[AirbusCapt]

Originally Posted by Thad E Ginathom

I didn't mean it quite like that. Not in the sense, "OK, so let's test it with people on board." I meant in the experience sense; every successful dive being further confirmation of its safety.

Thanks, thats where carbon fibre choice is a question. Unlike metals there is no predictable fatigue rate for carbon fibre based pressure hulls. There are some applications of carbon fibre based positive differencial pressure usage, but this is the first application of a pressure hull for negetive pressure differential.
In the bicycle industry no manufacturer offers a warranty for a carbon fibre frame, however warranties are offered for metal frames. This is because a carbon fibre based bicycle is known to suffer from sudden and unforeseen cracking and shattering during usage.

So counting each dive as a test would make no sense unless the company decided to ultrasound test the entire submarine post each dive. That would be too slow and too costly. But that is probably the only way they could prevent such a failure. Instead they went for a jugaad scheme. They had some sort of sensors rigged across the vessel to measure buckling/cracking. But these sensors make sense on a metal vessel with a predictable fatigue rate, whats the point on a vessal which could crack and shatter like glass?

[srini1785]

Originally Posted by Thad E Ginathom

I meant in the experience sense; every successful dive being further confirmation of its safety.

As explained by @AirbusCapt, to put it in perspective, the Young's modulus of carbon reinforced fiber is around 183GPa. For marine steel it is 200GPa. Carbon reinforced fiber's manufacturing process involves bonding the cloth like fiber with an epoxy resin and baking it in controlled environs.

The stress cycles, which @AirbusCapt is referring to is well known for marine steel while for Carbon fiber it is not fully understood. The problem, i am told, is due to the inherent nature of the bond between two dissimilar materials. The yield point of carbon fiber is sudden unlike marine steel which undergoes an elastic failure. This is probably the cause for an implosion and not buckling.

[Thad E Ginathom]

Originally Posted by AirbusCapt

In the bicycle industry no manufacturer offers a warranty for a carbon fibre frame, however warranties are offered for metal frames. This is because a carbon fibre based bicycle is known to suffer from sudden and unforeseen cracking and shattering during usage. ... ... ... They had some sort of sensors rigged across the vessel to measure buckling/cracking. But these sensors make sense on a metal vessel with a predictable fatigue rate, whats the point on a vessal which could crack and shatter like glass?

Shocking. I knew nothing about bicycle frames, but I'm vaguely aware that carbon-fibre tripods in photography can snap in a way that aluminium just would not.

Originally Posted by srini1785

... The problem, i am told, is due to the inherent nature of the bond between two dissimilar materials. The yield point of carbon fiber is sudden unlike marine steel which undergoes an elastic failure. This is probably the cause for an implosion and not buckling.

One seriously wonders at the choice.

In an early post, I asked a couple of technical questions, but no-one has yet answered:-

1. Risk of delamination in carbon-fibre composite, and whether the cylindrical structure would affect this. I'm pretty dumb with numbers, but plainly the interior diameter is smaller than the exterior diameter, the circumferences throughout the thickness are thus, different, and could that cause separation?

2. Focus is on the carbon fibre, what what about the titanium? ISTR reading that it work hardens quickly. Isn't work-hardening/metal-fatigue just different instances of same or similar phenomenon? All those deep-ocean stresses

<a bit later>

Originally Posted by adi.mariner

An informative article regarding how the hull of the Titan was built. Credits to the respective authors/website.

https://www.drass.tech/2023/06/27/ti...es-of-failure/

Thanks. After reading this, I now know that titanium seems to be a preferred material, so my doubts on that must be wrong.

[Jeroen]

Originally Posted by AirbusCapt

In the bicycle industry no manufacturer offers a warranty for a carbon fibre frame, however warranties are offered for metal frames. This is because a carbon fibre based bicycle is known to suffer from sudden and unforeseen cracking and shattering during usage.

Not sure where you get that from? These days many carbon frame bicycle carry a life long warranty! (Product life which comes down to 10-15 years easily)

Apart from that under EU consumer law, any product would have a 2 years warranty.

