Is cork a bad sandwich material.

Discussion in 'Fiberglass and Composite Boat Building' started by khaos, Feb 25, 2013.

  1. petereng
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    petereng Senior Member

    Graphite ligaments are not really a core but a microstructure. Obviously a core needs shear stiffness but it does not need or want flexural stiffness. I'm not talking about any industry segment. I'm talking about the physical requirements for a core in general. Skins are irrelevent to the conversation. What a lot of people do not know is that thick skins and thin cores do not work. There are rules to meet to make a good sandwich structure. ie the skins have to be very much stiffer in flexure then the core, the shear stiffness has to be adequate and other factors. If you look at the DIAB technical area they publish an excellent book on sandwich design. Many people describe timber, coremat and various things as sandwich cores but they are not, as they are too stiff in flexure so they contribute to the structures bending stiffness. If so they have bending stress in them and this has to be considered. A "core" only transmits the shear strain and therefore we can neglect calculating the bending stress in it. Also many people use a sandwich when a solid laminate would be better. A sandwich has problems with assymetrical loading in which the load only goes down one skin and leads to early failure. This typically happens in highly loaded boat panels across bulkheads. Sometimes called rolling shear failure. Sandwichs are only good in the through thickness direction to make a lighter structure. eg keeping water out of a boat. In aircraft structures sandwich does not always mean lighter!! as you have found out. Boats would have a lot less problems if they were solid in various spots as well. Cheers Peter s
     
  2. upchurchmr
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    upchurchmr Senior Member

    Petereng

    Call it what you want, the Graphite ligament microstructure was sold as and for the purpose of core.
    Of course thick skins and thin cores work. Not for minimum deflection. They are specifically used for maximum impact resistance.

    I have a friend at work who always wants to make up his own definitions of technical words. Usually to restrict the meaning to something he understands.

    Several high quality analysts told me that foam sandwich would not work as a core in a 2'x2' edge supported panel. It would fail in walking loads (200# man on one heel) because the shear strength was so low (they were only use to working with aluminum and fiberglass core). It took 4 guys of 250# without failing. They were only use to using materials which had similar modulus. What they failed to consider was that the significant difference in skin stiffness to core stiffness caused the shear load to spread over a much larger area than the 3" diameter heel size.
    Of course in Aluminum core, the material and foil size allowed the use of extremely thin skins which did not significantly spread the shear area, so for that specialized case they were right.

    There are so many statements you just made that directly contradict standard practice at work that I'm going to quit.

    "Sandwichs are only good in the through thickness direction to make a lighter structure" see the first paragraph above.

    Sandwich structure can be made of anything you want. Your narrow definition of Core does no one any good. Personally I like wheat bread and peanut butter.;)
     
  3. petereng
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    petereng Senior Member

    Hello Upchurchmr, this is not an easy subject and I've only been designing/working with composites for a short 35 years. I assure you I do not make up words and that my words are std lexicon. Sandwichs only improve the through thickness stiffness, they do not improve the in-plane properties. I'm not interested in a word war. The forum is "Is cork a bad sandwich material?" the answer is Yes if you use a proper cork core material in a proper way. The answer is also, No if you use a product designed for flooring and you stick it in a space shuttle. These sort of things need to be discussed in a very tight technical way if it is to all make sense :) Peter
     
  4. rxcomposite
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    rxcomposite Senior Member

    I tend to agree with Peter because what he is saying is based on engineering principles. The other members are also correct based on the application of engineering principles. Without digging into formulas, this can be easily demonstrated. It is all about load bearing.

    Let us call the horizontal center of the core as the neutral axis. This is where it receives the most shear. The more the core bends, the more shear it receives as the upper plane and the lower plane travels in opposite directions.

    To demonstrate, take your narrow cork sandwich and support it at both ends. Alongside place a piece of lightweight rectangular wood say about 1” x 2”. Lay it on its side and support both ends. Place a weight in the middle of each specimen. Both will bend slightly due to weight. Now double the weight both will bend more.

    To reduce excessive bending there are three ways of doing it.
    First method, double the thickness of the cork core. For the wood, rotate it so that it is now lying vertically, on its short side. This is called transfer of moment of inertia where the face/skin is now of greater distance to the neutral axis.

