Pull off testing

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, May 11, 2018.

  1. fallguy
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    fallguy Senior Member

    Bondline is the main concern.

    I appreciate your technical reply. I might need you to dumb down the reply for me rx. I assure you design is not the issue.

    I will expound on this subject after testing is done.

    Thank you all for the replies.
     
  2. fallguy
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    fallguy Senior Member

    Do you mean I ought to expect 50% of the foam rating for the bondline?

    I want to know what 22oz triax and 18oz biax ought to pull off Gurit M80 core using epoxy. A specific brand I won't name here at this juncture.
     
  3. rxcomposite
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    rxcomposite Senior Member

    Resin shear strength is chosen (minimum) 50% of the specified shear strength of the core. It is usually less as the design sheet says AVERAGE shear strength which is less than MAXIMUM shear strength of core. This is least of worry except in highly loaded design as most epoxy resin (RIMR 235) have 38 to 42 kN/mm2 interlaminar shear strength. Pretty high. Core shear strength starts at about 1 N/mm2 to 1.7 N/mm2. So if you are failing at bondline, there must be something terribly wrong.

    LR recommend four point sandwich beam test ASTM C393 or equivalent. That is for core shear strength. It is about deflection but if bondline fails, then the laminate fails. Am not sure about the Losipescu or V notched rail shear test if applicable because that is about shear strength of the laminate.
     
  4. rxcomposite
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    rxcomposite Senior Member

    At what load or pressure? What is the thickness of the core?
     
  5. fallguy
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    fallguy Senior Member

    12mm core

    I want to test the bondline adhesion objectively.
     
  6. fallguy
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    fallguy Senior Member

    Are you familiar with wetbagging rx?
     
  7. ondarvr
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    ondarvr Senior Member

    The actual number you get will be irrelevant, the only thing you are looking for is if the core failed or the bond line failed.

    If the bond line fails then you screwed up the process in some way, the core is the weak link in this type of test.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Yes I am.
     
  9. rxcomposite
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    rxcomposite Senior Member

    With a 12 mm Gurit PVC core and a shear strength of 0.79 N/mm2, density of 60 kg/m3, I can load it to 7.9 kN/mm2 (with an FoS of 2) panel size of 1 x 2 meter, 1,75 mm inner skin, 2.34 mm outside of WR. No derating on foam core.

    The numbers will all change with a change in input. It is all about design, not workmanship.

    What you are interested is did the skin bond correctly to the foam? Better do a large panel test or a series of test. Local voids (no bond) can influence your level of confidence on the panel. A "coin test" will work. Or holding up the panel to a light.
     
  10. rob denney
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    rob denney Senior Member

    If the core strength is lower than spec you can throw it away and start again or start a 10 year legal battle with the suppliers, which you will eventually lose. Both would be foolish. The core will be fine for your boat, which appears to be way over engineered anyway.

    Where you might, almost certainly will, have skin/core bond problems is if you placed the sheets of foam onto a resin rich surface instead of wetting them out first. If the former, there is nowhere for the air to go, so each cut foam bubble is a void. Vacuuming helps, but tends to only suck from a small diameter around the perforation. This is made worse if there is excess resin and/or it is starting to gel/thicken. Infusing is much better as the air is sucked out before the resin arrives.

    The test is to grab a piece of skin with a pair of pliers and peel it off. If the bond side is covered in foam, you have done well. If the surface is not covered with foam, send a photo to your core supplier, designer and us for several different opinions on whether you should throw it away or not. Mine will be not. There are hundreds of boats with poor skin/core adhesion from the days before epoxy, bagging and infusion. Unless they are pounding hard into big seas, they rarely come adrift. If they do, drill a couple of holes in the delam and squirt it full of resin and it will be fine.
     
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  11. fallguy
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    fallguy Senior Member

    I assure you the sheets were wetout to the Gurit recommendation.

    I would enjoy for you gents to read this datasheet and hear your opinions on their vacuum "window".

    Summit is merely hosting the Ampreg data and not the supplier. I did not use Ampreg. I just want to hear comments on the method. Pay close attention to gel times vs vacuum turn on times.

    The plier test fails for me. I suspect Ampreg realized this and developed a protocol.

    http://summitcomposites.com.au/wp-content/uploads/2013/06/Ampreg-22.pdf
     
  12. fallguy
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    fallguy Senior Member

    I had another fellow tell me the same thing rob; to accept it, but neither he nor you will be in the boat. And the ultimate question becomes will the craft be seaworthy in a major delam event. Answer that for me; please n thanks.

    And what is odd is Ampreg's method contradicts you. You said made worse if starting to gel/thicken. Ampreg wants it gelled unless I read it wrong; hours into gel actually.

    Thanks. I appreciate the responses.

    The work was done well; the vac turned on before gel times. I suspect the top of the line epoxy vendor has not done their homework as Ampreg has done.

    How do you plier test triax? It only peels in layers before getting to the last, but still no foam is pulled.

    My resin ratios are low, but on panels with less vac; the ratio increases with the same poor bondline.

    I have to test the Ampreg method with Ampreg and my epoxy.
     
  13. gonzo
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    gonzo Senior Member

    If (hopefully never) it becomes a legal issue, pulling with pliers will be viewed as anecdotal and not scientific testing. All proper testing starts with a protocol. That includes all the parameters like testing for yield, ultimate tensile, speed of force increase, angle of applied force, etc. That data can prove whether the materials or the laminating techniques were at fault. Also, it can prove if a material was abused beyond its intended use and it was the fault of the operator. For example, a boat was run aground crushing the core and the laminate failed later. Further, even if it never becomes a legal issue, objective data is very important for the continuing improvement and development of materials and design methods.
     
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  14. fallguy
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    fallguy Senior Member

    Gonzo hits one out of the park this time. And it isn't about his bias to rescue some work.

    Wouldn't it be better for the industry if techniques like Ampreg's were well known and these issues are discussed and known?

    Until Monday, I had never heard of the Ampreg method. And it was a lightbulb moment.

    Still would like anyone interested to comment on the Ampreg methodology.

    I am especially curious how any air pockets would leave the laminate if you wait that long. Or if the cost of the air pockets is better than the alternative?
     

  15. fallguy
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    fallguy Senior Member

    This is also assumptive. If the process is not properly defined, what say you?
     
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