The Truth About Carbon Fiber: Myths vs. Reality 

Im glad i saw this comment as i was coming to say the same. In a lot of cases where you see failure of CF parts you see things like rims or frames on bicycles. Objects that may have had sharp impacts in an area in addition to loads near yield. If a rim is perfect but in a single day hits 500 rocks thats a lot of impacts.

they should have mentioned in the video that carbon fiber composites are brittle (and as you said fail catastrophically), compared to the ductile behavior of steel. The brittle behavior of cfc isnt a bad thing per se but you need to engineer around its shortcomings

@@ak1s4 I think they were trying to avoid using the word "catastrophically." This video was partly intended as a defense of carbon because a lot of people have a lot of misconceptions after the titan sub implosion event. That is my take anyway. It is all fine you just need to make sure you design within the limits of the material. Carbon is still lighter even when you allow a more generous safety margin than you would with steel.

By definition, materials that exhibit behaviours similar to carbon are called brittle. These include concrete and glass, among others. Ceramics are also part of this family. In the case of carbon, the ultimate stress (fu) is very high, but it can't deform too much, so the overall energy absorption (ie the area under the stress-strain diagram) isn't that big. Like I said that doesn't mean anything bad.

Really important comment. There are three different failure processes here. One is single momentary overstress, another is fatigue damage (tens or hundreds of cycles of high stress), the third is cyclic stress (thousands or millions of cycles of less high stress). Carbon fiber: abrupt complete failure from momentary stress beyond yield limit, gradual loss of strength from accumulated fatigue damage, no change of properties from cyclic stress Steel, especially mild steels: gradual energy-absorbing yield from stress beyond yield limit, gradual loss of strength from accumulated fatigue damage (primarily corrosion-induced) with large critical crack size, no change of properties from cyclic stress Aluminum, especially high tensile aircraft aluminum: some energy-absorbing yield from stress beyond yield limit, gradual loss of strength from accumulated fatigue damage with much smaller critical crack size, work hardening from of cyclic stress. The move from aluminum to carbon fiber for long-life commercial aircraft is primarily because those aircraft have to stripped down to bare metal every 10-15 years and minutely inspected for cracks, especially around stress concentrators like windows and rivets. Not all composite fibers are the same. Kevlar is a bit lower tensile yield strength than carbon fiber, and more costly and heavier, degrades quickly when exposed to UV, and has substantially less compressive strength. But Kevlar absorbs a huge amount of energy when pulled beyond it's tensile elastic limit. In the high-cycle low stress regime it also absorbs energy, making it "dead". This makes it good for armor and rotor blades, or bits exposed to ramp rash like leading and trailing edges, or radomes (noses and antenna covers) where carbon's conductivity is a problem. Kevlar's lack of compressive strength makes it a poor choice for most aircraft structure. I'm frankly astonished that people build boat hulls out of carbon fiber, but it's so obviously superior for airplanes and wind turbine blades that the cost has come way down as the production volume has gone way up. $/stiffness for Chinese carbon fiber is now just 30% greater than fiberglass! Kevlar and fiberglass are a far better materials for a boat hull that should limp home safely after accidentally smacking a pier, rock, or reef (or another boat).

Sorry, really curious, so I'm asking every expert commenter here: If I'm designing a car's frontal impact structure out of a CFRP, with a titanium reinforcement within, such that it can appropriately dissipate a high speed collision's energy and then still have some composite material volume left for any potential secondary collisions or impacts.....what kind of fiber would I need? How would they need to be aligned? What resin? How much resin? Should the titanium pipe be wrapped by layers of carbon fiber threads around it or wound like a spring? Or should the fibers be facing the direction of the impact/compressive load of the collision head on?

@@corpsecoder_nw6746 I can’t really give any meaningful advice there- the composites ive worked with are pretty much done for once they incur any major damage- but that may not be the case as much with isotropically distributed fibers in a ductile matrix? Not too sure hopefully you get some good answers

​@@corpsecoder_nw6746it's a massive topic, search how carbon fiber is used in racecars. Most of them have a steel alloy roll cage, Carbon Fiber panels, and a lot of times they have special carbon fiber deformation zones, that dissipate (absorb)crazy amounts of energy in the collision (a lot more than steel would) . Also carbon fiber supercars handle high speed collisions very well, because of the energy absorbing qualities of CF.

Exactly! Concrete and Glue joints have some measurable tensile strength to them but you don't design anything to rely on those as the failure modes are sudden and unpredictable.

