Carbon Fiber Scantlings

Discussion in 'Materials' started by Asleep Helmsman, Feb 17, 2010.

  1. Asleep Helmsman
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    Asleep Helmsman Senior Member

    I have a basic question.

    How thick would a 152mm diameter pipe need to be to hold 12 bars (atmospheres) of pressure from the outside?

    Assuming 30% carbon and 70% room temperature cured epoxy.

    Any advice on carbon to epoxy ratios would be welcome as well.
     
  2. CDK
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    CDK retired engineer

    That is a very tough question to answer. 12 Bar is only 120 grams /sq. mm compression load, which is next to nothing and requires less than 1 mm wall thickness.
    BUT, to survive 12 bars the pipe must be perfectly cylindrical, the wall thickness uniform and there must be no dynamic forces like flow of water or vibration. If there are any dynamic forces, the unsupported length of the pipe is also a factor to consider.

    So success or failure depends largely on the precision the product is made with. On any 10 mm section the total force is 572 kg. If somewhere along the circumference there is a flaw, it collapses like a tin can.
     
  3. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Thank you CDK,

    I had been thinking something in the range of 3MM, It will need protection against impact loads and as you said dynamic ones while operating.

    There will be numerous (for its size) load bearing bulkheads and other sub-structures that will reinforce it.

    so I'm guessing that anywhere between 3mm and 4mm would give sufficient margin.

    In the 4 mm range would fiber glass / epoxy be an option?

    Lastly (for now) If I decided to make it out of glass and carbon would it be better to have the outer layer carbon or the inner?
     
  4. jehardiman
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    jehardiman Senior Member

    Just use epoxy, the carbon does nothing for you in external pressure. The only reason to use it would be in longitudinal bending if the shape is of any length. See Submersible Vehicle Systems Design and the '67 edition of Principals of Naval Architecture, both by SNAME, if you want to delve into pressure hull design.
     
  5. mark775

    mark775 Guest

  6. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Is that an actual suggestion?

    Luckily, I have been building things out of epoxy since the 70s; I don’t think I would build a cereal bowl out of unreinforced epoxy.

    Having limited, and only practical experience with carbon/epoxy composites I thought I would see if anyone knew about the application of carbon at depths.

    BAE seems to think carbon is the right application for UUVs.

    As it turns out my “intuition” was pretty much correct again. So I’ll go with 3 to 4 mm thick hulls, with thin layers of carbon sandwiched, depending on the needs of the local loads, over unidirectional or bidirectional glass.

    I think there will be much reinforcement from internal structures and the hard points will spread the loads around and actually add integrity overall.

    If anyone can tell me why this won’t work I would appreciate it.
     
  7. Hunter25
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    Hunter25 Senior Member

    Yes your layup will work, but jehardiman is right you do not need the carbon unless you anticipate flexure.
     
  8. sailor2
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    sailor2 Senior Member

    The dimensioning does not come from strength or stiffness, but from being airtight (or water tight or ... )
    1mm thich is unlikely to be tight enough, if it's about compressed gas, but would be plenty strong enough, if quality is perfect. In practise nothing can be guaranteed. You must test it to be certain. 2/3 of fibres in hoop, 1/3 along the length is optimum, for pressure load. If there is other external loading, change accordingly.
     
  9. jehardiman
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    jehardiman Senior Member

    As Hunter25 and the references I pointed to can show, pressure hull collaspe is due to buckling in compression and shear strength, both of which are uneffected by using carbon fiber in the matrix. Carbor fiber is stiff in tension for it's weight, not strong in compression for it's SM and therefore significantly poorer performing in pressure hulls than other materials, including epoxy alone.

    IMHO, CF is one of the most misunderstood and misused materials in the marine industry. There are application in which carbon fiber excells (such as the inner skin of FRP and the outer fiber of stiffeners), other applications where it is not the best choice but brings something else to the table (such as masts), and others where it is a gee-whiz "look-at-me-I'm-high-tech" sales gimmick used to run up 'added value'.
     
  10. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Thanks for keeping up with discussion,

    As I said earlier, the vast majority of my experience is with wood cored glass epoxy composites.

    So in your opinion would a sandwich of CF over glass be useful in the myriad of loads that a small powerful UUV would encounter at 100 meters?
     
  11. jehardiman
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    jehardiman Senior Member

    The people who build small, fast, powerful UUV's that must work first time out of the box would not, as far as I know, chose CF for pressure hulls. Think about it this way, we have been using CF for helo blades for the past 25 years so the technology is mature, so if it was useful, don't you think someone would use it?

    Don't trust me, see this paper:

    http://nopr.niscair.res.in/bitstream/123456789/6213/1/IJMS 38(3) 352-358.pdf

    Edit: BTW, my consulting fee is quite high. Send it in small unmarked bills to......:p
     

  12. Guest625101138

    Guest625101138 Previous Member

    Joe
    There is some data here:
    http://www.performance-composites.com/carbonfibre/mechanicalproperties_2.asp

    I have not been able to get the compressive strength better than 50% of the tensile strength in thin layups. I expect it would be possible to get the numbers in the table if the layup was well controlled like you get with commercial tube.

    The compressive failure in a CF laminate is inevitably due to buckling even in quite thick laminates. The failure usually occurs at some imperfection in the layup where the fibres have been creased or misaligned.

    I have used 40mm diameter CF tube from this place:
    http://www.cstcomposites.com/1_5_mm_wall.htm
    Mine was high modulus 2.1mm thick. It actually rings like steel tube. It is very well made with high fibre to resin ratio.

    You may be able to do better than commercial tube by careful selection of the fibre orientation for your application but that is a lot of work to design and make.

    A hull from the 1.5mm thick tubing will weigh about 1.5kg.

    Carbon fibre composite is not particularly abrasion resistant.

    Rick W
     
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