planes, IIRC they're X-Ray'd on a schedule to discover cracks and replace the relevant parts. I don't know if CF is expected to behave similarly. You certainly don't see it as an obvious quality of CF bicycle frames, as is visible with aluminum frames. The aluminum bikes have such thick beefy tubes and harsh/rigid ride quality to eliminate crack-forming cyclic stresses. CF bikes are considered more comfortable since they don't have to prevent all the flex, as well as being lighter. [0] https://en.wikipedia.org/wiki/File:S-N_curves.PNG https://en.wikipedia.org/wiki/File:S-N_curves.PNG
elromulous 3y ago
I would agree. Although one would hope they would learn from it and that "rebuild" includes improvements being applied. Of course rebuild is pretty ambiguous, so it could mean repair, which reminds of a famously incorrectly repaired aircraft[0]. [0]https://en.m.wikipedia.org/wiki/Japan_Air_Lines_Flight_123 https://en.m.wikipedia.org/wiki/Japan_Air_Lines_Flight_123
- naikrovek 3y ago
  I love reading about aircraft crashes when they're written like they are in that Wikipedia article. Very interesting reading, to me, for some reason.
  - seized 3y ago
    The podcast Black Box Down is also excellent for that, has a great overview of what went wrong and what was done to fix it or prevent it. And also the gold standard of podcasts Well Theres Your Problem has done a few plane crashes.
julosflb 3y ago
I m wondering what sort of cycles they are referring to. I would expect one very big full pressure cycle when slowly going down to seabed and returning up to surface. Unless they do hundreds of descent with the same hull...
nerdchum 3y ago
literally all other submarines are made out of metal. this sub was made out of a carbon fiber composite. This stuff is known to develop micro fractures.

3327 3y ago

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ender341341 3y ago

yes. Sometimes that can be OK, if the cyclic fatigue was from high levels of testing. If on the other hand it happens quickly and you haven't even tested it at anywhere near the depth you expect to take it and don't have tons of testing yeah, it's really bad idea. (which seems to be the case here)

tappaseater 3y ago
It reminds of the story of the de Havilland Comet, which had a bad habit of breaking up in mid-air. Engineers submerged the fuselage of the plane in and out of deep water to simulate pressure changes and it finally failed. leading to the diagnosis of metal fatigue. I mean that was 70 years ago. You'd think it might have crossed his mind.
- pengaru 3y ago
  I think aluminum is much better understood as having effectively no endurance fatigue limit [0] in that its threshold keeps dropping as the cycles progress. It's expected for cracks to form in aluminum parts of such planes, IIRC they're X-Ray'd on a schedule to discover cracks and replace the relevant parts. I don't know if CF is expected to behave similarly. You certainly don't see it as an obvious quality of CF bicycle frames, as is visible with aluminum frames. The aluminum bikes have such thick beefy tubes and harsh/rigid ride quality to eliminate crack-forming cyclic stresses. CF bikes are considered more comfortable since they don't have to prevent all the flex, as well as being lighter. [0] https://en.wikipedia.org/wiki/File:S-N_curves.PNG https://en.wikipedia.org/wiki/File:S-N_curves.PNG

elromulous 3y ago

I would agree. Although one would hope they would learn from it and that "rebuild" includes improvements being applied. Of course rebuild is pretty ambiguous, so it could mean repair, which reminds of a famously incorrectly repaired aircraft[0]. [0]https://en.m.wikipedia.org/wiki/Japan_Air_Lines_Flight_123 https://en.m.wikipedia.org/wiki/Japan_Air_Lines_Flight_123

naikrovek 3y ago
I love reading about aircraft crashes when they're written like they are in that Wikipedia article. Very interesting reading, to me, for some reason.
- seized 3y ago
  The podcast Black Box Down is also excellent for that, has a great overview of what went wrong and what was done to fix it or prevent it. And also the gold standard of podcasts Well Theres Your Problem has done a few plane crashes.

julosflb 3y ago

I m wondering what sort of cycles they are referring to. I would expect one very big full pressure cycle when slowly going down to seabed and returning up to surface. Unless they do hundreds of descent with the same hull...

nerdchum 3y ago

literally all other submarines are made out of metal. this sub was made out of a carbon fiber composite. This stuff is known to develop micro fractures.