leew2
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| posted on 10/5/16 at 09:35 PM |
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Impact absorbing foam front crumple zone
Hi, I am considering building a front crumple zone into my custom built car using layers of EPS foam. I am building with a rear engine and the driver
is seated quite close to the front end, I am going to have a bulkhead at the front end of the "pedal box" after which I want to create a
crumple zone. I reakon I will have around 300mm-400mm of space inside the CFRP/kevlar nose cone to fill with foam.
My current thoughts was to use some EPS foam of around 40km / m^3 density, perhaps in layers with a layer of kevlar between each layer of foam to
increase the amount of protection.
Just wondering if anyone else has constructed such a crumple zone or has any advice on building it. Does anyone know a good source of high density
EPS, similar to that used in a bicycle helmet but in sheet form?
Many thanks, lee.
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Sam_68
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| posted on 10/5/16 at 09:43 PM |
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Would it not be easier to use 2-part foam so that you can just pour it into place?
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twybrow
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| posted on 11/5/16 at 03:59 AM |
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I do this for a living (I have just finished day one of a technical summit reviewing just this sort of issue within the industry), and your proposal
scares me. Hopefully I have just misunderstood, but from how I read it, the space you would expect your structure to deform and fail into will now be
filled with foam, which would result in the bulk of foam being forced backwards in the event of a front end impact.
Composites are fantastic at absorbing energy under high strain rate failure, but they do so through catastrophic failure - lots of breaking is good,
as it all absorbs energy, hence why F1 cars turn to dust with a heavy impact. With your proposal, you limit the ability to fail, and instead you
transfer the load straight into your feet.
If you want to build in greater failure resistance, bulk up your laminate thickness and look at inclusion of off axis plys so that the load dissipates
over a larger area and it transfers around your feet. You may also want to design in failure points where the structure is designed to buckle and
fail.
Good luck. The large OEMs are not using composites in crash structures for good reasons, so I would be very wary of having a go with such a safety
critical part.
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Neville Jones
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| posted on 11/5/16 at 08:34 AM |
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I've dealt with these structures in the past.
It works but is dependant on foam density, and as put above, fibre orientations.
The very rear layer of foam needs to have a substantial layer of either laminate, or metal, behind it, to stop the scenario as Twybrow outlined.
The front needs to be low density, increasing as you go back.
I presume you are orienting the foam layers vertically, with the joints of the layers laterally?
You also need to find fire resistant foam as well, so that the driver isn't killed by toxic fumes if it burns. Breathe the fumes, and it can
take days to finally kill.
After all that 
Cheers, Nev.
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phelpsa
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| posted on 11/5/16 at 10:17 AM |
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The main advantage of the foam over composite is that the number of variables are low, it's pretty easy to predict the result and scale.
It's worth checking out the specs of the standard FSAE foam impact attenuator, that's a tried and tested method that could probably be
scaled quite easily. As pointed out anti intrusion is very important! It might also be worth looking at Ali honeycombe versions.
Having said that we did a composite structure as described by Tim, with staggered laminate to encourage predictable failure, and it was pretty much
perfect first test. There was quite a lot of FE work first and it was still probably more luck than judgement though....
My main concern would be that 3-400mm is not a lot, with energy absorption there really is no replacement for displacement. If you're looking
for something to stop you from 70mph with a reasonable peak g I'd think you'd need a fair amount more.
[Edited on 11-5-16 by phelpsa]
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phelpsa
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| posted on 11/5/16 at 10:25 AM |
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It can also be done quite neatly using carbon tubes of varying length. If constrained well enough they dissipate energy consistently. A similar
(single tube) structure features as side impact protection on all F1 cars.
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twybrow
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| posted on 11/5/16 at 11:49 AM |
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Have a look for the McLaren Mercedes SLR crash cones as inspiration. They were foam cores, overbraided with carbon with multiple passes, and ply drops
to give a tapering section. These were then tufted ro five through thickness reinforcement, and then finally injected with RTM. There are
presentations and papers out there describing the design and process - look for information from Eurocarbon who made the parts.
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Mr Whippy
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| posted on 11/5/16 at 12:05 PM |
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quote:
Originally posted by twybrow
Have a look for the McLaren Mercedes SLR crash cones as inspiration. They were foam cores, overbraided with carbon with multiple passes, and ply drops
to give a tapering section. These were then tufted ro five through thickness reinforcement, and then finally injected with RTM. There are
presentations and papers out there describing the design and process - look for information from Eurocarbon who made the parts.
I'm sure that works fine McLaren but how much testing and development would they have done into this and could something similar that works
properly in a crash to save your life actually be undertaken by someone in their garage?