You can not sell anything in Europe without warranty. But again, apart from that just about all carbon frame bicycles, be it a road bicycle, or MTB come with identical warranties as metal/aluminium frames. Not just in Europe, same is true in the USA and many other countries.

I think this thread is a testimony to how little most of us really understand when it comes to carbon fibre products.

How do they design, manufacturer and test carbon fibre parts in the aviation industry if they could shatter just like that?

Jeroen



Jeroen

[srini1785]

Originally Posted by Thad E Ginathom

Risk of delamination in carbon-fibre composite, and whether the cylindrical structure would affect this.

I'm no expert on properties of materials but i think that any material that has composite layered structure is prone to delamination at some point. Carbon fiber also has a laminated structure. Laminated structures in general have good strength in a direction perpendicular to their layers than to forces that are parallel to their layers.

The only thing that seems to be going for carbon fiber is it's weight although its beyond my understanding why lightness should be considered a favorable factor in a submersible.

Regarding the cylindrical structure, i can only say that any fluid would exert the same pressure on all sides. So a cylinder or sphere (having same radius of curvature) would experience the same force per square m at a given depth due to water.

Focus is on the carbon fibre, what what about the titanium? ISTR reading that it work hardens quickly. Isn't work-hardening/metal-fatigue just different instances of same or similar phenomenon? All those deep-ocean stresses

I don't know about Titanium. The point being repeatedly made in other technical forums (one of which i earlier mentioned) is that carbon fiber, in the case of this submersible, was bonded to a plexi glass viewing window using some kind of glue which was not rated for even 1/3rd the wreckage depth.

[srvm]

I have been following this thread for the past few days but was unable to find time to post here so far. Many posts in this thread have been about Carbon fiber composites and its failure modes, its suitability/unsuitability for the submersible applications etc. Some posts in this regard have been inaccurate, some are partially correct, some use wrong technical terms and some are completely wrong**. I won't get into which post is which type.

Kindly allow me to add a few points even though it is not specific to Titan's failure.

** I earn my bread by working in the area of Composite airframe structures. I'm fairly exposed (>14 years) to their design, stress analysis, material characterization, mechanical testing, Non-destructive testing (NDT) and do a fair bit of R&D in high performance composites.


1. First off, there are many types of composites in use (from performance and cost point of view): from use in ground applications such as pressure vessels, to automotive (F1 etc), to sports goods (cycling, tennis racquets), to Aviation and space applications. Naturally, the ones used in Aviation and space are of very high grade/quality and performance, and therefore costs more to produce. Also, these materials undergo rigorous characterization and testing before being certified for use in aviation applications. Also, the design of airframes (whether using alloys or composite materials) are also rigorously checked and reviewed.

2. The usage of the phrase 'Carbon-fiber' alone to refer to Titan's hull is incorrect as some have already pointed out. Composites are made of 2 or more constituent materials. In this case, Carbon-fiber is called the reinforcement and it is supported in a polymer matrix (typically epoxy for such applications). The raw material is layered or wound on a tool and is cured (polymerization in high-school parlance) in an autoclave under appropriate temperature and pressure conditions to obtain the part.


3. I would presume that for an application like hull of Titan submersible, the designers would have chosen a high-performance composite material similar to the ones used in aviation industry. And I presume/hope their designs were verified and validated thoroughly via tests on ground.

4. IF DESIGNED PROPERLY, Carbon fiber composites are capable of being used in this hull application. A few sub-points in this regard:

a. The cylindrical shape of hull is best suited for fabrication through filament winding method. If designed and executed properly, the structure should achieve its design strength and stiffness.

b. The challenging part of the design is going to be the joints. Particularly, the ones between Carbon fiber composites and Titanium/other alloys (dissimilar materials in many ways). Again, this is POSSIBLE to do, if due care is taken. Likewise for bonding / joining between composite and viewing window.

c. Delamination in composites is an area that worries designers (delamination = separation of various composite layers within a laminate). So, we take them into account during design to a certain extent. Also, an appropriate inspection plan (visual + NDT) is put in place during service to avoid unforseen failures. Typical NDT techniques used are ultrasonic inspection, infrared thermography, X-ray and so on. I'm not sure what kind of inspection plan and infra was in place for the ill-fated Titan.


