Tourist submersible visiting the Titanic is missing Part 2 69

[Brian Malone]

Tony Nissen left Ocean Gate in 2019, some time after David Lochridge was fired in 2018.

 

[IRstuff]

The hull that Lochridge was concerned about was the original Titan hull, which was replaced in 2021.

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[Brian Malone]

Yes, so the provenance of the hull that failed is an unknown. Probably not made by Spenser Composites, Possibly, Electro Impact or Janicki.

 

[Tomfh]

I really hope that they didn't use "injection repairs"....The difference in strength is dead set zero.....I am astounded that almost every composite manufacture still uses this technique.

What's your basis for claiming that? I did a quick google search and it seems a commonly used method with experimentally proven results. One such example:

https://www.nxtbook.com/nxtbooks/sampe/journal_20190304/index.php#/p/22

 

[TugboatEng]

From the charts in that link it appears that all cases converge quite closely on the right side which indicates that there is no difference between repaired and unrepaired.

 

[Tomfh]

which indicates that there is no difference between repaired and unrepaired.

That's well past ultimate. At ultimate load there is a marked difference between repaired and unrepaired.

 

[blakmax]

Tomfh

My assertion that injection repairs are ineffective is based on over 50 years of aircraft maintenance of bonded structures, and I have seen thousands of injection repairs inmetal bonded structure that have resulted in bond failure in later service. Like you, my assertion was challenged by a senior technician, so I invited him to select a range of examples on scrap parts that we could destructively examine. Of 14 samples, every single injected "repair" was easily separated from the mating surfaces. I have also seen an example of a production injection repair to supposedly bond a rudder core to the trailing edge of the rudder mast. Remember that this component had zero hours when the repair was undertaken, so there is no chance of secondary contamination interfering with the bond, I was indirectly involved with the investigation after the rudder departed the aircraft in flight. The injection was in the region where the core was bonded to the mast, with that bond being the mechanism to transfer shear loads into the mast. However, because the bond was ineffective all of the shear loads were forced through the skin to mast fastened joint, and that caused a fatigue crack that eventually led to the fracture of the skin and failure of the rudder.

The reason that people blindly follow the mantra of undertaking injection repairs is that the process fills the air gap such that NDT can no longer find the defect. In reality, the process only HIDES the defect. Understand that adhesive bonding requires chemical reactions at the interface, and that requires that the surfaces are chemically active. In the case of production voids in either adhesive bonds or composite laminates, the resin systems have experienced a full cure cycle, so the residual reactivity will be negligible. The only exception may be in cases where the surfaces have been separated by actual fracture and then repaired before the surface oxidises of becomes contaminated.

Now unless the test coupon used to validate the repair process replicates the fully cured glossy surface typical of the interior of a void, the test is meaningless. Bonding to a surface with a peel ply removed will not replicate the true situation. I have yet to see any valid test results
that would convince me. I really don't care how widely used the process is. It is wrong. Remember, the world was told as absolute facts that the world was flat and all the planets revolved around the earth.

Regards

Max

 

[Tomfh]

You seem to be saying that resin can’t stick inside a crack or a void. If that was true composites and adhesives wouldn’t work in the first place.

 

[SwinnyGG]

He's saying that once the resin is cured - including the inside surface of a void which existed during curing - it is not very chemically active and is very difficult to bond to without significant prep, which you can't do to the inside of a void.

Much depends on loading, void sizes/locations/density, etc - but I think generally saying that post cure void filing treatments always result in full strength is a generalization that isn't well supported.

 

[IRstuff]

Are these injection repairs cured the same way? If autoclave is the desired approach, that doesn't sound like something that can be done on an operational aircraft

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[dik]

I don't know about resins, but there are some coating systems that have a limited time in which to add another coating layer. How do you access the void? Drill a hole?

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-Dik

 

[TugboatEng]

That is correct, once cross-linking stops new cross-linked bonds can no longer form so there is certainly a time window within which a full strength repair can be made.