​@@corpsecoder_nw6746ok first off, titanium is way to rigid to use in crumble zones/crash structure. second, it is very rare and very hard to make a crash structure out of carbon fiber. There are only a handful of manufacturers that do that (i.e rimac) and even then it's still only partial. It's usually aluminum subframes combined with a carbon monocoque center. (for reference I would recommend looking at the Aventador chassis, it's a little bit older of a platform but it is still what most supercars cars look like underneath) Other than that I can go into specifics, I assume a larger weave would be be best for rigidity, but if you want it to "deform" (aka probably still shatter) on impact, probably a smaller thinner one

There are a lot of ways to introduce latent faults during manufacture.

The Titan submersible implosion is probably a good example of this.

​​@@bishopdredd5349the titan submersible imploded mainly due to ignorance, miscalculations, expired prepreg, improper layup method, design faults in particular of the titanium ring interface and all kind of different shortcomings. It has been determined the load capacity margin was next to non existent for the depth it was going to.

Anyway, the resin bond between the layers and "contamination" is not really the problem with compression. Instead it much more affected by the orientation of the Weave/fibres and the amount of resin during layup that limits the bucking load capacity of the fibres.

Yeah, his explanation about compressive strength is guided by their use of the material, I think. If they would make titanium planes, he'd talk differently.

I did the same; am now in my plastic deformation region and can never recover.

@@davidf2281 Sounds like you’re in a state of non linear deformation (aka slumped on the sofa) due to under estimating load cycle capacity resulting in insufficient ultimate strength.

@@MrFloneil dude i wish this thread was longer lol

if I'm making a car's impact structure with CFRP and say I have a titanium pipe for reinforcing said structures. Say, I want the internal occupants to not experience more than 10gs in a 60mph crash, the structure should have some "meat"/volume left for a 2nd crash like that (40mph) without being much harsher (under 20gs) on the occupants....how much CF do I need? What kind of fiber? What kind of resin? How much resin-to-fiber? What way should the fibers be aligned? How do I use the titanium pipe to maximize strength and resilience of the material for secondary impacts/compressions? How dense will the CF fibers and resin be? How much volume? How much weight will it be? How much would it cost? Can it be made to be inert to natural weathering or would I have to paint it to shield it from UV and rain and other gases?

so you're saying the titanium pipe and the fibers facing the direction of impact/crash?

+/-45 degree woven glass epoxy or carbon fiber tubes. Placed behind a bumper reinforcement bar. And mounted to the chassis hard points near suspension and engine mounts.

So many variables that a definitive video on this topic would be lengthy and even at that, incomplete.

Yea, that would be awesome and maybe those would work with glassfiber parts too?

Sorry, really curious so I'm asking every expert commenter here: If I'm designing a car's frontal impact structure out of a CFRP, with a titanium reinforcement within, such that it can appropriately dissipate a high speed collision's energy and then still have some composite material volume left for any potential secondary collisions or impacts.....what kind of fiber would I need? How would they need to be aligned? What resin? How much resin? Should the titanium pipe be wrapped by layers of carbon fiber threads around it or wound like a spring? Or should the fibers be facing the direction of the impact/compressive load of the collision head on?

What i know about composite repair, on experimental glider airplanes: Grind away the damaged section. Create a 70/1 incline on the good composite sections to glue on the new layers. This way, you can replace part of it without loosing strength. Also, to figure out what was in there, do a burn test. Cut out a sample of the whole structure. Pyrolyze the resin. Take a good look at the now visible layers. Yeah, i don't think the last bit is adequate for planes that need to pass certification :D but on those, you can look up the layers in your paperwork, of course.

Hahaha those damn PEO rules back at it again.

KSI is such a cursed unit. Going up to mega is MPSI, why are the pounds dropped from KSI? Why are you using metric prefixes with non-SI units? Why not TPSI for Thousand Pound-force per Square Inch?

if I'm making a car's impact structure with CFRP and say I have a titanium pipe for reinforcing said structures. Say, I want the internal occupants to not experience more than 10gs in a 60mph crash, the structure should have some "meat"/volume left for a 2nd crash like that (40mph) without being much harsher (under 20gs) on the occupants....how much CF do I need? What kind of fiber? What kind of resin? How much resin-to-fiber? What way should the fibers be aligned? How do I use the titanium pipe to maximize strength and resilience of the material for secondary impacts/compressions? How dense will the CF fibers and resin be? How much volume? How much weight will it be? How much would it cost? Can it be made to be inert to natural weathering or would I have to paint it to shield it from UV and rain and other gases?

