High shear elongation cores, hype or real world use?

Discussion in 'Materials' started by idkfa, Jan 8, 2011.

  1. idkfa
    Joined: Sep 2005
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    idkfa Senior Member

    Been trying to understand, got the PB#51 article etc.

    What is failure? surviving a bashing with the dock during a storm and having the core go into plastic deformation, is great, boat will not take on water but still needs to be repaired, the skins; both the fibres and resin will have cracks, even if still adhered to the core.

    Will dropping a hammer on to a thinly skinned, (600gsm) balsa cored deck, not experience less damage than with any (lower shear) foam cored deck?

    Do the slamming loads of a power boat require a core that has high shear elongation? Is the hull designed to flex? Is fatigue a failure at the bond line?

    Is there a standard test, say dropping a 5Kg steel ball, 1m onto 1msq panel supported at all sides? We use hydromat for fatigue testing.
  2. CatBuilder

    CatBuilder Previous Member

    In the real world, I'd define "laminate failure" as a debonding of the core/skin, cracks or other structural problems with the skin, or a disintegration of the core (in the case of friable cores).

    So yes... I think the outer skin has to be taken into consideration when you are talking about "failures."

    However, when comparing cores, I think most people end up assuming the same skin on all theoretical cores to simplify the problem.

    A very hard/brittle skin on a squishy core like core cell would result in easier "failure" due to impact than if you used a more rigid core. Of course, to make it easier, I am also ignoring the kinetic energy absorbed by a core vs. transferred directly through a core, which would be important as well.

    It's a very complex problem, but could be analyzed to completion by someone... somewhere... who had the time.
  3. Herman
    Joined: Oct 2004
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    Herman Senior Member

    In my opinion there are 2 failure modes:

    -failure where STRUCTURAL repairs are neccesary. (I am not counting a good buffing or slapping on some paint)

    -failure where INTEGRITY of the boat is jeopardised. (as in: a hole in the hull).

    Further, most hulls are designed for panel stiffness. To compare different options one should design them with the same stiffness, then test them with the same impacts.

    This will not give theoretical data, but data which come close to real life data.
  4. War Whoop
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    War Whoop Senior Member

    Cat here you may want to rethink the Squishy thing.

  5. CatBuilder

    CatBuilder Previous Member

    That's just a piece of foam though, right?

    Put a skin of brittle glass/epoxy on there and you'll get the damage I'm talking about because of the large deflection.

    If the skin is very hard and brittle, while the core is soft and squishy, you'll end up withe a cracked skin in the test in the video.
  6. War Whoop
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    War Whoop Senior Member

    No it does not work that way.
  7. TeddyDiver
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    TeddyDiver Gollywobbler

    I know we had a "conversation" of this very subject earlier but I really like to hear how it works :confused:
  8. War Whoop
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    War Whoop Senior Member

    Do not ever confuse squishy like foam rubber with a "Tough" material like a highly engineered structural foam.

    Let me get this straight the core should shatter at point of impact as well and also transmit the the load directly to the inside skin destroying that as well?

    The correct answer is the Core adsorbing and transmitting the load over a large area on the inner laminate,some coring can do this and others fail. some are cheap and inferior and the others that work ,you pay the price for.
  9. CatBuilder

    CatBuilder Previous Member

    There are two different phenomenon at play here:

    1) Transmission of the impact force

    2) Deflection of the outer skin

    My comment is only discussing deflection, which would be greater in a more "bendable" or pliable foam than say... balsa, which does not deflect, but does transmit all of the energy through the core and to the inside skin.

    So talking *only* about deflection-based damage, for the moment...

    You have a core that deflects exactly like in your video above. Let's say it's Core Cell, which it is. (everyone take a look at the video)

    If you put a hard, brittle skin on that core and it deflects to anywhere near the same amount in the video, the hard, brittle skin on the outside will break. Why? The hard, brittle skin can't be deflected like that without breaking. That's all I'm saying. I might turn it around and ask you how a hard, brittle skin will not break when bent to the same degree as that foam in your video?

    It's a question of over-deflection in this case because there is less to support the impact, as the foam gives way.
    Now back to phenomenon #1...

    If the outer skin of our laminate is "softer" and less brittle than before (is that high elongation epoxy?), the outer skin will bend with the foam, the inner skin will bend with the foam and the forces will do just like you say. Instead of cracking due to extreme deflection, the skins will bend with the foam and energy will be absorbed and dissipated. This is the best scenario.

    Balsa performs very poorly in this scenario of transmission. When it's hit, the impact transfers directly from the outer skin to the core, straight through the core and to the inner skin, breaking and debonding the inner skin. This is the case of a harder core material.

    What I'm saying is all things are trade offs on boats (the old rule still holds true).

    Unless you have a softer outer skin that will take the high deflection when you bend something as extreme as the video you showed, you will have outer skin failure due to the brittle qualities of the outer skin.

    On the other hand, if you have balsa core and do the same test, the energy is transmitted straight through the outer skin, to the balsa, through the balsa (with little deflection) and then all ends up on the inner skin, shattering it or breaking its bond.

    It's just a tradeoff, like all things boats.

    Where you really win, IMO, is if you use a softer outer skin over the Core Cell which can take the deflection load, but is somehow not soft enough to make your hull into a noodle. Then you can have the advantage of the foam taking up the energy without the skin breaking from deflection.

    Don't forget... I *did* choose Core Cell! :D
  10. rberrey
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    rberrey Senior Member

    While we,re on cores cat what will you use for your hard spots? I bought renecell 240E for mine. rick
  11. War Whoop
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    War Whoop Senior Member

    Soft ?? what the hell is that? Put whatever laminate you wish on it,Stiff ? We do S glass and Carbon laminations, I fail to see the problem with a material that resists failure, you can use less laminate and not have to worry about the moment of inertia as a person using one of the second rate cores would.

    It behaves exactly the same EXCEPT when overloaded, then the damage tolerance of corecell becomes very evident.
  12. CatBuilder

    CatBuilder Previous Member

    I used 12lb Core Cell for my hard spots in the hulls, which are simply the composite chainplate connections.

    My deck is balsa, because I didn't want to fuss with taking out core and planning out exact locations for everything. Balsa can stand up to the compressing loading, so I used it. I just have to do the epoxy fill around any bedding holes and use a backing plate. To me, that's easier than re-coring or planning every piece of hardware out in advance.

    Also, I'm the maintenance guy as well as the builder, so no water will be getting in my deck core.

    War Whoop: I'll wait a bit to see what happens in the thread. I can't argue... just putting out what I was thinking.
  13. War Whoop
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    War Whoop Senior Member

    Cat I do not care what anyone uses,they can stick home depot plyrot in there.
  14. CatBuilder

    CatBuilder Previous Member

    Hey, I respect your opinion a lot. You helped me out tons already, so no way I'm arguing with you. :)

    BTW: Slight thread drift... I had a couple stations that were a bit narrow to route out at the top of my mold. I figured, "what the hell?" I'll try putting a couple straight pieces of 1x2 in to cover the 3 or so stations that were a little thin. Came out all lumpy and ugly. Not fair at all. I took them out and putting a new real, scarfed 1x2 in like the rest of the battens.

  15. War Whoop
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    War Whoop Senior Member

    LOL you learned about saving time so you can do it over the right way.
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