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v8kid
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| posted on 11/5/16 at 02:01 PM |
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Why not just buy the FSAE foam impact attenuator for $150?
Someone else has already done the sums and testing. Why reinvent the wheel!
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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phelpsa
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| posted on 11/5/16 at 02:35 PM |
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quote:
Originally posted by v8kid
Why not just buy the FSAE foam impact attenuator for $150?
Someone else has already done the sums and testing. Why reinvent the wheel!
Cheers!
Because that's only designed to absorb a fraction of the energy of a full size car. 300kg from 30mph IIRC.
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twybrow
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| posted on 11/5/16 at 07:17 PM |
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quote:
Originally posted by Mr Whippy
quote:
Originally posted by twybrow
Have a look for the McLaren Mercedes SLR crash cones as inspiration. They were foam cores, overbraided with carbon with multiple passes, and ply drops
to give a tapering section. These were then tufted ro five through thickness reinforcement, and then finally injected with RTM. There are
presentations and papers out there describing the design and process - look for information from Eurocarbon who made the parts.
I'm sure that works fine McLaren but how much testing and development would they have done into this and could something similar that works
properly in a crash to save your life actually be undertaken by someone in their garage?
No, see my first post. I think this is a crazy idea, but that should not stop the OP reading up on the way it has been done before. Are you reading my
post above as an endorsement of buying a braider, tooling, robotic tufter and an rtm machine, because I did not suggest that!?
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bi22le
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| posted on 11/5/16 at 07:34 PM |
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What are you trying to protect and at what speed.
Knowing these mileage marathon high mpg cars you wont see over 30mph and they weigh naff all. For this reason I would go with the thought of
protecting through rigidity and strength not dispersion of energy and deformation.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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twybrow
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| posted on 11/5/16 at 08:13 PM |
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quote:
Originally posted by bi22le
What are you trying to protect and at what speed.
Knowing these mileage marathon high mpg cars you wont see over 30mph and they weigh naff all. For this reason I would go with the thought of
protecting through rigidity and strength not dispersion of energy and deformation.
That is a good question. The OP didn't actually state this, and I assumed it was typical road speeds/crash scenarios, which if this is a track
only mpg challenge car, then the requirements might be very different.
I still would not fancy crashing anything that does not absorb/dissipate energy - you are advocating designing a brick, to crash into a brick, but at
30mpg you are still going to be subject to 20G, which plays havoc with your hair if not also your internal organs! You could look at low speed crash
cones for inspiration. These are typically around 300mm long, steel structures, designed to be deformable, that will handle crashes up to around
15-20mph (in a road car) with minimal transfer into the rest of the body in white. So in a very lightweight car, you might be able to push the 15-20
mph to 30mph (remembering that the kinetic energy = 1/2.m.v^2, so double the speed and you quadruple the energy).
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leew2
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| posted on 11/5/16 at 09:33 PM |
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Many thanks for all the comments, quite a few ideas to research further. That FSAE foam impact attenuator is interesting, thanks for that, I will
have a deeper look into them as it is along the lines of what I was thinking, I was not aware they even existed.
Some details I missed from my original post, at the moment I am working on an MK1 car, which is a single seater which will weigh about 80kg and have
an electronically limited top speed of 28mph. I then may work on a mk2 in 2017/18 which will be a heavier 2 seater, perhaps around 300kg with a top
speed of 56mph.
I want to build the main chassis to be rigid and stiff with a solid front bulkhead designed to prevent any foam from being pushed through into the
passenger compartment. I then want to have something which could maybe be described as a giant helmet strapped to the front. A bit like how your
skull is rigid and a helmet is designed to absorb impacts with a hard outer shell (most likely CF/Aramid) and foam liner which is designed to deform.
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v8kid
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| posted on 12/5/16 at 04:11 AM |
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quote:
Originally posted by phelpsa
quote:
Originally posted by v8kid
Why not just buy the FSAE foam impact attenuator for $150?
Someone else has already done the sums and testing. Why reinvent the wheel!
Cheers!
Because that's only designed to absorb a fraction of the energy of a full size car. 300kg from 30mph IIRC.
We're making a lot of assumptions here
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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phelpsa
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| posted on 12/5/16 at 08:44 AM |
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quote:
Originally posted by v8kid
quote:
Originally posted by phelpsa
quote:
Originally posted by v8kid
Why not just buy the FSAE foam impact attenuator for $150?
Someone else has already done the sums and testing. Why reinvent the wheel!
Cheers!
Because that's only designed to absorb a fraction of the energy of a full size car. 300kg from 30mph IIRC.
We're making a lot of assumptions here
Cheers!
Apologies, I forget how diverse the range of projects on this forum are!
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