It is amazing the nonsense spouted since the submersible implosion.

Any half decent in depth video has talked about the way the haul was wound, not just the fact that it was carbon. Also, not that carbon can’t do it or doesn’t have any compression strength, but rather that it wasn’t the optimal material…which is all true.

@@gpaull2 yeah, WET strands woven around a titanium core?! That's just asking for a disaster. Now if it was properly engineered dry carbon, the results would have been WAY different.

The carbon fiber pressure hull was a stupid use of carbon fiber. I remember i was at a conference like 10 years ago. And some one did a presentation on a carbon fiber pressure hull and we where talking about how it was a dumb idea back then. I would maybe have used it for un-manned rov or auv maybe. If it actually made it lighter or better in some way.

@@mshepard2264 Not necessarily. The buoyancy added by the carbon fiber is a good attribute for a submersible. Every material has its limitations, but generally these can be overcome by good design. I think the main issue in this case was arrogance that led to poor design and fabrication. It is no different that the issues with aluminum that were discovered early on with pressurized airlines such as the De Havilland Comet. Saying that carbon fiber shouldn’t be used for a submersible due to one failure is like saying aluminum shouldn’t be used for airliners given that the Comet had multiple failures before the cause was found and mitigated through design.

I see what you did there

Not a good choice for submarine pressure hull, good choice for F1 and aeroplanes.

This is exactly what I was looking for. If I'm making a car's impact structure with CFRP and say I have a titanium pipe for reinforcing said structures. Say, I want the internal occupants to not experience more than 10gs in a 60mph crash, the structure should have some "meat"/volume left for a 2nd crash like that (40mph) without being much harsher (under 20gs) on the occupants....how much CF do I need? What kind of fiber? What kind of resin? How much resin-to-fiber? What way should the fibers be aligned? How do I use the titanium pipe to maximize strength and resilience of the material for secondary impacts/compressions? How dense will the CF fibers and resin be? How much volume? How much weight will it be? How much would it cost? Can it be made to be inert to natural weathering or would I have to paint it to shield it from UV and rain and other gases?

@corpsecoder_nw6746 You're asking me? Sorry I couldn't answer even if I was an engineer with a masters in material science focused on composites. Too little information on your design. That isn't a suggestion for you to provide more detail. I like to help but this well beyond my knowledge.

I second your comment! As I watched the video, I couldn't help but think about the Titan -- and how three things likely led to its demise: first, using insufficient and untested material for the task it was put to (if I recall correctly, the shell was only 6 inches thick), second, not using any techniques to check for stresses in the hull between dives, and third, having no idea how many times something like the Titan could dive before failing. Titan was a failure of sloppy engineering, not a failure of carbon fiber materials!

yea same. People's faith in carbon fiber has gone. Meanwhile motorsport fans have seen so many drivers walk out of multiple impact pileups/crashes/collisions thanks to a carbon-fiber reinforced polymer w/aluminium-honeycomb-matrix safety cell and crash structures. Although I do feel like crashes like Hubert Spa 2019 and van't Hoff Spa 2023 were cases where CFRP wasn't strong for 2nd crash after the material dissipated the 1st one.

@@alpheusmadsen8485the carbon fiber used on titan was apparently unairworthy carbon fiber bought from Boeing, so it was doomed from day 1

I heard that weaving sometimes reduces compressive strength

The direct compression strength of glass FRP is much lower than carbon. Pure carbon fiber aligned with the compression forces is the way to go. This has been tested.

Thats one of the reasons they use Basalt fiber in Boats and sporting equipment as well as artificial limbs. Its also natural ..has a huge temp tolerance ballistic and abrasion resistance .. its cheap green and requires or produces very little like water or carbon dioxide in its manufacture.. it makes up a huge part of the earths crust and its been squeezed out by the tens of millions of Tonnes daily... and even more in the past. If you need it lighter combine it with flax hemp or jute fibers.. Its actually more resistant to delaminating than carbon epoxy and highly chemical and salt resistant. Its cheaper to make.

