Length of glass strands !

Discussion in 'Fiberglass and Composite Boat Building' started by tunnels, Mar 23, 2011.

  1. tunnels

    tunnels Previous Member

    Been wondering if any one has ever done any comparisons of the strength of panels laid with csm and a chopper gun ?? :p
    The strands on the csm we have on hand off the roll i measured to be 50mm long mostly !:rolleyes:
    The length of stands from a 3 blade Venus chopper are close to 33 mm long but by running 2 blades you get a combination of long and short strands .:D
    I noticed it was more bulky when sprayed but still rolled down ok and wet out very quickly . ;)
    I would say the longer strands would be better than short strands allowing more and better flexability and even better tensile strength !! What say ye all ??:confused:
     
  2. Herman
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    Herman Senior Member

    Not tested, but plausible.

    Although if you are aiming for best properties, you keep the chopper gun or the CSM in the closet, and use more "high tech" materials. (ranging from wovens, multiaxials to UDs, and whatever fancy stuff they construct now, including 3D stuff)
     
  3. tunnels

    tunnels Previous Member

    I hear what you are saying and i to am not in love with the situation BUT im stuck with it . Backward thinking is an understatement for the guys i am working with and trying to drag out of the prehistoric ages . .
    The point is a general interest one ,the lengths of glass strands would make a differance of any sort and has any one got any info or constructive thoughts on this subject !!
    I would say yes it will make a differance but by how much i dont know !! A small test panel will bend much more that a standard chopped panel with short strands before breaking !!And that was noticable !!. I am a great fan of unidirectional glass in construction and used it very successfully over the years . :D
     
  4. Herman
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    Herman Senior Member

    You could do some simple tests:

    Make a long and wide strip, with either short, or mixed fibers, or CSM (or Multiaxials, etc).

    Make them all the same thickness. If you have the tools, you could even sand them down to the same thickness if needed.

    Now support 2 ends, and place weight in the middle. Things do not need to be very scientific to be able to draw conclusions.

    There was even an article in Professional Boatbuilder, descibing a tensile and bending tester, which could be made with simple materials. One of my customers built one, and is enjoying it at times.
     
  5. petereng
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    petereng Senior Member

    Hello Tunnels- 33mm vs 50mm won't make much difference in your laminate properties. If the fibres were down to 10mm I'd then be worried. Theoretically somewhere around 3mm is the required length for fibres to transfer load within the laminate. What will make a difference is resin content. The chopper gun will generally add more resin to the laminate then if you laid up the part with CSM by hand. Unless you have the resin drum sitting on scales and the glass rovings sitting on scales and these have big readable screens so the operator can figure exactly how much resin they are adding then the CSM cloth laminates will be better performers then the gun. Cheers Peter S
     
  6. AndrewK
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    AndrewK Senior Member

    Hi Peter,
    A question not relating to CSM but multiaxial cloth style and overlaps.
    Say you have a long cored panel with 750g triax faces at +45/-45/0 (catamaran hull).
    Option 1 is to laminate with warp triax running along the length of the panel to give you a continuous 0 deg fibre along the length of the panel.
    Option 2 is to laminate with weft triax transversely with standard 50mm overlaps.
    How does the theoretical 3mm fibre length (overlap?) relate to real world choice of manufacture, ie would the two panels have the same stiffness and strength?

    Cheers
    Andrew
     
  7. tunnels

    tunnels Previous Member

    Hello
    So far i have found the longer starnds from the chopper wet out quicker and lay better so theres a couple of plus'es . bUT what i am really looking for is why is there some choppers that have 3 blades 4 blades and have even a chopper in the box with 6 blades so the chopped glass must be quite short like down about 12 mm long .
    One company years back we used all chopped glass for the whole boat , Am building some Australian boats and they have loads of chop in them . Gradually i am adding more fabrics to do away with the chop . And no you dont use more resin with a chopper gun if you get used to rolling what you spray you can judge whats wet , what just right and what is dry . So its completely up to the gun operator as to what you get . Dont blame the equipment . I had a stand up argument with a know it all forman in one company on that very mater so . They always complained that there gun was to wet so i just reached in the box and found a smaller nozzle and changed it with the same pressure settings ,the glass just fell off with not enough resin .
    A gun is a completely adjustable spray tool and does what the operator makes it do . He is the one pulling the trigger and has total control. !! If he dosent completely understand what it capable of he should have his *** kicked !!! Hard !!!Get my drift !!!