    Second method if you do not want to increase core thickness- Replace the sandwich skin with a higher modulus material such as plywood. For the wood, screw or glue a metal plate on the top and bottom. The face is now taking the stress more than the core and reduces bending.

    Third method, increase shear strength. For the cork, drill vertical holes and fill it with epoxy so that the top and bottom skins are tied together. For the wood, screw or glue a thin sheet (not plate) of metal on both sides. The aggregate (sum) area of the vertical epoxy filled holes increases the shear property of the cork core. The sheet metal on the sides of the wood absorbs some of the shear stress.

    For those who proposes filling up the holes or “dimples” on the cork, you are effectively increasing face thickness as in method 2.

    To sum up:
    1. Bending and ultimately failure is dependent upon the weight (or pressure) it will support (load bearing).
    2. To increase stiffness,
    a. Increase modulus of the skin, transferring the load bearing to the outer skins.
    b. Use a core with higher shear properties
    c. Increase core thickness as far as practicable.

    Low tech or high tech, whatever satisfy the requirement will suffice.
     
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  5. rxcomposite
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    rxcomposite Senior Member

    Consistent with the explanation, a better method would be to extend the "skin" to the sides effectively making a box as seen from the cross section. the rules accept this method.
     
  6. upchurchmr
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    upchurchmr Senior Member

    Petereng,

    Sorry I don't have your experience, only 32 years in Aerospace.
    My primary problem is that the narrow definition of sandwich for a single goal ignores a multitude of different goals and requirements.

    Lots of statements being made are perfectly true from basic physics/ engineering principles, but don't always reflect the needs/ goals for the wide variety of situations in practical design.

    I noticed you still have not talked about the walking load issue which can be a limiting factor in some design cases, including aircraft and boats.
    One caution in a Klegecell design brochure years ago was the potential to leave permanent footprints in a foam sandwich deck if the outer skin was too thin and with a dark colored deck.

    I also talked about analysts who could not understand the effect of differing physical properties compared to what they were use too.

    Too narrow a view will lead you to say things like, "man can never fly". No matter how precisely you limit the meaning of your words you still are cutting off different solutions to real world situations.

    You are right, no real purpose in a war of words, no more comments from me.
    Ignoring the basics can lead to some astounding failures. From that view I agree with virtually everything you say.
     
  7. upchurchmr
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    upchurchmr Senior Member

    RX Composite,

    A few nit picking comments about your list.

    1. Bending and ultimately failure is dependent upon the weight (or pressure) it will support (load bearing).
    2. To increase stiffness,
    a. Increase modulus of the skin, transferring the load bearing to the outer skins.
    Increase the section modulus which includes material type, thickness and for composites the orientation of the laminate if you can analyze that closely.
    b. Use a core with higher shear properties
    You can also use a core that has flexural capabilities -if the material there has stiffness it will pick up load (more load) until something breaks. No sense in ignoring actual capability that exists. This works if you allow the concept of wood as a core. If not I guess we need to invent a new description of a laminate of high strength / stiffness skins over lower density/ stiffness/ strength material in the middle. I.E., glass/ epoxy skins over balsa, cedar or even oak just as an example.
    c. Increase core thickness as far as practicable.

    One problem with all these discussions is the lack of weight as a criteria.
     
  8. SamSam
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    SamSam Senior Member

    This was discussed here a few months ago in this thread....

    http://www.boatdesign.net/forums/fiberglass-composite-boat-building/anyone-tried-45617.html

    about this 3M product (formally Nida-Core, or as Herman says 'Structiso')...

    [​IMG]


    It is made in a rigid foam and a soft flexible foam. Although they only suggest it for RTM, or resin transfer molding, using matched molds, it works with infusion or plain old wet vacuum bagging. Even in a straight open mold layup, I would think it might work.

    Flexible core...

    http://multimedia.3m.com/mws/mediaw...vTSevTSeSSSSSS--&fn=3M RTM Core Data Page.pdf

    Rigid core...

    http://multimedia.3m.com/mws/mediaw...TSeSSSSSS--&fn=3M Infusion Core Data Page.pdf

    BTW, it came up in the other discussion about if those loops were just sitting on the surface of the foam or what and didn't get answered well. The strands of roving go all the way through and when wetted out and cured form solid reinforced pillars between the two sides of the sandwich laminate. They work in compression and tension to resist the various forms of stress the sandwich is subject to. They pretty much eliminate peeling of the laminates from the foam.

