XPS core again

Discussion in 'Materials' started by Owly, Sep 27, 2021.

  1. Owly
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    Owly Senior Member

    This topic has popped up here and elsewhere repeatedly, with the same objections repeated again and again, some I feel are valid, others less so. As a sandwich core "peel strength" is not the issue...... Nobody is going to grab some overhanging end and try to peel the glass away from the foam. Shear strength IS the issue. A sandwich construction requires the core to operate as a sheer web between two load bearing faces. The bond matters in shear. The foam itself is quite strong but will fail when bent beyond it's elastic limit....... but so will Divinicell. By the time you reach that limit, you have deformed it far beyond what it will experience in real world application short of a major wreck, where most materials will have failed also.

    A foam core sandwich operates like an I beam. The surfaces carry the loads in compression and tension, the foam joins those two surfaces in shear so that they work as a uniform beam or panel.

    I propose a couple of possible ways to enhance the connection between the two surfaces beyond just the strength of the foam. My favorite is to puncture the foam in a grid with a device similar to a gang of sewing awls, each needle carrying strands of fiberglass thread, the tails of which would extend such that they will lay beneath the glass when it is applied. In experiments using an ordinary sewing awl this stops peel entirely..... again, peel is not a real world scenario. The pieces I've made fail in buckling on the compression side... the foam will wrinkle and the glass will delaminate, but this is with extreme bending and thin glass skins.
    Proposal 2 is to use planks edge bonded together, with glassed edges, which essentially act as stringers. This could be done easily and fairly quickly by ripping the foam planks out of 4x8 sheets, lining them up edge up on a table with cardboard spacers, and applying glass on the bias, then cutting them apart living a small scrim of glass fiber (without resin) to join the surface plies of glass. This would give you a very positive connection between the two faces when assembled.

    The low cost for very thick foam makes this an attractive core material. It will yield a very stiff structure because of the thickness.. 1" is the thinnest I can find locally. The insulation value of thick foam core for both thermal and sound would be of great value IMHO.

    It is strong in compression if distributed....... it does not take point loads well. This can be mitigated as others have by applying a thin ply layer, or simply adding a layer glass mat, and or more layers of fabric where actually needed.

    I'm hoping to begin a "small" boat project... 18' tri.... this winter, and utilize blue XPS as the main material. I would not do this if I didn't think it would work. I've built fiberglass XPS sandwich projects in the past, though small.

    The objections I have read, are all the same, and virtually none seem based on actual use..... pretty much hearsay.

    H.W.
     
  2. cracked_ribs
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    cracked_ribs Senior Member

    Curious to see it done.

    Bill Schwabe of Lightning Yachts told me once that they found peel strength with XPS to be basically equivalent to Divinycell and that the paddle boards his employees made from it held up well, but obviously they'd never use it as structural core in one of their boats.

    Of course, if you paid Lightning Yachts money for a boat and discovered they'd put XPS in the hull, that would probably be the end of Lightning Yachts. But he thought it was not worth the savings for most applications given what could be done with plywood and considering the percentage of the budget the core makes up. Still, he wasn't nearly as dismissive of the concept as I would have guessed, and it's not like he doesn't have a hundred times as much experience as any amateur builder, that's for sure, and specifically focuses on light weight composite construction. If anyone would know, you'd have to figure it'd be him.
     
  3. Owly
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    Owly Senior Member

    That's pretty interesting....... What I hear most of the time is stuff like "XPS has no place in boat building", "it isn't construction foam", and various ignorant statements to that effect and similar. One of the big drivers of this is the confusion between XPS and EPS, both polystyrene, but very different. EPS is the stuff garbage products like throw away food containers and coolers is made of. It is thermally expanded beads done in a mold. XPS is the closed cell extruded product, generally found in blue or pink, far stiffer, and far stronger.
    The peel strength comment was very interesting..... but again I don't consider peel strength a huge issue considering the usage. One surfboard builder on this forum I think, advocated sanding with 20G sandpaper to scar and pit the surface, and I've read other similar advice. Most of this boat cost will be core, glass, and epoxy. With ply construction I could get away with polyester resin as this boat will not spend much of it's life in the water... it'll be in the water when being sailed, otherwise it will be on a trailer. The thickness of the core will result in very stiff hull sides and bottom, which means no stringers needed, and fewer bulkheads. Wood will be used in a few strategic places.
    I'm an aviation guy from way back, and admired the work of Burt Rutan who designed his EZ series aircraft with XPS foam core wings. Designed in the early '80's, I have yet to hear of a failure.

