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 An other crazy idea regarding sandwich composites
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Balor
human power supergeek

Russia
1164 Posts

Posted - 06/10/2021 :  14:37:22  Show Profile  Reply with Quote
I've already did some EVA-core composites and it seems to be pretty nice - if you take something comparable to density to typical XPS foam (bit heavier, but much more resilient!) it also works like foam sandwich, maybe a bit flexier, even with only 5mm thick core.

Now, a question - how flexy can you actually go?

I've been thinking about recycled foamed *polyurethane* core.
No, not as soft as seat cushion of course, but there are pretty tough ones used as, say, exersise mats - very high resilience!

Not sure about english sources, here is a local one, use google translate if you care enough:

As far as I understand, PU foam is chopped up and bonded under pressure, creating a foam that is denser and semi-closed pore (the finer then initial materual - the more closed the structure becomes, obviously).

It is not termoplastic (so no vacuum-forming as much as I understand), but PU has great affinity to epoxy and core bonding should be absolutely top-notch, I've run a few experiements - and unlike XPS foam should be capable of glueing with contact (solvent-carrier) adhesives that otherwise attack XPS (same goes for EVA-core though, but eva is not anywhere near as resilient as PU).

What's particularly good for me, being recycled materual it's very cheap for what you get. (1cm thick, 2000 by 1500cm large sheet for basically close to 5 or 10 bucks, depending on how dense the foam is).
I wonder how low in core stiffness can you go when it comes to non-stuctural... or maybe even structural cores?
How thick can you go? Are there any guidelines?

Balor
human power supergeek

Russia
1164 Posts

Posted - 06/10/2021 :  14:38:24  Show Profile  Reply with Quote
Link got lost somehow, here it is:
https://foamline.com/catalog/specials/item/vtorichno-vspenennyy-ppu-rbd/
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Balor
human power supergeek

Russia
1164 Posts

Posted - 06/10/2021 :  14:50:06  Show Profile  Reply with Quote
What I've also been thinking- Aerovelo been using elastomer mounts for the upper shell, to prevent road vibrations killing laminar flow.
Can same effect be had by using relatively thick core of soft, yet resilient material? Of course, would not work for BM liners, they need every millimeter of area, so thick core is not an option, but for something streetable... well, likely something streetable will not benefit from laminar flow design too much anyway (it would be very hard to maintain it even at much lower speed than at BM I presume), but a vibration isolating, *heat-isolating* and highly resilient (even if a bit on flexible side) shell might be better fit for a streetliner.
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Balor
human power supergeek

Russia
1164 Posts

Posted - 06/10/2021 :  15:54:30  Show Profile  Reply with Quote
An interesting link regarding graded core stiffness and impact resilience, btw:
https://link.springer.com/article/10.1007/s10853-018-2799-x
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warren
human power expert

USA
6742 Posts

Posted - 06/11/2021 :  07:04:00  Show Profile  Visit warren's Homepage  Reply with Quote
John Tetz and some other folks built very nice foam shells a few years ago. They used Zotefoam which isn't cheap but you could use other similar materials.

http://www.recumbents.com/mars/pages/proj/tetz/VFS/projtetzVFS00intro.html
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Jeroen s
New Member

Netherlands
60 Posts

Posted - 06/11/2021 :  07:36:46  Show Profile  Reply with Quote
If u ad a soft compresable material in the mix, like an elastomere it provides a spring. Elastomere also has a damping effect. I think it would be a lucky shot if u hit the right vibration frequency. If u ride over the same surface at different speeds, it will vibrate differently. If u are unlucky u can hit a frequency where the rebound and new movement start to combine in to one bigger movement.

I think it would be difficult to build a reliable soft foam and hard material sandwich. U need a vacuum and then something to prevent the foam from deforming.

There have been several Foam/cloth fairings back in the 90ies in the racing scene here in Europe. The cloth provides a rub resitance cleanable outer surface. It also can take some pulling strenght. The foam keeps the shape right and prevents deforming or flapping in the wind.

