Stiffness vs Tensile Strength in Foam Sandwich Construction?

Discussion in 'Fiberglass and Composite Boat Building' started by Iridian, May 16, 2021.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    It seems there is a lot of "talking over" each others heads, and misuse of terms.

    Strength = is the maximum nominal stress in a typical tensile test.
    Which is typically the force/area... and this leads to the 'yeild' strength of a material. In other words, the 'safe' limit of usage of said material, under a given applied load.

    Stiffness = relates to the 'E' of a material, which is a ratio of the stress/strain in the linear part of the stress-strain curve of said material, up to its 'yeild' point.

    In structural design, the terms strength and stiffness have different applications in their understanding and how it is applied to a structural design. Using a material which has a high strength, does not = stiffer or even safer. It just means you are using a material which can withstand a high load for its given cross sectional area. A 5ox5mm Flat bar shape of wood when pulled will fail at a load much less than that if the same shape is made from steel. The only variable here is the material's mechanical strength.

    Stiffness relates to the 'EI', which is the amount of deflection under an applied load.
    It is a measure of the structural response under a given load scenario and the deflection is dictated by the material property and the second moment of area of the structural arrangement.

    Deflection may not, on its own, be a measure of a serious issue. Since the deflection, is what?.. it is a response to an applied load and if the response is "within" set limits or assumptions, then the structural arrangement is ok.

    Consider next time you are on a flight (once you've had your jabs :D) and you look out the window. What will you see - you'll see the wing tips of the aircraft moving and moving rather a a lot.
    So this deflection has been engineered into the response and is considered acceptable, despite it seeming counter intuitive.

    Now consider being on a catamaran. The two hulls are joined together by box beams alone.
    Under an applied load - let's say a quartering sea, so a torsional bending moment, the material you use has sufficient strength and the stress experienced by the box beams is sufficient and inside yeild/elastic limits of the material.
    However the two hulls are now, like the wing tips of the aircraft,. moving and moving a lot retaliative to each other. Clearly this is not suitable or satisfactory, you don't want to be on a jelly mould of a boat...it ode snot instil any confidence.!!
    So in this case the stiffness is insufficient, despite the strength being acceptable.
    Increasing the stiffness of the box beams will reduce the deflection until you consider the amount of deflection - acceptable.
     
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  2. rxcomposite
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    rxcomposite Senior Member

    Glad you brought it up. I was going to say the same thing. It is going to go sideways very quickly.

    With so many things on his mind like S2 glass exotics, Carbon fiber, Specific strength, elongation, ect, all at the same time. This is going to be a long thread.

    We have to start somewhere. Sandwich Core design maybe? Core sizing?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    All very good, but the specific interest is in GRP and GRP sandwich, and it is obvious with reasonable reinforcement selection, the sandwich is designated to mitigate the stiffness shortcomings, and it isn't tensile strength that is a prominent concern, it is frequently the problem of point impact that mandates thicker laminate of the outside skin, than is needed for strength, otherwise you would have identical skins inside and out. Balsa core is different though, to synthetic cores, and less a problem for point impacts
     
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  4. fallguy
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    fallguy Senior Member

    How about explaining how Gerr uses a scantling number and how it is derived.

    The scantling number must mean something like a combination of stiffness and uts.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Has anyone ever seen a GRP boat that "broke its back" ? No, I haven't either, so worrying your head about tensile strength of GRP is not time well spent.
     
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  6. rxcomposite
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    rxcomposite Senior Member

    To quote myself "It is going to go sideways very quickly.". It already did.

    Seems it is going to be Composite Design 100. Not 101, That is advanced.;)
     
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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Its like this, there is no need to introduce overly-technical complication where it is not needed, it is virtually taken as read by anyone reasonably au fait with GRP, that stiffness concerns trump tensile strength worries just about every day of the week. Otherwise you would never have seen heavy glass boats with gun lay-ups that are far from tops tensile strength wise, but are that way to get some stiffness.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    And of course, sandwich construction only exists because of a desire to use the high tensile strength of GRP (potentially very high with the right reinforcements) without having to go heavy. Why would anyone think tensile strength is a problem, given that background
     
  9. Iridian
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    Iridian Junior Member

    @jehardiman

    Unfortunately no background in Structures. I have an MS in Systems & Operations Research, but that's not overly helpful here. Probably the closest thing of use is calculus based physics, but its been a while. I'm following with your explanation on the forces on a boat hull though, excellent explanation. Thanks! @Ad Hoc , also tracking on your explanation of stiffness and deflection; are there any concerns about material fatigue?

    In order to hopefully clear some stuff up, I'm pondering the idea of starting the build of a 50' catamaran in 4-5 years (considering Raku 52 from kit?), or maybe repairing a used one. Short term, I'm trying to improve my understanding of composites and the benefits of each, and hopefully do some small scale projects to see how much I actually like working with composites. I've built a couple of small wooden boats, but nothing that great so far.

    Thinking about building a composite truck shell, motor mount for my H20 Miracle, and a canoe so far.

    The canoe build and motor mounts specifically led me down the path of some of the more uncommon fabric options, but it seems like either S Glass or Basalt Fiber might not be prohibitively expensive, even for a larger boat, at least for some of the layers, should it be value added.

    @Mr Efficiency , thanks for grounding the conversation. For point impacts, is resistance to damage a function of stiffness and elongation before failure? My assumption is that a stiff material capable of some elongation would be superior in protecting the core but not shattering?
     
  10. rxcomposite
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    rxcomposite Senior Member

    Chopped Gun method is GRP. Sandwich construction is Advanced Composite, squeezing the last ounce of potential in composites. It is complicated.
     
  11. rxcomposite
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    rxcomposite Senior Member

    That is one topic that has been discussed in length over this forum.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    There really isn't much stiffness in a skin, obviously the remedy for reducing penetration and crushing has been to thicken it, not to mix and match laminate components, although Kevlar might be an exception to that. In any event, point impact outer skin penetrations are not the end of the world, they are repairable, the point impact that would crash through the whole thing, would just as likely do the same to a thicker single skin boat. Crushing of the core without fracture of the skin is a slightly different matter, but again the sort of collisions that cause serious damage that way, will also have likely badly damaged a single skin boat. All would require repair. Generally speaking, cored boats tick a lot of boxes, but are more expensive for mass production.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Always!
    One must never forget fatigue when designing structures that are ostensibly designed to 'flex' as part of its MO.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    I think you'll need a magic wand, some LSD and a whole lot of time to chill and contemplate the universe ... to get to the bottom of that one! o_O
     
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  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Fatigue afflicts GRP boat owners more than their boats, the stuff will bend a million times (well, a little poetic licence, perhaps) within its limits, with no discernible deterioration, ( as the fishing rod attests) but one overload outside its limits will crack or snap it.
     
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