    H.W.
     
  4. upchurchmr
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    upchurchmr Senior Member

    Owly, you don't know nothing.
    An important material property for foam core is shear stiffness - as opposed to strength.

    I one time made a XPS sandwich with glass/ epoxy skins and measured the deflection.
    It was very little better than 2 separate skins of glass bent together.

    A sandwich with a boat building foam (can't remember which) had 1/4 or less deflection.

    If you are willing to have your boat hull deflect a great deal, then go ahead. Or if there is another load path to take the shear then OK.

    The reason a surfboard works is that the strength and stiffness is in the outer skin. The skin on the sides takes the majority of the shear stiffness (and strength). The core in this case does very little for bending - but it does something of course.

    You are right this keeps coming up. And you keep getting it wrong.

    The only thing you have done with your idea of putting glass on the edges of core is to reinvent glass honeycomb. Take out the foam and the product will be lighter.

    Did you ever work with Rutan? I did.
     
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  5. fallguy
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    fallguy Senior Member

    Sadly, the xps thing keeps coming up because people still fantasize about its capabilities.

    The 'stuff' has 2 main problems and a third process limit.

    1. Low shear strength. There are higher density xps foams available that will work better. This, btw, is also the peel problem. Unless Rutan published the shear ratings of the xps he was using; we need to be very careful because it is possible the industry reduced the density of xps since Rutan's famous video. Also, a caution, Rutan's work was not on thin cores iirc. A thin section of xps will delam far faster than a thick one in any dynamic setting. This idea can be used in boat building a bit.

    2. Low compressability. This is a problem for certain applications. Especially underfoot. Stomp on a piece of it and see what happens. Bump it against a dock and it will smash easily. Then, for any vessels traveling at speeds, hydraulics will take over.

    3. For manufacturing, how it behaves with esters. Melting is not worth the trouble.

    All that said, xps has a great place on boats.

    It is an excellent insulator and has low water absorption.

    It works great for amas.

    It can probably be used as a core between ply panels well. Not going down that rabbit hole.

    Inside the boat; there are certainly places it could work. A bulkhead, for example. Another rabbit hole.

    I am the person who recommended scuffing the surface for use as amas. The reasoning is simple. If you make more surface area, the shear strength improves and the bond, done correctly, does as well. But in order to advantage a rougher surface, the surfsce must be puttied or hot coated before glasswork. Otherwise, glass standing on high ridges of xps are even easier to shear away. This also thickens the glass shell and makes it harder to compress.

    As for creating a shear web, fine. Do it. But I'd still want to use a better xps foam, higher density, lower compressability, higher shear strength. Unless you are insulating a cooler or a ceiling or building amas.

    In the case of amas, the xps is really only a mould. It you run it into a tree; you will be fixing it.

    Xps is unfriendly on the table saw, be careful ripping it. It loves to skate off sideways...lots of experience avoiding sawing it!!

    As for the strip building with it...I think it sounds like a hell of a lot of work to develop an inferior hull. Yes. Inferior. The savings on the hull will be about say 100$ per 32sqft. For an 18' hull, I see about $600 savings or a grand. So, for me not worth it.

    pay some attention to the critics..I tried to bond a piece of 1708 to unprepared (no rough sand) xps (2'x3') and it peeled off easier than paper backing on a sticker..In fact, I used peelply, and pulling the peelply delaminated the edges, so I had to pull test the other side even. If the product can't withstand peelply; that ought to tell you a thing or two.

    Here are some variations on xps that highlight differences among 'highload' types. Standard xps pink has a compression rating of 30 psi only.


    https://www.dupont.com/content/dam/...e-Building-Solutions/literature/179-02548.pdf
     
  6. skyking1
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    skyking1 Junior Member

    The high load line of XPS is more costly, but has much greater compression and flexural stiffness. The highload 100 is 4 times stronger in compression and twice as stong flexural strength then the blue square edge that everybody dreams of using.
    https://www.dupont.com/content/dam/...ighload-40-60-and-100-pis-43-D100079-enNA.pdf
    None of that fixes the shear strength. It will be splendidly strong right up until it isn't.
    I would not risk a hull design with it, but have seen some really nice pickup camper shells and the like. For that I think it is suited well. The pickup is not going to break in half and sink on you if it fails in shear ;)
     
  7. upchurchmr
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    upchurchmr Senior Member

    There is a good place for the stuff.
    Just not where you want strength and stiffness.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    It is used in many structural applications, it is just a matter of the load that will be need to be carried.
     