    If anyone is familiar with sewing machines, the 3M core strands are 1/2 of a simple machine stitch. The other half would run a thread through the loops and then the "knot" would be pulled to the center of the foam and not be seen.

    Here is a nice technical research on the concept...

    http://www.sciencedirect.com/science/article/pii/S0266353805003441

    .
     
  9. rxcomposite
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    rxcomposite Senior Member

    We did not discuss optimization. Criteria such as productivity, cost, and weight was not discussed but it can be factored in.

    The problem with so many variables happenning at the same time is it hard to predict the outcome. I am attaching again the cored laminate design in case you missed it. It is very flexible to use. If you go to material property section, you can play around with the fiber to matrix ratio to arrive at the modulus of the skin to be used. You can also arrange the core database according to specific weight. That means you can choose the core that gives you the most bang for the least weight. If you add unit cost to the core, and filter the arrangement, and find the cheapest core that will fit the bill. Although the SS is an LR formula based, it will not analyze impact strength.

    I added the cork material but am not sure about the accuracy of the data. Just whatever I can find in the net. Feel free to change it.
     

    Attached Files:

  10. khaos
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    khaos Infusion Padawan

    As the original poster I want to say that, after some initial unhelpful answers :( , this thread has turned to a very informative one. This was more of what I was looking for. :) So many great links that broadened my understanding of core material and how it ultimate use should determine its acceptance. Including end weight, sheer, isolated pressures, and desired skin thicknesses.

    The stitched core study results are surprising to me it seemed like with so many 'pillars' to spread the load that it would have been more durable. However, in some applications that may be a good solution.

    Not every application has the same requirements. So my opening question was/is crazily unframed. So in the end cork may be fine. Or not. :)
     
  11. rxcomposite
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    rxcomposite Senior Member

    The truss brigge has been around for a long time. They were applying the correct principle.
     

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  12. Eagleburger
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    Eagleburger Junior Member

    Thanks for all the info.

    Suck and see, that is my approach. I will be picking up some cork soon for vacuum infused radio control aircraft undercarriage. Low absorption, flex and memory make it desirable to try. It is very wet here so I dont know when it will happen.
     
  13. petereng
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    petereng Senior Member

    upchurchmr - aircraft flooring is not a trivial problem. I do fea (composite and metal) most days of the week for various structures from automotive to aerospace to electrial equipment. I've learn't not to say "it won't work" from the numbers. I have a perpetual struggle in getting accurate material properties. Testing composites takes well over $2500AUD to characterise a single composite ply. So it takes a lot of money to generate the base data if you have many different materials in the stack that you are dealing with. But I've been doning this ffor a long time so I can estimate pretty close. Commercial aero floors have o deal with haevy feet and stilletto type shoes so I understand the delemas. For those out there that do calculations the only way to model sandwichs correctly is to use a solid brick element for ther core and a contigous plate element for the skins. preliminary work can be done entirely in palte elements but at the end of the day you need to move to the correct approach. Cheers Peter s
     
  14. Herman
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    Herman Senior Member

    Just from a practical point of view: one of my customers is making canoes with 3mm corkcore, and it holds up nicely.

    Besides the technical aspects, you sometimes also need to look at other factors, like price, and workability. If you need to apply 3mm PVC foam to a short radius mould all day, you will start to like CorkCore... PVC foam snaps, or needs heat, CorkCore can just be slapped in place.
     

  15. ASM
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    ASM Senior Member

    Core cork is being used widely in the yachting industry. We have introduced it as a core material in places where acoustic enhancement is needed. TH damping loss factor of a cork core composite is higher than a foam cored one, we have tested this via impact hammer tests. It is true that infused panels do loose some of the damping because of rigid bridging between the outer skins, but yet it outperforms foam and balsa core. There are several types of core ranging from 110 - 400 kg/m3, latter one being used for engine foundations mostly. Core cork is a green product (trees are harvested not cut down... and live for 250 yrs) and in combination with bio resins it could take over a large portion of the composite boatbuilding world.
     
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