Shear calc help

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Nov 9, 2018.

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  1. fallguy
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    fallguy Senior Member

    9 layers of 600g with mat shear strength?

    Anyone know the calc?

    Thanks.

    Just developing a torque spec for my own use. Dan
     
  2. fallguy
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    fallguy Senior Member

    Epoxy.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    The calculation can only come once you know the material properties of the laminate.
     
  4. fallguy
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    fallguy Senior Member

    Thanks John.

    I torqued things down to 20 ftlbs and feel 'good' about it.

    These have no lock washers, but here is the application.

    It is Richard Wood's design, but I wanted to do this one with my son who is majoring in ME and Richard is on vacation.

    The wood spanner in the foreground was just there for bonding. We waited a week to torque them down. B55B847B-5E90-4F11-A5FF-3A9FA4C91CA9.jpeg
     
  5. rxcomposite
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    rxcomposite Senior Member

    Do you want to know the shear strength of the ply or do you want to know how much torque a bolt when fastening it?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Dan,
    Sounds more like a compression question than shear??

    What size and grade of bolts are these?
     
  7. gonzo
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    gonzo Senior Member

    The force generated by the torque on the bolt is dependent on its diameter, pitch, finish and whether it was lubricated. There are online calculators that will give you the answer.
     
  8. fallguy
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    fallguy Senior Member

    I want to know a good torque spec and just wanted my son who is getting a degree in ME to help on the question. And I didn't really want to bother Richard for an exercise which is largely a matter of semantics. Richard is very busy right now.

    There are several factors at play. One of them is obviously going to be the limiting one.

    1. The strength of the bolt, the bolt is 16mm 316 ss. Not likely the limit.
    2. The strength of the strapping. The strap is 3/8" 316 slotted to receive the stud. The strapping will bend before it breaks.
    3. The modified stud/bolt head. The head of the stud is welded 316 to an adhesive styled washer. The washer is made from 1/4" 316ss and it is about 30mm square with holes for adhesive and the bolt. The stud head is only useful in determining force applied to the plate or glass laminate. The stud is epoxied into the boat with a 50/50 cabosil and milled fiber mix. This glue keeps it from spinning or falling out, but does little else for the strength.
    4. The plate. In this case, the plate is the fiberglass. It is nine total laminations of 600g glass with mat. Three of the laminations were in a primary bond and vac bag, the next six laminations were hand done on the boat, all epoxy. The thickness is somewhere between say 3/8" and 1/2" thick, but I didn't measure it. We made lunch table assumptions between 300 and 2000 psi for the shear strength of the laminates. Of course, using the 300.
    5. The ally beam. The beam is about 5/32" thick aluminum with a very thick top where the beam strap centers. If we torque the bolts down unevenly, the risk would be kinking or denting the ally which might compromise the beam and that is a big issue because the cost of the beams is the cost of a full mast or $4,000-$5,000. Repairing the damage would be difficult as well, especially on the inboard strap as it is pretty far up the tube for any internal work.

    When we started we assumed the weakest link would be the glass and that the glass might shear. But once I started to torque the nuts down, I realized the torque of 20ftlbs or 240inlbs was quite a bit and so we stopped. We made some assumptions and actually felt really good about torqueing to 50ftlbs, although it is unneeded. During the torqueing to 240 in lbs, we noted very little movement or change to the plate or glass laminate. This is because I have a neoprene liner under the beam that is certainly taking up some of the pressures and silences the beam/socket interface, and under each strap, I have a piece of rubber inner tube to protect and silence the beam/strap interface. And if anything moved at all, it appeared to be the linings or the strapping, or taking up voids in the interfaces. The rubber linings are not permanently installed if you look closely at the picture because we are going to oven cure the hulls, so the rubber can't be in until after we paint.

    thanks for any comments .. Dan
     
  9. fallguy
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    fallguy Senior Member

    We are looking for a torque spec for the bolts and wanted to try and come up with it as my son is studying mechanical engineering in college.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Dan

    Ok... first as noted, need to work out the torque on the bolts. But not forgetting what we calculate is the max toque for THAT bolt. So we know that if we torque it beyond this, it'll damage the bolt.

    The general metric formula used for the max torque of a bolt = [Yield (proof) in MPa of bolt ]x FoSx Area (mm) x Diameter (mm)/5000 = Nm

    The FoS commonly used is 0.7, or 70% of the material property value.

    If you prefer this in imperial and more exact, the Torque is T = cdF
    d = diameter of bolt, in.
    F = initial preload, Lb
    c = coefficient. Usually taken as 0.2 for an unlubricated (dry bolt) or 0.15 lubricated (not dry).

    You can work out exactly the value of 'c', but it relates to the angle of helix and coeff of friction and thread half angle etc etc...

    You are then face with, well, ok, if we torque the bolt up to its max limit what effect on the glass? That is dictated by the area of the washer. Since the washer will be forced onto the glass by the tension in the bolt.

    So you can calculate the force, or initial preload, as noted above from the torque. Assuming you have a calibrated torque wrench? Once You have this value, it is a simple matter of compression stress = F/A, the A being the area of the washer.
    This stress is then compared to the max compression load the GRP can take before cracking/yielding as such.

    So knowing the material properties of the glass behind is important as you then work backwards, knowing your washer size to what is the max torque to apply.

    You are then faced with...well once or torqued up...will the 2 surfaces slide apart?

    That comes down to the coeff of friction between the 2 surfaces, nominal tables will provide that, or you could simply calculate it using a light spring balance.

    So the total clamping force must be greater than the force, normal to the bolt, to slide the two surfaces apart.
     
  11. fallguy
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    fallguy Senior Member

    Well, I need a better torque wrench. The one I have is suitable, but not ideal.

    We have all been wondering about the sliding. The boat is held together more absolutely by other means, but the beam sliding would be bad. Torqueing too low would be trouble.

    Of course, this could also be managed by a bolt at the top of the beam that would absolutely stop sliding.
     
  12. fallguy
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    fallguy Senior Member

    Can anyone tell me the max compression load of 9 layers of 600g mat s glass?
     
  13. rxcomposite
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    rxcomposite Senior Member

    Comprehensive Biax data is hard to come by but here is one with epoxy laminate biax with a 50% glass content.

    Longitudinal Tensile strength= 17.4 ksi
    Longitudinal Compressive strength= 15.7 ksi
    In Plane Shear strength= 21.8 ksi
    Note that the data is for lateral test. For Thru thickness, interlaminar shear strength, with a force perpendicular to the surface of the laminate:
    Vshear= 1.5 x shear s= 32.6 ksi (a rough approximation for a short beam test)

    or looking for compression thru thickness
    1.2 x Longitudinal compressive S= 18.8 ksi

    Strength is always higher at thru thickness because the force is perpendicular to the fibers. For composites, strengths are low so it is advisable to use large fender washers when bolting a laminate to spread the load and anti compression bushing to prevent crushing the laminate.

    Data given are units of measure so you have to find the force/area on the material you are using.
     
  14. rxcomposite
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    rxcomposite Senior Member

    Sliding is shear. Torquing a bolt is compression. The best solution is add metal anti compression bushing. The wall of the bushing adds shear strength and the height prevents over compression of the laminate.

    The best laminate schedule for a laminate subjected to shear is 0/+45/-45/90 degree. Strength Equal in all direction.
     
    Last edited: Nov 12, 2018

  15. Ad Hoc
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    Ad Hoc Naval Architect

    RX
    I believe Dan was referring to my comment :

    The 2 surfaces being the aluminium beam and the GRP. Not the fibres in the laminate, per se.
     
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