Bolting in composite

Discussion in 'Class Societies' started by fcfc, Apr 14, 2011.

  1. fcfc
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    fcfc Senior Member

    for assembly that work in shear.

    Are there accepted rules ?

    Typically a metal chainplate bolted on single skin plating.

    I have found some methods for bolting chaineplates on plywood bulkheads, but found nothing for bolting in composite.

    Thanks.

    Also what I find strange is that bolts are working in shear, where my engineering backgound tells me that bolts should work in traction only, and shear is to be handled by contact friction.
     

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  2. Ad Hoc
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    Ad Hoc Naval Architect

    fcfc

    DNV, HSLC & NC Rules, Pt.3 Ch.4. Sec.9
     
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  3. fcfc
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    fcfc Senior Member

    Thanks.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Can you explain further what you mean by this?

    However, you're correct, the bolts are in shear.
     
  5. fcfc
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    fcfc Senior Member

    The contact pressure excerced by the bolts between the 2 parts should be enough that the in plane load is hold by the friction between the 2 parts.

    The problems of multiple bolts working in shear is that you need a precise fit: in DNV 0.1mm fit gives you a gamma = 1 . 1mm fit gamma goes to 1.6. (60% more bolts) and more than 1mm is not in the rule.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Ah ok..i see where you’re coming from now.

    Ok, image, an infinitely long bolt. Place part 1 on the bolt and part 2 on the bolts shaft. Push the two parts together.

    The parts are not in direct contact, the parts are not even close. Now lift, what happens?

    The load will go from part 1, via the bearing –contact area- of the bolt face on part one, into the bolt as shear. Then the load will go from the bolt, as compression/bearing, on the face of contact area on part 2, and then from there into part 2.

    If the parts do not make contact, the shear in the bolt also becomes bending, since the bolt is like a point load on a beam, the beam being the length of the bolt (between the two parts). Therefore, if the distance apart is too large, the bolt is in shear and bending.

    Now bring the two parts together. Not touching but close enough, sat 1mm apart. There is no bending in the bolt. The distance between the two parts does not allow for the load to be transferred as anything other than pure shear. The stiffness of the bolt dictates the laod path a shear alone.

    Now, push the two parts together tight fit, so close that you cant even get a thread of hair between them. The load path is the same as above. The contact of the two parts makes no difference, because the load path is via the bolt alone.

    If you now glue the two parts together, how much load is taken by the adhesive and how much by the bolt? Well, what is the contact area, and what is the allowable shear stress of the adhesive. Can the amount of adhesive support the shear load? If the answer is yes, why bolt? If the answer is no, then the remaining load, allowable, shall be taken by the bolt. So split, some by the bolt and some by the adhesive.

    However, in practice this gets messy and proving it categorically is a theoretical exercise, since it relies on a good bond etc etc….. Therefore we assume the total load is going via the bolt alone.

    By tight fit…do you mean the diameter of the bolt to the diameter in the part?..if so, then yes, the bolt must be a near tight fit. This ensure a double shear on the material directly above. The extreme case is if the bolt is much smaller, like a thin strand of cheese wire. Then the load is single shear and can potentially tear right through….

    Good luck
     
  7. powerabout
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    powerabout Senior Member

    most modern race boats make a loop in carbon all glassed in the hull and you just use a pin to hold the rigging,
    No more bolts
     

  8. Tim B
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    Tim B Senior Member

    Indeed, the best solution is to use continuous fibres (Carbon or Kevlar etc.) to effectively "tie" your point load to the hull. You will find that the "effective" UTS of the fibre reduces, so you need to design it rather carefully.

    With regard to bolting in shear or tension, shear is usually prefferred. That is not to say that it is always the solution, but when there is a choice it is usually the one to go for. Simplistically, it's down to the amount of metal available to take the load. When in tension, the thread (which is a pretty small amount of metal) has to transfer the load to the shank of the bolt. Comparatively, the shear-loaded bolt has much greater area through which to resist the load. In the shear-loaded case, it is likely that clamping forces only really come into effect for joining very thin materials.

    There is plenty of info around on bolted joints. I suggest google.

    Tim B.
     
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