top of page
  • Writer's pictureNed Patton

How to Make Sure it Doesn't Break - Part Deux

In a sense this post is about the loss of the Titan submersible. But mostly it is about how to make sure whatever you design out of composites doesn’t break – using the failure of the composite portion of the Titan as an example.

Warning – this one may get slightly technical because I am going to talk about stresses at joints between composites and metal and what care must be taken in order to ensure that your composite part doesn’t fail at the most likely place – where there is an interface between the composite and anything else – especially a metal part. And, of course, this is also about when that interface or joint has to transfer load either from the composite to the metal or metal to composite – depending on whether it is in tension or compression and what part is being pulled on or pushed on in the axis of the joint.


First, however, a little pictorial is in order. The pic to the right here is a good schematic of the overall design architecture for the Titan pressure hull. What it shows is the central cylindrical section which is carbon fiber / epoxy, and the two hemispherical titanium ends, making this thing look sort of like a medicine capsule. You can even see the acrylic viewport at the front of the hull in this view. While that is also an important factor in the loss of the Titan, since we really don’t know what failed first until forensics on the wreckage that was brought to the surface is complete, it may or may not be the culprit. My vote is no – it was the carbon fiber part of the pressure hull that failed.

And that is what this post is about. Why and where do you surmise that the pressure hull failed. What I’m going to give you is what immediately came to my mind when I heard about the disaster and why I think I have a good idea what failed and how it failed. And that’s why I need to talk about joints between composites and metals so that everyone gets a reasonable idea of why I suspect where the initial problem was with this pressure hull.

First, a little about what we know from the news reports and from OceanGate itself. Apparently there was only one way in and one way out of the submersible. To enter the hull, the forward titanium hemisphere was unbolted and removed so that the occupants could enter the capsule of the pressure hull. Then the forward hemisphere was bolted back on and sealed up with the occupants inside. This is a sort of a strange design for a submersible – most of them have at least a hatch that can be opened to let people in, and some of the shallower depth mini-subs even have what is called a “lock-in / lock-out” chamber or a way to exit the sub even when submerged.

Deeper diving submersibles like Titan usually don’t have this sort of arrangement, but they at least have a water tight hatch opening at the top to allow the occupants to enter and exit the sub. Not so with Titan, but then that isn’t why I’m writing this – just to note that since Titan did not have this hatch, the carbon fiber cylindrical section could just be a simple tube with titanium rings bonded into the carbon fiber which bolted to the front and rear hemispheres.


So, this brings me to the real point of this post. To describe what I think happened. The pic to the left is a notional concept for a composite to metal joint. Note that the carbon fiber composite overlaps a tongue of sorts sticking out of the metallic part that is bonded to the composite. This is one of very many variations on a theme, and there is a company in Sacramento that did the original design work on this carbon fiber hull many years ago whose founder has on the order of 100 patents in just this sort of joint. The company is Spencer Composites, and the founder, Brian Spencer, did the original design of this hull.

And here I need to dispel most of the news reports about this, Spencer DID NOT BUILD the Titan hull. In fact, they only did the original design, which changed many times before OceanGate finally built it. My guess about this is that when OceanGate got Spencer’s design Brian had done a very credible job of the design, but OceanGate priced it out and it would have been too expensive to make it. This is just a guess, but since I’ve been in this business as long as I have I get a feeling about things like this since I have had this happen on more than one occasion. So, maybe, just maybe this was one of the contributing factors? Only time will tell about that.

And also, only time will tell about the actual root cause of the failure. But, since this is a post about my suspicions, let me lay them out for you. If you look carefully at the pic above you will see discontinuities in the composite. First, the composite is made up of many layers of thin carbon fiber / epoxy with the fibers going in different directions. From what I understand, most of the fibers were wound on in the hoop direction – around the circumference of the hull. This means that there were not as many fibers in the axis of the cylinder as there were in the circumference of the cylinder. And this makes sense from a stress perspective, because in a cylindrical pressure vessel, the hoop or circumferential stress is twice the longitudinal stress. So, a 2:1 ratio of hoop to axial fibers might make sense.

That is – except at the joint, where there is a much more complex stress field. The problem is that the stiffness of the carbon fiber composite in compression is different than the stiffness of the titanium ring in compression. That means that when external pressure is applied the inward displacement or strain of the carbon fiber tube will be different than the inward strain of the titanium ring. This sets up a very complex set of stresses that need to be analyzed and taken into account in the layup design of the composite and how it is bonded to the titanium end rings.


The pic to the right is an FEA representation of a composite to metal joint showing sort of how such a design is analyzed using the finite element method. If we surmise (whether we are right or not) that the light blue central piece with the darker blue end is the metal tongue, the rest of the material is the composite. Note that there is a rather complex stress field right at the end of this joint. I believe this is probably pulled in tension rather than being in compression the way the Titan joint is designed to operate. In compression, this joint behaves in a similar fashion, but there is still a fairly high stress at the interface between the composite and the metal.

And one more wrinkle to this is that the titanium ring to composite joint is not loaded purely in compression, because of the stiffness and strain mismatch between the composite material and the titanium. There would be an added component of shear occurring across the joint because there is a displacement mismatch. Effectively the titanium ring shrinks in diameter at a different rate than the carbon fiber composite cylinder. So, there is an end compression from the external pressure, and either an outward or an inward tension (displacement mismatch) across that joint, putting one side or the other of the joint in tension. So one side or the other of the metal tongue is pulling away from the composite.

So, here is where the problem lies. The composite is in layers whose only strength is the resin that holds the layers together. And at the interface between the titanium tongue of the bonded in titanium ring, one side of that bond joint goes into tension as soon as external pressure is applied. This is the driving force for the beginning of delamination of the composite and eventual failure of the pressure hull. Delaminations began on the tension side of the metal to composite interface which eventually led to complete collapse of the pressure hull.

At least that is what I think happened. And since the news reports have said that initial passengers heard cracking noises coming from the pressure hull as they were going down to see the Titanic, I would be willing to wager that I am right about the incidence of the first delaminations in the pressure hull and how that led to eventual collapse.

And this may or may not have been taken into account in the final design of the pressure hull. I'm sure that the first design of that hull had taken this into account, but since Brian Spencer was only involved in that first design (and he only had 6 weeks to do it) I'm pretty sure that OceanGate did not use that joint design, and instead used something else. Only time and forensics will prove if I'm right or wrong about this.

Finally, I welcome any further discussion from anyone out there in the audience. And if you know of someone who might be interested in this and if you are interested in starting a conversation about it, please let me know. Since this is being posted both to my website – www.nedpatton.com, as well as to LinkedIn, you can reach me either way. Just drop me a line and if you go to my website, please provide your email address and we can start an off-line conversation about this.

And, as always, please take a look at my upcoming book from McFarland Books. The pre-sale link is here - https://mcfarlandbooks.com/product/The-String-and-Glue-of-Our-World/. You can also find this link on my website. And when the book is finally published, I will be selling both unsigned and signed copies on my website as well.

Talk to all of you again next week. Stay tuned. And if you hear anything about the Titan this week and want to talk to me about it, please feel free to chime in either on my website or on LinkedIn.

TTFN

Comentários

Avaliado com 0 de 5 estrelas.
Ainda sem avaliações

Adicione uma avaliação
bottom of page