5. High-performance composites (such as used in aviation and space) are actually better in fatigue compared to traditional Aluminium and Titanium alloys. (I saw some posts in this thread mentioning the complete opposite). Without getting too technical this is the simplest explanation: Crack propagation is easier if the material is homogeneous and isotropic (like alloys). It is difficult (needs more energy) in non-homogeneous, anisotropic materials like composites.

Also, designers take fatigue effects into consideration during design, whether it is alloys or composites. Also, loading-unloading cycles of few tens or hundreds times is different from cycling for thousands of times. For instance, critical sections of airframe which are subjected to fatigue loads will be typically designed to withstand a million or more load cycles (basically it is designed to be safe for its entire operational life). So, I presume designers of Titan's hull would have considered few 100's to 1000's of load cycles while designing it.

Someone mentioned that composites are poor in fatigue and that is why they aren't used in turbine blades. Turbine blades are typically made using Nickel or Cobalt based superalloys because of the extreme thermal and fatigue environment. The temperatures in turbine can easily reach about 1500 deg C. Most composite materials cannot be used at such high operating temperatures. And thats why they are not used. Not because of poor fatigue manners. Composites are widely used in fan blades of gas turbine engines (E.g. GE90, GE9X etc.) which are subjected to fatigue loads but at lower operating temperatures.
{Turbine is different from Fan. Fan comes before the combustion chamber. Turbine comes after the combustion chamber, hence subject to higher gas temperatures.}


6. Most metals and their alloys are DUCTILE. This means that, up to a load, they come back to original shape and size when they are unloaded (known as Elastic limit or Yield point). Above this load, they elongate/deform permanently even if the load is removed (known as Plastic deformation). If one continues to load it further, then the material ruptures. In a way, this DUCTILITY is considered beneficial because the material provides some sort of 'warning' in the form of plasticity before it ruptures completely.

Now, Carbon fiber composites are not ductile, they are BRITTLE materials. They do not undergo plastic deformation. Instead, they are elastic upto some load and then BOOM (ruptures suddenly). Engineers who were trained in traditional methods, consider(ed) this behavior as non-ideal/inconvenient. But that was 30-40 years ago. Composites are now better understood and more predictable, which is why most modern aircraft structures (both civilian and military) are nearly 50% composite by weight.


7. Since this post is already too long and technical, let me end with one final point. Foreign body impact = Kryptonite for composites. They can cause (sometimes severe) delaminations / disbonds on the inside of composites while looking perfectly fine on the outside. This can fool inspectors during routine visual inspections.

For e.g. if Titan's composite hull got hit/banged by some part of Titanic's wreckage during its final or previous trips, then it could potentially cause delaminations with fatal consequences. Once again, a detailed inspection plan and execution should be in place to catch such damages.



My sincere apologies for the long & pedantic post. I could not resist posting in a detailed manner since some earlier posts were adding to some common misconceptions about high performance composites (which is my daily-bread )

srini, much of your previous and this post is correct but there are some mistakes.

Originally Posted by srini1785

I'm no expert on properties of materials but i think that any material that has composite layered structure is prone to delamination at some point. Carbon fiber also has a laminated structure. Laminated structures in general have good strength in a direction perpendicular to their layers than to forces that are parallel to their layers.
....

Fact is exactly the opposite of the above. Composites are great when you load them in their plane. Poor when loaded perpendicular to its plane. Which is why they tend to delaminate when some object falls on them.

Originally Posted by srini1785

.....
The only thing that seems to be going for carbon fiber is it's weight although its beyond my understanding why lightness should be considered a favorable factor in a submersible.
....

That's because, such vehicles have to be made, assembled, trialed on ground (outside-of-water) - where heavier the weight, more laborious, painstaking it is to move it around etc.