Secondly, epoxies blush. Uncured amines remain on the surface. They form a greasy feeling layer that can make it difficult for subsequent bonds to be made. Mechanical prep is required to remove amine blush which is not possible to do with an injection repair.

 

[dik]

Thanks, Tug...

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

-Dik

 

[Tomfh]

He's saying that once the resin is cured - including the inside surface of a void which existed during curing - it is not very chemically active and is very difficult to bond to without significant prep, which you can't do to the inside of a void.

Yes that seems to be the claim. The composites industry disagrees with his claim. Resin injection repair is routine, and provides experimentally proven restoration of strength. Does it provide 100% restoration? Probably not. But blakmax is saying it provides virtually no restoration of strength, and is merely snakeoil used to pass NDT. That’s a fairly bold claim.

 

[hpaircraft]

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

Tensile strength for raw carbon fiber of reasonable quality generally runs up to about 400ksi. When combined with a resin to form a composite, the tensile strength is of course reduced by the resin's contribution to the cross-sectional area.

The compressive strength of carbon fiber composites depend almost entirely on how they are processed. Tests by sailplane designer Jim Marske in the 1990s showed that hand-laid carbon tapes could achieve something like 40-60ksi. When heroic measures are taken to ensure fiber alignment, tows and tapes could sometimes achieve almost 90ksi.

The carbon fiber pultruded strips I use in the wing main spar flanges of my sailplane designs are good for about 320ksi in tension, 200ksi in compression. To get non-scary-looking wing deflections under maneuvering loads, I generally design to about 120ksi at ultimate load of 8g. Using the pultrusions allows for good strength values without going to heroics in terms of materials or processes.

 

[SWComposites]

provides experimentally proven restoration of strength
oh really? please provide specific references. and the SAMPLE paper listed above is for one very specific condition, with delams at a fastener hole (due to poor drilling) and the results are not representative of most "injection repair" cases (the paper tested clamped up double shear conditions which are also not representative of most aircraft joints, which are single shear) such as edge delaminations, voids between core and facesheets, etc.

i've been involved with many injection repair cases thru MRB and have even run some tests, and frankly there isn't much data (beyond some "service experience") to back up what most MRB organizations do (which is basically justified by "it now passes NDT").

 

[Tomfh]

please provide specific references

You're in the industry. I'm sure you can find references. I'm not really interested in quibbling over double shear vs single shear test results.

I'm just curious as to why you and blakmax think resin injection is essentially worthless. It's an odd claim.

 

[blakmax]

Tomth

I've been in the industry for longer than most but I have yet to see a VALID test result that shows that strength can be restored through a few drill holes. I have tons of evidence that the injection repair for bonded joints in metallic structure simply does not work. Since these bonds were performed on fully reacted epoxies, I fail to see how they can suddenly work on laminates. I will be very interested to see other reliable test results. Show me!

The only way injection repairs have a chance of forming bonds is if the injected adhesive has a strong acid-base reaction to break the surface tension of the resin in the void.

Regards

Blakmax

 

[Tomfh]

I will be very interested to see other reliable test results. Show me!

From the earlier link I provided, which was resin injection at holes:

In that instance the strength of the repaired specimins match the pristine specimins.

SW has said no no no, that test is irrelevant, because it's not a delamination failure.

So here's another link, on flexural delaminations, which when repaired have 93% strength compared to pristine specimins:

https://www.sciencedirect.com/science/article/abs/pii/S0263822318339485

I'm sure you and SW will have your reasons why this test too is a load of rubbish, but honestly it's pretty hard to believe you when you're not providing any counter evidence. Since you're disagreeing with industry practice, how about you provide some test results which back up your theory that resin injection is of no use.

 

[TugboatEng]

I commented earlier that the towards the right sides all of the specimens converge. You mentioned that the ultimate strength of the repaired specimens was better than damaged. Do we typically design around ultimate or yield strength?

Honestly, I don't think the chart is very clear. What is the failure criteria? The specimens seem to be yielding but never yield to a conclusion.