Increase shear ???

Discussion in 'Fiberglass and Composite Boat Building' started by jorgepease, Feb 19, 2018.

  1. jorgepease
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    jorgepease Senior Member

    Do you think creating shallow dimples in a foam core, say up to .5 inch deep, gradually tapered from around a 2 in diameter and spaced every x inches in a grid ... would that increase shear?

    I am thinking in a 2 part process, infuse first skin so that it forms to the craters, fair over the holes and infuse the finish skin. That would put a lot more glass in contact with the core possibly making it viable to increase it's thickness and eliminate stiffeners and bulkheads.

    Just an idea that crossed my mind...

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    Last edited: Feb 19, 2018
  2. TANSL
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    TANSL Senior Member

    The shear stress does not depend on the composition of the laminate but on the external loads to it. What you could increase or decrease is the ability of a laminate to withstand such shear stresses.
    Frankly, it does not seem like a good idea, but I can be wrong. The best thing you can do is two samples, one with holes and one without holes, and check the resistance to the shear stress of each one. It seems to me that a "weaker" zone can be produced, from the point of view of the shear stress, at the interface of the two infusions. On the other hand, the union of the layers of fiber with the core is not, if well done, so "weak" as so that it is necessary to think about reinforcing it. It's just an opinion, keep that in mind, please.
    Regarding the bulkheads, it must be borne in mind that the mission of the reinforcements, resist bending and compression efforts, can not be replaced by the resistance of the core.
     
  3. jorgepease
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    jorgepease Senior Member

    Thanks TANSL,

    I would think the infusion between layers of glass have greater shear strength than glass to foam providing of course that an aggressive peel ply was used. If it's not, then all these kit boats which are taped together must be weaker than a boat infused in one shot, though others claim they are not.

    My goal is not to eliminate completely, the stiffeners, but to increase the span between them. In a single shot infusion it would seem that would be possible, though maybe not worth it from a weight and labor perspective ... but the idea of having a cavernous uninterrupted space with 2 inches of insulation as a side benefit appeals to me. ))

    I think you are right, I have to make some samples and see where I can get them tested.
     
  4. TANSL
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    TANSL Senior Member

    I have thought that you make an infusion with fabrics and core and, later, a second infusion with the first, let's say, as core and more layers of fiber. Naturally the first one should be dry before receiving the second (maybe what I say is nonsense). In that case, an "interface" between them is created that is much weaker than if everything had been infused in one go.
    Note that the core increases, but little, the flexural / compressive strength of the panel. The reinforcements do that, or at least they get a greater effect with less material.
    I do not understand this well:
    Could you explain it better, please?.

    Of course, it is possible that the reinforcements can be eliminated but, normally, it will be cheaper (material plus labor) to make a bulkhead with reinforcements.
     
  5. gonzo
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    gonzo Senior Member

    Firstly, it depends on whether you cut or press the dimples. Pressing them would break the foam structure and weaken the core. Secondly, it you take it to the extreme, it may be better to use scored foam which creates a solid resin bridge between laminates
     
  6. jorgepease
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    jorgepease Senior Member

    I was just saying ... In subsequent infusions, if the bond between those layers is not as strong as the bond between the initial glass to core, then this idea would not work... but that confirms my suspicion on kit boats being inferior because they are completely made up of secondary bonds. Proponents of taping hulls together will tell you there is absolutely no difference in strength.

    gonzo, good point actually, after the first infusion those scores and perfs would be filled so infusing the second layer in a mold wouldn't work anyway.

    Edit ... actually if you infused the panel flat then you would just redrill the perfs for the second infusion but its a moot point if the bond between layers is weaker than the bond between first skin and foam
     
  7. TANSL
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    TANSL Senior Member

    In my opinion, that is what could happen. Also, as Gonzo says, the core can be weakened by dimples.
     
  8. jorgepease
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    jorgepease Senior Member

    Last edited: Feb 19, 2018
  9. Ad Hoc
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    Ad Hoc Naval Architect

    In a word....no!

    The semi-circular shaped (or whatever they end up being) dimples, will create localised stress raisers and actually be the initiate point for cracking.

    What is reistaning the loads then...thin air??!!
    Then you need to address the whole structural design from first principles then. That is to say, what is the span what is the breadth and where are the loads coming from. Then, design your structure to absorb and then shirk the applied load without the need for additional support/stiffeners. But not forgetting as the load is transferred, where does it go too...!
     
  10. jorgepease
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    jorgepease Senior Member

    But if the bond between layers wasn't an issue, then would the second skin covering the dimples prevent those stress points? The first layer of glass is stressed and transfers some of that stress to the second layer. Since more glass is in contact with the core, the shear strength is increased or do we still have stress points.
     
  11. TANSL
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    TANSL Senior Member

    I think I expressed myself correctly so I do not understand your comment or the intention of it
    The shear stress existing in an area of a panel does not depend on the composition or material of the panel. Another thing is the capacity that panel has to withstand certain sharp efforts.
    Take into account, among many other things, that it does not increase the contact surface of the glass with the core, but of the resin with the core (the resin is much less resistant than fiber). And for this, you have introduced irregularities in the core mass that always, as I think Ad Hoc says, will suppose points of concentration of stresses and less resistance to shear force that is how the core really should work.
     
    Last edited: Feb 19, 2018
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Firstly there are 2 planes of direction to consider, in-plane and out of plane stress. This means we need to look at the vertical and horizontal load paths.

    1. Horizontal.
    Imagine a road full of potholes. Very uneven and bumpy. Let's say this represents the surface of foam once those dimples have been cut into it. If you flood the road so all you can see is water, the potholes are covered. Great! Nice smooth surface.
    Let's now say the temp drops and the water freezes. Still smooth surface. But if you were to now push or pull the massive ice sheet so that there is no bending, just a direct shearing action....what do you think will happen to the ice that is inside those potholes (dimples)? Will the ice slide easily over the shape, or will it on the leading edge gets ever so slightly crushed as the pothole shaped ice is now being forced to slide into the bulk of the road rather than smoothly over it. To slide over it, something must give.

    2. Vertical
    If you take a slice of the foam between the dimples you have area A.
    If you take a slice of the foam concincent with the dimples, the depth is less ergo the area A is now less.

    Lower area = high stress.

    And in both cases. It is a localised stress raiser, since the contact area is low, ergo, the stress is high.

    Ugh....why take up band width....
     
  13. TANSL
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    TANSL Senior Member

    I do not know, I do not understand you, but I'm afraid it does not matter.
     
    Last edited: Feb 19, 2018
  14. Yellowjacket
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    Yellowjacket Senior Member

    Don't bother putting in dimples.. the bond to the core is already stronger than the shear strength of the core.. The surface area where the skin is bonded to the core is large and you'll just shear the core material in the middle before you tear off the skin...... Adding dimples, assuming that you don't damage the core putting them in, will increase the surface area, but that isn't going to buy you anything since the core material shear strength is lower than the bond surface.
     

  15. jorgepease
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    jorgepease Senior Member

    I think I get it now, thanks everyone ))
     
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