Is a carbon roll cage impossible?

[PintOfKittens]

Not sure if this has been covered, but when CF fails, it fails spectacularly, throwing CF splinters everywhere!

[JonnyVTEC]

Its not as good as metal in pure compression type loaded, ala roll cage.

The compressive material properties are dominated by the resin rather than the fibres and hence its not really that much better than having a plastic roll cage. Nice bit of homogenous metal is fine, usually steel or T45, even ally alloys are questionable for this application.

[thinfourth2]

Think of it another way.

All materials have roles at which they are good at and things that aren't good at.

Take cake for example, brilliant in some areas but terrible as a rollcage, same with carbon fbre

the driver keeps eating the rollcage

[Glade]

ThomasSoerensen said:

Don't they make car and bike helmets of CF these days?

They should be able to take a bang and still protect.

Edited by ThomasSoerensen on Friday 17th December 15:34

Helmets are designed to deform to absorb the impact.

If you have a big hit you need to replace them, so it's a different type of application.

I think if you drop a helmet from much above waist height it's probably had it.

[tyranical]

Aircraft are being made out of carbon fibre these days, they have had CF parts for years (The tail plane has been CF since the 70's), the boeing 787 is mostly carbon fibre and so is the airbus a350XWB. The wings are all carbon fibre on the 350 but obviously they are only going to get hit from the front.

The A400M has carbon fibre wings, thats a military aircraft and you would have to say that there is a possibility of that being hit from all angles.

[mmm-five]

tyranical said:

Aircraft are being made out of carbon fibre these days, they have had CF parts for years (The tail plane has been CF since the 70's), the boeing 787 is mostly carbon fibre and so is the airbus a350XWB. The wings are all carbon fibre on the 350 but obviously they are only going to get hit from the front.

The A400M has carbon fibre wings, thats a military aircraft and you would have to say that there is a possibility of that being hit from all angles.

Are you sure they're made out of carbon fibre and not 'composites' - i.e. glass fibre, kevlar, aluminium sandwich.

[tyranical]

CFRP.

I work there.

Edited by tyranical on Saturday 18th December 10:18

[flemke]

ThomasSoerensen said:

If not of the woven type then what type are the constructions on monocoques then?

I agree completely that a structure designed to protect should not shatter on first impact.

Don't they make car and bike helmets of CF these days?

They should be able to take a bang and still protect.

A tub will be made from a sandwich of aluminium honeycomb covered by a CF skin on both faces:


A typical wall of a tub might be 10 mm thick, but it's mostly air.

In the McLaren F1 GTR racing car of the mid-'90s, although the car had a completely CF tub, they used steel for the cage:


They also use CF tubs with steel cages in DTM today:

[NightDriver]

F1 cars have used carbon rollover hoops for years - as part of the monocoque structure. When they were first being introduced there were alot of testing requirements to prove 'structural equivalency' to a metal based hoop. It took many iterations, many tests and many material designs (with composites, you are normally designing your material as well as the part with fibre direction, resin choices, cure cycles etc).

I was briefly involved with a GT team who were interested in developing a composite rollcage. The trouble was, to prove the structural equivalency in a test environment would have been horrendously expensive.. Firstly we would have to design the tubular structure (filament winding techniques with the right resin and some glass wound in would have been the theory) and test this - it could have been done in a couple of iterations, or 100... With my experience of composites, it would be 100! Once that is proven you need to look at joining the tubes, mounting them to a vehicle (most likely bonding to foot plates welded/bolted to points on the chassis). This would all have been trial and error testing as well, at first. A huge number of tests, with expensive parts (bodyshells and low volume carbon parts..£££).

The costs of the project would have potentially been horrendous, and way more than they teams yearly budget! They stuck with a steel cage, optimised for stiffness and weight.

[mad4amanda]

RobM77 said:

Obviously F3 (or FRenault now?) upwards have carbon tubs now, and they're often rebuilt after accidents. I don't know much about running carbon cars though, but they might not need much of a knock to need a new tub - not something that a club racer on a budget would be able to do!

Yes Im sure the latest Frenault tub has a CF roll hoop built in and the F3 ones but they are not really tubes. There was a scare when one broke earlier this year either at oulton or cadwell?

[k-ink]

I'd far prefer titanium to cf due to it's toughness. You'd need it to be a bolt in cage obviously. Of course steel is dirt cheap and that's probably why it's used. But it may also have more desirable properties - you'll have to ask some metal experts!

[Scootersp]

someone hit the nail on the head earlier when talking about toughness.......toughness is a recognised and measurable material property and not just a throw away "it's tough" comment.

http://www.ndt-ed.org/EducationResources/Community...