    Anyway back to the subject ,Longer strands must be better , just looking for some one that has tried it !! and knows a little more !!
    :confused:
     
  8. Herman
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    Herman Senior Member

    No, he/she should be educated...

    As for shorter strands: Used in many (non boating) products...
     
  9. petereng
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    petereng Senior Member

    Hi Andrew - In terms of stiffness and strength there should/would be no difference. Comes down to logistics and stackability to the choice you make. Cheers Peter S
     
  10. petereng
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    petereng Senior Member

    Chopper Guns - Are outdated and inefficient processes so you need to move away from them asap. Thats end of story. All the companies in USA and Europe who have implemented VIP have improved either production output with the same number of people or same production output with less people. Plus it has been at less cost. Chopper guns are wastful in terms of materials and time if you do a proper time and motion/materials study. Chopper guns and their labour have lots of hidden costs that do not show up in a cursary analysis. They produce very dirty environments (apart from environmental pollution a dirty environment is an inefficient work environment and does not foster good worker attitudes) and poor laminates and no amount of training will fix these issues. Move along. Cheers Peter S
     
  11. AndrewK
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    AndrewK Senior Member

    Peter, thanks for the reply, I was after an answer from someone with authority. The reason why I asked is that frequently I see first time builders take on laminating jobs in the most difficult way. That is they try to do a large laminating job on their own in one hit using far too large lengths of cloth. As a result end up putting them selves under so much pressure that often the quality of the laminate suffers.
    Many are also very paranoid about secondary bonds, how much of an overlap is required for a laminate with a secondary bond to be equal in performance to a laminate made in one shot? assuming epoxy resin is used.

    Cheers
    Andrew
     
  12. petereng
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    petereng Senior Member

    Hi Andrew - See attached calcs. The first calc shows the pull-out strength of a typical glass fibre is 0.75mm. So the smallest fibre required is say 1mm. The second calc is the overlap and shows that a low strength laminate requires 5mm overlap and a high strength requires 50mm overlap. This is at ultimate load so need to apply a design factor somehow. Most secondary strength problems lie in the preparation or doing of the work more than the geometry of the joint (assuming the joint geometry is suitable). Hope this helps. Cheers Peter S
     

    Attached Files:

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  13. AndrewK
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    AndrewK Senior Member

    Thanks again Peter, always very helpful.
    My general guide has been 50:1 taper/overlap.

    I see that you say typical sheer strength of epoxy is 20MPa, this is the property that is missing from many data sheets.
    Is the 20MPa from your testing?
    What numbers are you getting for VE and iso-PE shear strength?

    Cheers
    Andrew
     
  14. War Whoop
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    War Whoop Senior Member

    Can you get any gun roving with the red tracer? We used this in training years ago ,as for chopper skinout with a skilled operator it is just fine, as you lost the binder present in sheet mat and have pure glass.
     

  15. petereng
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    petereng Senior Member

    Hi Andrew - Resin Shear Strength will only be published for secondary bonding resins. 15-20MPa would be typical for laminating resins whether they are PE,VE or epoxy. Specialised toughened secondary bonding resins VE, PMMA and Epoxy will be about 30MPa. There are some out there at 40MPa, SP have a 40MPa bonding epoxy. There is a good argument for a 10,000:1 taper ratio ;) Cheers Peter Schwarzel
     
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