Tourist submersible visiting the Titanic is missing Part 2 69

[dik]

I'm not familiar with it... the only guy I know that is, and this was the subject of his PhD thesis. He has not responded to my query (maybe for a good reason). I gather that CF is really susceptible to internal flaws.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SwinnyGG]

I wonder how an unreinforced acrylic cylinder would perform

UTS of cast acrylic is only around 10,000 psi... I'm sure you could make a hull big enough to hold a person out of purely acrylic, but the wall thickness would have to be pretty crazy

 

[Brian Malone]

If such a water-clear acrylic hull could be fabricated, the view would be wild!

 

[MechinNC]

I read an interesting piece today in Vanity Fair. I'd post a link but I just burned up my free article. Now behind a paywall for me.

A few interesting take-aways:

1. There were basically no substantial pieces of CF found in the wreckage. They picked up some shards, that is it.
2. Some of the Ti pieces were "bent".

3. Very little to no CF left adhering to the Ti pieces.

Going off memory from this morning, so may not have the quotes exactly right.

 

[SWComposites]

https://www.vanityfair.com/news/2023/08/titan-submersible-implosion-warnings

 

[IRstuff]

TLDR, but your bullet points mostly match the article, but didn't see anything where they recovered any shards of CF, just the following

https://www.vanityfair.com/news/2023/08/titan-submersible-implosion-warnings[/URL]]“There was no piece I saw anywhere that had its original five-inch thickness,” he said. “Just shards and bits…. It was truly catastrophic. It was shredded.”

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[TugboatEng]

Swinny, I read that the compressive strength of carbon fiber is quite low, maybe 10% of a UTS of 175ksi. Meanwhile, acrylic can have a compressive strength upwards of 18ksi. That's why I had the thought that they may have been better off building the tube out of acrylic, possibly even with the same dimensions. The fiber reinforcement doesn't have much contribution to compressive strength, think of each fiber as a very slender column. The fiber does reduce the effects of creep which may allow the structure to operate at loads closer to it's ultimate strength.

Apparently, acrylic turns opaque before it fails so if their submarine suddenly turns white it's time to get back to the surface. Maybe this is more reliable than listening for popping sounds.

 

[SWComposites]

compressive strength of carbon fiber is quite low, maybe 10% of a UTS of 175ksi. Absolutely wrong. We have used carbon fiber composites in aircraft structures for 40+ years, including 787 and A350 fuselages and wings, both which have significant compression loads. One just has to known what they are doing, and have appropriate test data.

 

[btrueblood]

That article is terrifying.

A quote from Lochridge on inspecting the submersible prior to its first dive:

"The carbon fiber filament was visibly coming apart, riddled with air gaps, delaminations, and Swiss cheese holes—and there was no way to fix that short of tossing the hull in a dumpster."

and further:

“Non-destructive inspection is required to be undertaken and subsequent results provided to myself prior to any in water Manned Dives commencing,” he added, digging in his heels on the scanning. This would reveal any weak spots and provide a baseline that could then be used to check for signs of fatigue after every dive.

Scanning the hull shouldn’t be a problem, should it? Lochridge noted in another document that OceanGate had previously stated the hull would be scanned. (Spoiler alert: The hull was never scanned. “The OceanGate engineering team does not plan to obtain a hull scan and does not believe the same to be readily available or particularly effective in any event,” the company’s lawyer, Thomas Gilman, wrote in March 2018. Instead, OceanGate would rely on “acoustic monitoring”—sensors on the Titan’s hull that would emit an alarm when the carbon fiber filaments were audibly breaking.)

Oceangate subsequently fired Lochridge and then sued him, mostly to shut him up.

 

[Red Corona]

He might have truly bought into the notion that everyone else in the world was wrong and he was the only one that was right, and that his craft was not risky

I think that's very likely given that, as you say, he was on board himself

 

[Brian Malone]

March 2018 news story from a Everett, WA area TV station:

https://komonews.com/news/local/eve...s-submarine-to-be-used-for-titanic-expedition

Tony Nissen, director of engineering, with the Titan.