That soft, camping mat foam can take little more than its own weight. At higher speeds it will deform, unless it has a support structure.
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Balor
human power supergeek

Russia
1164 Posts

Posted - 06/11/2021 :  11:39:46  Show Profile  Reply with Quote
As far as I understand, EVA foam is pretty much the same as Zote foam, and yea, not cheap in higher thicknesses (about 1/2 inches like Tetz used) - recycled PU is 2x cheaper at the very least.
Camping foam is PE foam. The crosslinked variety like Zotefoam due to matt surface actually has pretty damn good adhesion to epoxy, and EVA foam has even better adhesion.
Like I said, with harder grade of foam, resulting sandwich is really stiff:
I've made a sort of 'potato chip seat' using a layer of ud carbon (got dirt cheap from aliexpress)/merely 5mm thick EVA foam/GF sandwich and it is pretty damn stiff, even w/o stiffening ribs! In fact, even stiffer than I may need for non-structural fairing I suppose!

My idea is to (which takes WAY longer than planned) have a male mold (in two or likely 4 halves/quarters) cut from expanded foam, cover with PE film, drape cloth over it, epoxy and let it set, adhere foam to it - the harder part, I really hoped for a vaccuum forming over a male mandrel but it turned out harder than we planned), than cover it again with cloth completing a sandwich... also cover it with aluminium foil before epoxy fully sets, for better surface quality AND heat reflection (and some additional visibility). Evev if you paint it with something afterwards, heart-reflecting (IR) properties will not be lost. Foil does not have very good adhesion, you can peel it away if you really try to - but it will not peel away on its own.

But the 'graded foam stiffness' gave me an idea. If you want a HPV that can take hard knocks and provide impact absorbtion, than I think an interesting idea would be to make a double sandwich - an inner, stuctural one (in case of structural fairing), and outside made of low-stiffness, but high resilience foam and relatively light GF/basalt/kevlar cloth that is also very resilient - it will not provide much to actual overall stiffness, but will serve as shock absortion outer layer in case of falls and impacts.

I will experiment with that using scraps of material and simulated impact testing... (likely a highly scientific hammer blows)

Edited by - Balor on 06/11/2021 19:17:10
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warren
human power expert

USA
6742 Posts

Posted - 06/11/2021 :  15:32:35  Show Profile  Visit warren's Homepage  Reply with Quote
The Zotefoam fairings did hold their shape pretty well and were reasonably crash resistant. They did get a little scuffed up in a crash. Tetz was able to form the foam over a male mold using a heat gun I think. I remember racing with somebody racing one of Tetz's faired bikes and it crashed in a hard corner but the rider escaped any road rash and the fairing was only abraded in a minor way.

The PE foam will soak up a lot of epoxy, so if you do make a fairing using that as your core material, don't expect it to be light! It should be very tough though so if you are going for impact resistance that should do the trick...
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Balor
human power supergeek

Russia
1164 Posts

Posted - 06/11/2021 :  19:31:14  Show Profile  Reply with Quote
PU you mean? I've already made a similar mistakes. Yea, it makes sence due to semi-open pore structure, but like I said going for higher density foam compared to typical seat/matress foam.
I've also been thinking about memory foams, not cheap at all but super damping properties. Can they be used as a part of foam sandwich composite leaf spring as dampers?
Unfortunately, as far as I understand, unlike typical PU that retains most of its properties in wide range of temperatures, memory foam is higly temperature dependant... and very squishy, too.

What I intend to do first is use some thin packing PU foam I've got for free and make some fenders with basalt fiber.

A test piece (just two layers front and back) is reasoably stiff and completely indestructble! You can bend it nealy in half (with effort) and it just springs back. You can see "core shear deformation" taking place allright, but it is 100% reversible.
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Balor
human power supergeek

Russia
1164 Posts

Posted - 06/12/2021 :  09:27:36  Show Profile  Reply with Quote
I've been thinking: why am I limiting mysef to PU or Eva? There are other elastomers and their foams.
https://www.hepako.de/en/blog/elastomer-mechanical-properties-and-how-they-are-measured/

According to this article, it seems that CR (cloroprene, neoprene) has the best balance of mechanical properties.

https://www.researchgate.net/project/Investigations-on-the-sandwich-composite-beams-and-panels-with-elastomeric-foam-core
I've also found this! Makes a lot of sense.

...
Well, I know why am I limiting myself to PU or EVA - costs :(
Btw, the in the article foam used is EVA foam with 200 kg/m3 density.

Edited by - Balor on 06/12/2021 17:44:43
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