High shear elongation cores, hype or real world use?

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

  1. CatBuilder

    CatBuilder Previous Member

    Sure did. That tip you gave about only doing it with scarf joints was really clear when I tried the shortcut. No more short cuts.

     
  2. War Whoop
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    War Whoop Senior Member

    Yes now it is critical you hold the humps and hollows to a minimum.
     
  3. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Most of the answers given by the members are correct except that it did not come in order.

    The impact response of a composite structure can be divided into 3 categories.

    1. Elastic deformation-the entire energy of the impact is absorbed by the structure and released when the structure returns to its original shape. No breakage.
    2. Plastic deformation- at higher energy levels, the structure deforms permanently and does not return to its original shape. Failure in other words.
    3. Catastrophic failure- the impact levels exceeding the exceeding the capabilities of the structure wherein the ultimate strength of the material is reached.

    The topic in discussion is a cored laminate design in which the choice of materials (core, fiber, and matrix) will prevent the catastrophic failure or at least minimize it. The design approach would be to have a choice of materials that works in unison with each other, while preserving the strength needed.

    There are two types of impact, the bulk and the local. The most common (wave slamming) wherein the impact is distributed over the panel and the local concentrated load such as hitting a floating log (or dropping a hammer into a balsa cored deck). In real life, wave slamming is common for high performance boat but hitting a solid immovable object is a remote occurrence but is a possibility.

    In choosing core, a core that is stiff will transmits the load to the other face of the laminate causing fracture or delamination while a core that deforms will absorb the impact preventing further damage.

    In a fiber matrix interphase, a fiber loaded with low elongation resin will fail first because the “brittle” resin is the dominant factor. The result is a catastrophic failure. If the fiber is loaded with high elongation resin, theoretically, the fiber will break first and the resin takes over. Because of high elongation, the “tough” resin will take more strain before ultimate failure increasing its ability to hold things together.

    While flexing is a boon for impact strength, repetitive flexing will cause failure at the edge of the stiffener because this is where the greatest shear is located. As the panel flexes, the edge of the stiffener resist.

    The design thus calls for the synergy of all the materials involved in the process.

    Another interesting issue raised by WW is the use of CF. Will a relatively stiff, high modulus carbon fiber work when we are concerned with flexing? Yes!

    Stiffness of a panel is a cubic function of thickness. Using high modulus carbon fiber for the skin will reduce core thickness. Reduced overall thickness will result to flexing. Increasing span length also increases flexure. Test result showed that carbon fiber fared slightly better than E glass and Kevlar because the higher flexural stiffness of the carbon distributed the impact load over a greater area of the foam core. Energy is absorbed by the skin and distributed and the overall flexing of the structure helped absorb the excess impact.

    The conclusions are 1.That strength is still the most important issue, followed by 2. The fiber-resin interaction is influenced by the resin toughness (elongate under strain) before catastrophic failure and 3.High stiffness of a core material comes with high brittleness.

    As for the test, the water bladder test or hydromax approximate the real life performance of a boat laminate subjected to uniform water pressure, flexural beam test approximate the performance of a laminate under transverse load, while an Izod or Charpy impact test is for resin toughness. A dropped ball projectile is for testing local impact as in PB 51 article. Most unlikely scenario but in case it happens, you will be limping home with a failed laminate but not breached.

    1. Marine Composites- Chapter 4
    2. Fundamentals of Composite Manufacturing
    3. Composite Fabrication-Choose the Right Core, April 2003
     
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  4. TeddyDiver
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    TeddyDiver Gollywobbler

    So in short, to enjoy the benefits of high shear elongation cores, skins have to be engineered accordingly.
     
  5. rxcomposite
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    rxcomposite Senior Member

    Yes. But is the easy part. I posted an excell spreadsheet in one of Catbuilders thread about Core Shear Formula which include skin sizing. The program even suggest using CF or substituting a process to get a high fiber content. In one of the sheet, it is shown how increasing fiber fraction increases modulus of fiber.

    It is a Lloyds formula/methodlogy so the safety factor is 3+. But like all programs, it does not think so the input must be tempered by all the intelligient discussion that rose from this forum.
     

  6. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Idkfa,

    After re reading PB 51 issue, I quite agree with your thinking.

    What is the reality of a boat getting hit perpendicularly with a 100 lb steel ball 6 times and at the same place everytime? Unless probably Captain Jack Sparrow is alongside and pummeling the boat with his cannon.

    As the article state, it was a series of test. It is a RELATIVE test to find out how much force was needed to break some panels while the foam cored panel does not. A lot of panels was probably used while the author was adjusting the ball height or force.

    What is the relation to the actual world wherein a boat would struck a log, a buoy, or a reef? And yet it will be a glancing blow as the boat is moving forward.

    Any members out there who can enlighten us?
     
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