 

[SwinnyGG]

I read that the compressive strength of carbon fiber is quite low, maybe 10% of a UTS of 175ksi

It's more like 50% or so. We'd be able to put a pretty good number on that value if we knew the combination of materials used.

 

[TugboatEng]

Yes, I'm making a big assumption with the acrylic adhesives and resins. They are certainly the most tolerant to application and error which is why I assume they are what was used. I called two specific application errors on day 1.

 

[MechinNC]

I am no stress analysis type engr, but on that 5" thick CF cylinder in compression, are the stresses uniform throughout the laminate? I think not, but too lazy to research it.

I know our Navy subs use ribs internal to the pressure hull.

Wonder if this thing could have been built up with prefab CF ribs and then hull wound around them. The inside surface of the rib would be in tension where CF shines and hull could be thinner. But still hull would have to take the compressive forces from the end caps...

Just thinking aloud...

 

[IRstuff]

no ribs whatsoever; that's evident from the YouTube videos on the winding process.

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[MechinNC]

Yea, I know it did not have ribs. Just wondering if ribs would have made a better design.

 

[3DDave]

Solid carbon fiber would have worked, except for the passengers.

Ribs in submarines are there to prevent oil-canning, where local deformation leads to global buckling.

This appears to be compressive buckling - if ribs could be in tension they would add to the compressive load in the hull, causing it to to buckle sooner.

What was needed was a reduction in compressive stress by making the wall much thicker. Local reinforcement produces stress concentrations, unless the designer is very careful, which this guy was not.

 

[blakmax]

3DDave,

what would have helped even more was a better composite failure criteria than the commonly accepted Tsai Hill or Tsai Wu criteria that are widely programmed into many FEA packages. FROM MEMORY: (and it is a 75 year old memory!!) there was a widespread assessment of composite failure criteria conducted by the International Conference on Composite Materials (ICCM) in the3 early 2000's whereby candidates were invited to submit strength assessments for known but undisclosed test results. Candidates were asked to provide predictions based on known material data, lay-up details and given loading conditions. I may be wrong, but my memory was that the above failure theories were unconservative for compression-compression loading by a very significant factor (from memory it was about eight).

Quote (Lochridge, VF article by S. Casey)
"The carbon fiber filament was visibly coming apart, riddled with air gaps, delaminations, and Swiss cheese holes—and there was no way to fix that short of tossing the hull in a dumpster."

If this quote is valid, then the issue of failure criteria would then impinge on damage tolerance considerations. Fr goodness sake I really hope that they didn't use "injection repairs" to correct the defects. I have alw3ays said that you only achieve two things from injecting fresh resin into a defect: 1. you hide the defect so that NDT or visual inspection can not find it, and 2. you get a warm fuzzy feeling that you have repaired toe problem. The difference in strength is dead set zero. During the cure cycle the surface of any void is fully reacted out, so the surface of the void is not capable of reacting with the fresh resin, so no bond is formed. I am astounded that almost every composite manufacture still uses this technique. It HIDES the defect, but it does not REPAIR it!

Blakmax

 

[SWComposites]

Anyone wanting to go down the wormhole of composite failure theory madness should start with these two papers:

Recommendations for designers and researchers resulting from the world-wide failure exercise, P.D. Soden, A.S. Kaddour, M.J. Hinton, Composites Science and Technology 64 (2004) 589–604

A comparison of the predictive capabilities of current failure theories for composite laminates, judged against experimental evidence, M.J. Hinton, A.S. Kaddour, P.D. Soden, Composites Science and Technology 62 (2002) 1725–1797

 

[SWComposites]

And Max I completely agree with your comments about "injection repairs", but I doubt they tried that on a large 5" thick part.