Carbon on the linked curve will be a bigger spike than the High Carbon steel so may withstand a higher load but won't cope with high strain (stretching/deforming of the material).

There is no one material or design for impact protection that is better than all others it's all about the best combination for the particular role.

I'm pretty sure the motorbike helmets are designed with a thin carbon shell with various cell foam below to absorb energy....so you have a very hard shell that provides good one off impact to blunt/solid objects but immediately below this poor impact but high energy dispersing foams......so clever design using a mix of materials to provide protection whilst remaining light enough for the user.

steel/metals can bend elastically (bend underload but then return to their original shape) or plastically ie permenantly bend/dented etc. The plastic range of a metal can be influence by the alloy used and/or production method (forged alloys deform far more on impacts where cast ones usual fracture). I'm not 100% on carbon fibre but it will have little to no 'plastic' deformation it either returns to its original shape or catastrophically fails.....I fish and fishing rods bend enormously and are highly stressed when casting and some occasionally 'blow up' and fracture during casting or playing a fish.....if they survive a stressful cast/fight none of them end up out of shape afterwards.

So for rollcages when factoring cost, ease of fitment (welding) to the car and that the only advantage carbon would bring is possibly a weight saving then I think they are pretty here to stay.

[Serg2K]

Reading this thread I remembered watching a video of a F1 nose cone crash test. To say CF turns to dust on impact is not an understatement.

It will basically expend all its energy dissipation potential on the first impact very well, by vaporising itself. Obviously there is nothing there to help you out on a second impact.

http://vodpod.com/watch/2921194-lotus-f1-racing-cr...

S

[Ultrasound]

Is the reason for the lack of carbon roll cages or the addition of steel cages to carbon cars not more to do with FIA regs than any other factor?

[AnotherClarkey]

doogz said:

The thing about steel is, as well as being very strong, it's also very tough, nice property to have to dissipate energy and save your ass.

Could make it from titanium, but i don't see the point. You're going to have to come up with some sort of interface everywhere the cage is attached to the shell, since the shell will be steel, and you can't just weld Titanium to steel.

It'd also be hideously expensive, and titanium isn't that much stronger than steel, although it's lighter. So if you're trying to build it down to a weight, you'll end up with very thin walled tubing, which will compromise the strength at any welded connections.

This is the right answer. Steel is (usually) ductile and will absorb energy by deforming, not by splintering into sharp fragments.

[rpguk]

Ultrasound said:

Is the reason for the lack of carbon roll cages or the addition of steel cages to carbon cars not more to do with FIA regs than any other factor?

I suspect it's a bit chicken/egg, if there was a demand for CF roll cages amongst teams then the FIA would see they were available but as it is there is no demand.

[Rotary Madness]

I seem to remember reading somewhere that titanium is quite brittle when compared to steel, so even though its lighter and stronger, itll make a big mess when it does start to deform.

My thinking would be, why not use a hybrid system? Steel cage, wrapped in something like CF. The CF provides more strength, but when it does let go, the steel cage beneath alows it to deform in a less catastrophic way?

[k-ink]

Rotary Madness said:

I seem to remember reading somewhere that titanium is quite brittle when compared to steel

I don't know much about it, other than it's use in cycle frames. On my limited quick research it showed up two different methods. The cheap frames has rolled titanium, ie the tubes were formed by rolling flat sheets around, then welding them. The far superior method was drawn titanium - with no seams and hence much stronger. This is why some titanium bike frames are half the price of the proper kit.

But as you say I wouldn't be suprised if plain jane steel was simply tougher overall, regardless of weight issues.

Edited by k-ink on Saturday 18th December 14:09

[NightDriver]

The reason composites can sometimes 'explode' on failure is due to pre-stresses induced during forming. On tubular structures such as fishing rods the fibres are wound in at high tension, at failure this tension is relieved and *boom*.

There's plenty of ways around it though; fibre tension, mould design, material layup etc etc. You can do most things by compositing carbon, it usually just takes a fair bit of time to design the correct material for the parts!

[JonnyVTEC]

tyranical said:

Aircraft are being made out of carbon fibre these days, they have had CF parts for years (The tail plane has been CF since the 70's), the boeing 787 is mostly carbon fibre and so is the airbus a350XWB. The wings are all carbon fibre on the 350 but obviously they are only going to get hit from the front.

The A400M has carbon fibre wings, thats a military aircraft and you would have to say that there is a possibility of that being hit from all angles.

They dont have the same impact concerns as automotive roll cage.

The covers or skins will be UD tape laid in the orientation to deal with tensile loads. The spar will be diaganol bias so it can deal with the shear loads between the skins, etc etc. They very much engineered for the application. A400 d nose has interchangable panel exactly for battlefield damage. The stiffeners on the spar are also still metallic.