[Jeroen]

Originally Posted by srvm

. Many posts in this thread have been about Carbon fiber composites and its failure modes, its suitability/unsuitability for the submersible applications etc. Some posts in this regard have been inaccurate, some are partially correct, some use wrong technical terms and some are completely wrong**.

Here, hear! At last somebody that knows it’s onions! Thanks very much for your elaborate reply, much appreciated!

That's because, such vehicles have to be made, assembled, trialed on ground (outside-of-water) - where heavier the weight, more laborious, painstaking it is to move it around etc.

Not exactly. Metallic hulls, however, because they are not buoyant in designs for depths of more than 2,000m, present challenges when it comes to managing ballast for ascent and descent. In particular, metal-hulled craft require the use of syntactic foam attached to the outside of the craft to achieve neutral buoyancy.

So it is all about buoyancy and how to deal with it at such extreme depths. Using composite materials solves this problem and allows for a much simpler design. It is much cheaper.

Jeroen

[udzgodfather]

Recent news from the company who made the Submarine for visiting Titanic, someone at the board level has to be really nuts to not understand that this is not working for the company.

[Thad E Ginathom]

Originally Posted by srvm

... Kindly allow me to add a few points even though it is not specific to Titan's failure.

Super post. Please don't worry about the length. This kind of information, presented in a form that those of us who are not engineers and/or in the field can understand, is gold dust

Originally Posted by Jeroen

Here, hear! At last somebody that knows it’s onions! Thanks very much for your elaborate reply, much appreciated!

it is all about buoyancy and how to deal with it at such extreme depths. Using composite materials solves this problem and allows for a much simpler design. It is much cheaper.

Thanks for your input too.

Originally Posted by udzgodfather

Recent news from the company who made the Submarine for visiting Titanic, someone at the board level has to be really nuts to not understand that this is not working for the company.

It's so crazy it looks like something from The Onion! They could advertise as anniversary dive! Couldn't get much more bad taste.

But seriously, this suggests that they are under no regulatory control whatsoever. If this was an aircraft, wouldn't it be grounded until completion of full investigation and release?

Hmmm... Am I suggesting they should just scrap the whole venture, company, and all? Not really, but full investigation and transparent testing should come before future plans and advertising.

[MinivanDriver]

Originally Posted by srvm

** I earn my bread by working in the area of Composite airframe structures. I'm fairly exposed (>14 years) to their design, stress analysis, material characterization, mechanical testing, Non-destructive testing (NDT) and do a fair bit of R&D in high performance composites.

Thank you for the detailed posts. Great to see an SME clarifying the details and correcting misconceptions.

There's one point that has been nagging me. I would be very thankful if you could throw some light on this aspect.

In an aircraft hull use case, the composite hull is under tension ( or rather I intuitively think of it that way) as the outside air pressure at altitude is less that the inside of the pressurized cabin.

However in the deep sea submersible use case, the compsite hull is under compression since the outside pressure is many fold higher than the inside.

Now intuitively I feel that the composite material being made out of strands of fibre that is woven, then reinforced and layered would withstand tension better than compression.

Would really like to hear your expert opinion on this. Thanks.

[locusjag]

Originally Posted by Thad E Ginathom

But seriously, this suggests that they are under no regulatory control whatsoever.

What I've gathered is that this company was a fly by night operator who operated with the knowledge that they were not in any one country's control when out on international waters. In international waters, only the International Maritime Organization's (IMO's) laws apply, which I believe are more about sailors' duties toward fellow sailors in distress and the likes of that. (If there's more to the IMO's laws, please do enlighten me!)

So yeah, they've had operations in international waters with no one to police them.

This has to be some weird legal loophole - because, why not just regulate them based on where such companies are incorporated? I don't understand this bit.

[srvm]

Originally Posted by MinivanDriver

Thank you for the detailed posts. Great to see an SME clarifying the details and correcting misconceptions.

