Carbon over honeycomb

Discussion in 'Composites' started by Scheny, Apr 23, 2019.

  1. Apr 23, 2019 #1

    Scheny

    Scheny

    Scheny

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    Hi!
    Recently I read a paper about asymmetric composite thickness optimization, where it said you should use a coupling layer as ort of bonding help over honeycomb. They used an additional layer of 80g/m² of glass as it is "loosely woven" and therefore has alot of excess resin.

    I checked some planes including the Extra 300 which is said to be one of engineerings finest and could not find any similar laminate.

    Only thing I could find was a coupling layer which was utilized over prepreg carbon, as prepreg seems not to stick too well to hneycomb on its own.

    I would be interested if any of you uses a coupling layer and why? (weight is of essence, so would rather not)

    Best regards, Andreas
     
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  2. Apr 23, 2019 #2

    wsimpso1

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    I have never played with honeycomb. Divinycel foam cores are way robust in my home workshop. You may be looking at production and can justify the process development you will be going through.

    80 g/m^2 is a little heavier than 2 oz cloth. Saturate that with resin to make the bond and use it on both sides of the core, and you just added 320 g/m^2 or about 9 oz/yard to your part while being almost meaningless for stiffness and strength in a graphite faced laminate. That puts the combined core weight (core plus glass/resin) on the same order as foam cores... You will want to do some tests to see how your process works out on that before committing to it.

    The literature on composite construction usually advocates using a film adhesive between honeycomb and pre-preg, and has to be lighter than having an additional layer of cloth on each side of the core.

    Billski
     
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  3. Apr 23, 2019 #3

    Jay Kempf

    Jay Kempf

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    Film adhesive is the safe way to do honeycomb. It is very hard to control liquid adhesives around honeycomb and so without a great process you will use up all of your advantage. Foam can only absorb so much resin and can be done in one bagging operation. If you had infinite funds for tooling you could CNC cut and mold and inner and outer skin, spray adhesive on them, then vacuum bag them together over a core in one of the molds.
    Rutan started using a geometric pattern of ribs towards the end of his career at Scaled. He would rip a trapezoidal shape, put it inside the outer skin on a hex pattern, then bag strips of composite over the ribs to stiffen a skin. When I saw this the first time I stored it away as a potential use for stiffening a fuselage shell. Not sure about using it for wings but maybe. Spacing of the stiffening ribs is sized to ward off buckling. It's sort of metal thinking in composites but still clever.
     
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  4. Apr 23, 2019 #4

    Scheny

    Scheny

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    What would be the "official" process for honeycomb? The outer layer can be done in vacuum infusion, then you would wet it again and add the sandwich with vacuum. But then what? The outer layer is added wet with vacuum I guess, I'll ask our composite guy how they did on formula cars.

    As for Divynicell, Rohacell etc. I am not sure about how good you can drape it in 3D form. Otherwise my composite guy also favoured it.
     
  5. Apr 23, 2019 #5

    BoKu

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    Some general notes and comments from a composites guy who has seen but never had to use honeycomb:

    * Honeycomb appears to be something akin the price tag on a yacht: If you have to ask about it, you probably shouldn't be using it. Or maybe you should be paying someone else to use it on your behalf.

    * The honeycomb I see the most of is used in the fuselage of the ASK21 two-seat training sailplane. It's there to stiffen the fiberglass monocoque fuselage shell in highly-loaded areas. What I've been told (and this should be taken with a grain of salt) is that Schleicher's production artisans (you can afford to have those when your country has national healthcare) developed a variety of techniques for using honeycomb in hand laid ambient (no vacuum) cured shells. Apparently they'd lay up the fuselage in the warmer part of the day, make the last ply a bit soupy, then put down the honeycomb. Then, as the shop was cooling down they'd put down a saturated closeout ply of fiberglass over the honeycomb. As the air inside each honeycomb cell cooled, the pressure would decrease, helping pull the closeout ply down onto the honeycomb and the honeycomb down onto the base plies. That's all well and good when you've got the process dialed, but repair shops are pretty uniformly unhappy with trying to repair the stuff.

    * If it was me, I'd definitely use some sort of Divinycell core foam. You can buy it pre-scored into squares with a mesh backing, and that appears to conform reasonably well to shallow compound curves.

    * Where the curvature is more acute than the pre-scored stuff can handle, I'd experiment with prefabricated triangles and hexagons of Divynicell in a vacuum-bagged wet layup. I'd probably drop-ship sheets of Divynicell to someplace with a waterjet cutter and have them zap it into something like 2" hexagons. It would be a bit of a puzzle to fit them together during the layup, but probably easier than trying to mess around with honeycomb.

    --Bob K.
     
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  6. Apr 23, 2019 #6

    BoKu

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    In the airliner parts I sometimes see, the general layup schedule appears to be:

    * Pre-preg outer plies
    * A sheet of film adhesive
    * Honeycomb
    * Another sheet of film adhesive
    * Pre-preg inner plies
    * Vacuum bag consumables and membrane
    * Cycle under heat and pressure in autoclave

    The heat that cures the pre-preg laminates also softens the film adhesive so that it bonds the honeycomb cell ends onto the laminates. When done right, the adhesive gets soft enough so that surface tension forms a smooth fillet between the cell ends and the adjacent laminates. The special materials and consumables are pricey and the autoclaving is pricey, but the results are consistently very good, and high-end aerospace companies seem perfectly happy to pay for it.
     
  7. Apr 24, 2019 #7

    Scheny

    Scheny

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    Thanks Bob, this was very enlightening!
     
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  8. Apr 26, 2019 #8

    flyboy2160

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    If cost is no object, you can use what's called 'Flex Core' to get the HC to conform to curved surfaces. But it's still far cheaper than machining solid blocks of the stiff HC.
     
  9. Apr 29, 2019 #9

    PiperCruisin

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    Have used and specified honeycomb (nomex and aluminum). Film adhesive works as BoKu described it. The not-so-fun part of using honeycomb core is the 2:1 to 3:1 taper on the edges. Foam core is easier to put in the taper.
     
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  10. May 18, 2019 #10

    foolonthehill

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    We use Gurit M80 4mm core exclusively in the Arion Lightning, and other products we build. We do infuse just about everything, although we do some wet layup vacuum bagged parts. The Gurit is double-scored, which allows it to lay down nicely on compound cured surfaces, and provides great transfer channels for the infusion process. If I were using pre preg again (since 1981 and up until 2015, I'd never made a part that wasn't pre preg!) I'd certainly go with honeycomb core. For most folks, though, the Gurit is worth looking into.
     
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