Sandwich material testing! XPS vs Divinycell

Discussion in 'Materials' started by Jussi_2, Jan 5, 2014.

  1. Jussi_2
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    Jussi_2 Junior Member

    Hi!
    I am going to make some material testing soon. I will take a video, and report how it went. I have choose two material for making Sandwich panels.

    1: Finnfoam(xps) + fiberglass or carbonfiber skins.
    2: Divinycell + fiberglass or carbonfiber skins.

    I wanna see the result that is the Divinycell really so much better, and worth of paying multiple time higher cost.

    I was thinking to make hammer / bending test / how much weight I can load over it.

    If U have any other ideas of how to test these materials U can suggest.

    Finnfoam is super cheap here in finland, and it is very good insulation material, so I wanna test how it could be suitable for hull or deck material.

    I were thinking to use 1220*600 sheets. and maybe 2mm thick skins over it. or just 1mm, that it wont be too strong.

    Have anyone test this xps? So I dont do these for nothing, if someone already has this information? But anyway it would be nice to see it with own eyes.
     
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  2. Jussi_2
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    Jussi_2 Junior Member

  3. rasorinc
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    rasorinc Senior Member

    Jussi_2 Please keep us informed on what your doing as I am very interested in this. Thanks
    Do you know if Finn Foam has a flotation factor?
     
  4. Jussi_2
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    Jussi_2 Junior Member

    In the attachement is some details of finnfoam. What is flotation factor?.. my english is not so good. heh
     

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  5. groper
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    groper Senior Member

    You don't need to do testing... You can calculate it.... The formulas for calculating it can be found if you google " hexcel honeycomb design technology". Download the PDF and read it thoroughly.

    The xps has a lower shear strength than divinycell. Thus the laminate schedule will be limited by the core shear before the same occurs with divinycell. So, reasonably thin laminates can be used successfully before the core shear prevents further increases in panel strength by increasing the laminates schedule.

    You can create reasonably strong panels with it none the less, you simply need to understand why and how it can be done. You will learn this if you search out the above reference I gave and run some numbers to see where and when it fails. I recommend putting the formulas into an excel spreadsheet so you can quickly manipulate them. The relationships then become apparent just as quickly...
     
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  6. SandEngXp
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    SandEngXp New Member

    Xps

    XPS has acceptable mechanical properties for sandwich core material. But I agree the performance you can calculate from Hexcel, DIAB, etc design manuals. Keep in mind to check the test methods are the same for compression and shear values for different material manufacturers.

    1) is has a low operating temperature. 55-60C max with mechanical load.
    2) is not Styrene resistant so requires a barrier coat to laminate with Vinyl Ester or Polyester.
    3) Long term water absorption can be significant for your climate where you cause a dew point inside the core.

    Good Luck
     
  7. DennisRB
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    DennisRB Senior Member

    Cool test. Would this dense XPS be a good choice for a large hard bimini for a cat? It needs to be strong enough to walk on, just for sail pack up duties. Does not need impact resistance etc.

    [​IMG]
     
  8. bjn
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    bjn Senior Member

    Did you make a test?

    Since you started a thread about Easycell I assume that you didn't chose XPS. Why?
    http://www.boatdesign.net/forums/materials/easycell75-closed-cell-pvc-foam-50414.html
     
  9. SamSam
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    SamSam Senior Member

    I don't use any of the stuff, but to spend thousands of dollars on materials based on what some guy did in his garage, who performed tests that can't really be related to an engineered structure and the stresses they encounter....I don't know. Haven't legitimate people done this before with just about any combination of materials imaginable?
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    In my opinion if something is dirt cheap and does the same duty in a particular application as a much more expensive alternative, it will sweep the competitor away quick smart. Which leads me not to study engineering reports about the properties of cheap foams as it pertains to cored boats, I assume those that work professionally in sandwich structures have already done that, and that is why they have not adopted it. I suppose something new might just pop up that does the job, but XPS is not that new thing.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm no expert in the matter of cored structures, though I've built boats that way, but I sometimes wonder whether there would be scope in having a core of varying density, such that the outside had the greater integrity, and the middle the least, or even a laminated core of varying densities where it might save weight and cost, but do the same or similar job to a uniformly high density core. That problem has likely been well considered too, but I'm not familiar with any examples.
     
  12. groper
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    groper Senior Member

    It's like anything, there can be a place to use it and a place not to. The properties of xps allow it to be used in engineered structures such as cyclone rated roofing sandwich panels and truck bodies etc. it's mostly used with sheet metal skins and typical core thicknesses of 75mm and greater for good reasons. It's localised buckling and impact strength are poor due to low compressive strength. It's shear properties are similarly lower than structural boat building cores and thus it can only pass significant shear stress calculations with considerably large core thicknesses. Core shear stress decreases as sandwich panel thickness increases. Thus it could be used in a boat provided enough thickness is used and also if there is no localised impact loads - such as a Bimini roof for example and using reasonably heavy laminate to prevent foot traffic dents.

    Yeah won't find it's much cheaper once you work it all out as the extra laminate costs inc resin etc balance it all out. Same goes for weight, it might be lighter for the same thickness but if you need twice as much thickness you loose what you gained. I can see it's use if sandwiched between a couple thin layers of ply which provide a low cost alternative for lightly loaded structures etc...
     
  13. bjn
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    bjn Senior Member

    I think most opinions about XPS are based on the properties of the standard low density qualities. They have about 32kg/m3 density, a compressive strength of 250kPa, and shear strength of 200kPa. That is relatively weak for a core material.

    But there are higher densities of XPS. And it seems like the relationship between density and strength is not linear. The higher densities seem to be stronger in relation to their density. Like the Finnfoam above, with 42kg/m3, and 700kPa compressive strength. The shear strength isn't listed, but I found a document which has properties of a 45kg/m3 XPS board. 700kPa compressive strength (10% deflection), and 500kPa shear strength. That is about the same as Divinycell H 45 which has a compressive strength of 600kPa, a shear strength of 560kPa at 12% deflection, and 460kPa at 8% deflection.

    If you look at the data for Divinycell, there is also a non-linear relationship. It seems like if you increase the Divinycell density by a factor of 2, you get an increase in shear strength by a factor of 3.

    Groper, if a beam core has half the density and twice the thickness, shouldn't the sandwich become 4 times as stiff for the same weight?
    Lower cost, and less deflection, same weight. Seems good to me. =)
     

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  14. DennisRB
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    DennisRB Senior Member

    I'm still thinking I will make my Bimini top out of it.
     

  15. groper
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    groper Senior Member

    It depends... You have to calculate the shear stress and also other stresses in all parts of the sandwich including the skins which also depends on the skin modulus aswell. All of the equations are located in the Hexel honeycomb design theory PDFs. The design process is also clearly written there. I suggest putting the equations into an excel spreadsheet so you can quickly manipulate the input values and you will quickly understand what you can and can't do with any material in a composite sandwich. It's a well written document and I found it quite easy to understand ... It also lists many typical material properties in the appendix so you can play around with different materials in your sandwich using realistic numbers...

    http://www.hexcel.com/Resources/Dat...eets/Honeycomb_Sandwich_Design_Technology.pdf
     
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