  9. Owly
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    Owly Senior Member

    "don't know nothing"??? No I can't claim to have worked with Burt Rutan...... though I did attend one of his workshops where we built a wing. The core material was 2 lb XPS foam........ Obvious in your opinion a grossly inadequate core. We hot wired it to shape, laid unidirectional fiber down the groove for the spar structure, etc. Obviously you know something I don't...... Perhaps a series of wing failures and crashes I have yet to hear about... perhaps airworthiness directives? Lawsuits? The ONLY structural failure I know of was a quickie........ which has the landing gear at the tips of the canard which was landed so hard it failed the canard wing.

    All your dire warnings seem to conflict with what I've seen and with the results of tests I've done over the last few years...... It's interesting that even daring to suggest use in a small boat results in a veritable dog pile which I find interesting............ though really not very informative. Nowhere on the internet have I found and account of someone who actually ignored all these dire warnings and did it.......... and had it fail.... not even an umpteenth removed account..."My drinking buddy's former brother in law's co-worker's uncle's cousin 4 th removed tried it and his boat broke up in rough water and he drowned according to a Gypsy woman who contacted his spirit in a seance..."

    H.W.
     
  10. Owly
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    Owly Senior Member

    How you use a material is key of course.... as has been said here. I'm well aware of the higher density XPS foams..... but they are not easy to come by, and they are also heavier. In the context of what I plan to build, the dimensions of the panels including thickness, I think it will prove completely adequate. It only makes sense to design according to material properties, and needless to say there will be multiple materials used as appropriate. I have built a few sandwich structures using epoxy and blue XPS, some of which were test panels, and a couple are things I actually use....... The performance I've seen does not match the dire warnings I'm hearing from some people here.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Aside from the poor chemical resistance issue, what would be the main shortcomings of a higher density polystyrene foam ? Poor shear strength ?
     
  12. fallguy
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    fallguy Senior Member

    Actually, the shear ought to improve. Simple physics that a material with more stuff in it will be harder to pull apart. The downside is just that it is heavier. But 2# foam is still just a weak foam at the end of the day.

    The idea of an epoxy/cabosil shear web is fine. The problem is that the core is still highly compressible. Denting the hull bottom on the ground or a trailer which is more than common in a boat does not happen in an aircraft, but would happen almost certainly on an xps hull. If you wanted to try the xps on amas or hullsides; that would be okay, but the bottom is asking for trouble unless the outside skins do significant work.
     
  13. fallguy
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    fallguy Senior Member

    Actually, the shear strength will improve as the density increases. Not to overstate things, a heavy powder has no shear strength, this is relative to xps density...
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    So the compressive strength seems to preclude any application where point loading on a thin skin might happen, even with the heavier foam ?
     

  15. fallguy
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    fallguy Senior Member

    The main issue would be the hull on a rock on the beach or hitting a floating log at speed where the forces are greater, or a vessel designed for higher speeds and forces of wave impacts with poor core characteristics.

    A 200# man standing above a rock in the boat on a beach could theoretically be putting a point load exceeding 200 psi, and the xps is rated at 30, so the loading is about 10x the materials capability. That said, many hulls would fail the same test. How much becomes the next question and how fast does the hull travel, etc. because the next phase of failure is done by water pressure.

    My boat, for example has a solid glass beaching keel with wood sacrificials to deal with these potentials. A deep shoreline and I'd be impacting a solid glass section of the hull. Skegs and strakes can protect a more fragile hull as well. During my build I dropped a couple heavy items onto deck or hull and did delam the boat a bit from point loads striking the hull. So careful attention as well must be made to the walkable decks if one uses xps for a hull, not from dropping things, but walking or jumping. If one lands on the ball of the foot and say that is 3 sqin, 200#/3 is 67# without any jumping, so the xps rating is too low for even walking, for example.
     
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