Resin, after it cures, is a type of plastic. Composite construction with polyester, vinyl ester or epoxy resin and glass, aramid, graphite or basalt fibers is an example of fiber reinforced plastic (FRP) construction. Working with abs, polycarbonate or other plastics you may be thinking of usually requires high pressure injection molding. The molds are quite costly and usually the cost can only be justified if the production volume is high. It takes a lot of money to mass-produce cheap plastic crap. Also, epoxy carbon composite is stronger than all the other stuff I mentioned.

Resin was definitely plastic last time I checked, unless the definition changed

Yes, in certain applications. Standard modulus CF tubing is stronger and lighter than 4130 Chromoly. It flexes similarly. The challenges come down to how to bond it to metal and resulting galvanic interactions, how to bond it other carbon fiber structures and other materials,and cost vs benefit. Please don’t take my word for it and do your own research. There are lots of resources on the web including manufacturers data, testing videos, as well as the excellent videos by DarkAero. I have looked into this extensively and this is all just off the top of my head.

Depends on how you plan to attach tubes together. A full blown monocoque should be lighter than a similar sized steel tube frame anyway, and for chassis rigidity I'd go with the former

Have you looked at laminations and composites using Basalt fibers.. its not as light but its ballistic and ability to resist tearing shattering and temperatures and chemical attack make it an excellent partner in such roles ... Im coming at it from the hammering and flexing of boat hulls in the ocean and Kit cars ..military uses.. and civil engineering as a light super stable replacement for steel in structures exposed to fire hurricanes ..earthquakes. Its also a Zero added carbon material beyond its melting to form a fiber. Temp stability -250dec C to 800C plus. You can wrap exhaust manifolds in it its none hazardous/itchy easy to work.

Lmao

Yeah. Everyone in the world now thinks they are an expert on carbon fiber. Glad the dark aero guys are trying to correct the record.

You do not speak for everyone. Many of us are patient, and would prefer that they take as much time as they need to make sure their aircraft is safe.

@@user2C47 providing a progress update doesn’t preclude safe progress numbnuts

It has a bunch of nearly unresolvable design flaws. It's not going to fly safe without some major changes, a hard pill to swallow when they've been painting themselves as "experts".

​@@ccengineer5902Would be instructive to have a short list of these defects. I am sure there are many aircraft and composites engineers out there would be interested to read through it.

@@ccengineer5902 this is new to me. What design flaws does it have?

Yes, I do think there's an important subtlety here around the definition of failing "without warning". Most people's intuitive notion of that phrase is that a "warning" is human-perceptible and will give you a chance to act when your structure is failing. I get that ultrasound scans can reveal fatigue/damage in carbon composites, but I think at a fundamental level the plastic deformation of metals makes people feel better about them from a human perspective.

This statement does not consider most scenarios. Especially aluminum parts made from rolled sheets have a microstructure leading to a crack-nucleation life of 0. This means, that any loading, no matter how small, will damage the material - you mentioned this point. However these type of defects usually propagate insde the material and cannot be picked up through visual inspections. Without preventive maintenance, such a part will become brittle and eventually fail abruptly. On the other hand theoretical models show, that a well designed and manufactured CF-part may even have an unlimited crack-nucleation life. And once the part fails, it usually does not just snap but it will rather absorb a lot of energy during destruction. This does not make a difference from an engineering point of view, but I guess on my bicycle I rather have a frame, that gives me a couple of seconds reaction time before completly falling apart.:D

The need is never eliminated, it's pathetic to assume so

An interesting question and I don't know the answer. But most ceramics are fairly brittle. You would want something with fracture strength for longevity. And it must elongate at least as much as the carbon fiber before yielding. Even though carbon fiber is brittle, I think it may stretch more than most ceramics.

@@mckenziekeith7434 i was thinking that the carbon fiber layer would essentially act like reinforce bar like rebar does in reinforce concrete. Where the fiber stop the prorogation of crack. Effectively eliminate the fracture weakness. Carbon fiber has almost no thermal expansion, so i thought that would pair with ceramic pretty well, since ceramic have very low thermal expansion as well.

@@keenheat3335 for sure about the tempco. About the rest I don't know for sure. The idea of a ceramic "glue" with graphite fibers is for sure interesting. It would be an interesting material. I have often wondered if sodium silicate could be used instead of epoxy to wet out composite fibers. Sodium silicate can be fired in a kiln. It is used as a ceramic glaze.

Your thinking of cmc’s.

More details on the aircraft's fuel system here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-kVOEJPPR60U.html

Great question! We made a video about the fuel tanks and fuel system here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-kVOEJPPR60U.html