There's one point that has been nagging me. I would be very thankful if you could throw some light on this aspect.

In an aircraft hull use case, the composite hull is under tension ( or rather I intuitively think of it that way) as the outside air pressure at altitude is less that the inside of the pressurized cabin.

However in the deep sea submersible use case, the compsite hull is under compression since the outside pressure is many fold higher than the inside.

Now intuitively I feel that the composite material being made out of strands of fibre that is woven, then reinforced and layered would withstand tension better than compression.

Would really like to hear your expert opinion on this. Thanks.

Thank you.

You are quite right that internally pressurized vessels are predominantly under tension loads. And vessels/submersibles subjected to external pressure are under compressive loads.

And indeed, composites with their layered construction have better tensile strength (about 20-25% higher) compared to compressive strength. But this is known and hence factored-in during design where compression loads are to be carried by the structure.
*Slightly advanced topic* = Also, most thin structures (not just composite ones) such as ones used in aircraft when subjected to compression loads tend to BUCKLE first before getting crushed under compression. Once again, there are ways to predict onset of buckling and prevent buckling by design.

Composites are regularly designed to carry compression loads. for example, top skin of aircraft wing is dominated by compression loads. So, if designed properly with care, it can be a safe structure.

[Formula1]

Originally Posted by Thad E Ginathom

I don't recall how many deep-ocean trips this thing had done, but it was not its first. So survival was not against the odds. Even if they were odds you or I would not like.

On the contrary, this is exactly what makes it more probable to fail. It's a matter of time. The numbers will catch up with you, on a sub like this.

If a touring company offers you to take you to the Titanic, run.

If a touring company that prides itself on cost-cutting offers you, run further.

If a touring company that prides itself on cost cutting offers you a seat on an already used untethered cyclindrical carbon fibre-titanium sub, just laugh. It's about your life, not about whether something's possible. You could make it that deep in a phone booth, if "it's made well enough".

If you're a billionaire who really wanna see the Titanic, commission a sub yourself.

[Brumby]

Originally Posted by locusjag

In international waters, only the International Maritime Organization's (IMO's) laws apply, which I believe are more about sailors' duties toward fellow sailors in distress and the likes of that. (If there's more to the IMO's laws, please do enlighten me!)

So yeah, they've had operations in international waters with no one to police them.

This has to be some weird legal loophole - because, why not just regulate them based on where such companies are incorporated? I don't understand this bit.

IMO laws are designed to protect every human stranded at sea or in distress. It's a duty of all the registered merchant vessels to save any one who needs help out at sea.

Regarding the incidents/accidents/crimes that happen at sea in international waters, the maritime authority of the country where the ship/boat/yacht is registered and the country where it enters after the crime happened, are required to take required necessary actions. Local police too is involved and supposed to take required actions.

OceanGate is a registered company and their ocean going crafts must have been registered somewhere. They must be having license to operate international VHF radios registered with service provider so, legal action proceedings will be there. They must have made the clients to sign an indemnity to cover themselves.

Incidents and accidents happen across the industries, that doesn't stop the business.

In case an accident happens on a ship or a marine craft, initial evidence is collected, investigation is done at different stages depending on the nature of the incident, if the nature of incident is not serious, the ship goes about its business immediately after the initial investigation is completed. Loss of 2-3 lives is not considered a serious incident to stop the business.

We submit an indemnity for our kids, when the school is taking them out for picnics, tours etc, so OceanGate is well covered by such indemnity, stating that the customer is well aware of risk in case of an untoward incident.

The above statements are purely based on how the business works, and as far as I understand, emotional values have no role in business, except to attract customers (business).

Hundreds of lives of seafarers are lost at sea every year, response from the employers is standard as per the procedures, no emotions, no feelings of remorse.

Harsh but thats the truth.

[McLaren Rulez]

Originally Posted by udzgodfather

Recent news from the company who made the Submarine for visiting Titanic, someone at the board level has to be really nuts to not understand that this is not working for the company.

I had to laugh at the fact that there's not even a discount on the $250,000 price tag!