Pull off testing

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

  1. gonzo
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    gonzo Senior Member

    I got it. However, they treat the failure as crack propagation and not as a maleable material failure.
     
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  2. fallguy
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    fallguy Senior Member

    The failure is interlaminar; not bondline, friends.

    Sorry, but I made a mistake and was in a huge rush to leave last week.

    I drafted the thread away from the shop.
     
  3. fallguy
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    fallguy Senior Member

    16ABA2C8-73AD-4791-A044-A8289D4FD298.jpeg

    This pic is upside down, but you can see the interlaminar parting is rather easy with the plier test.

    Comments welcome.

    Sorry about the mistake.

    I got a little foam on the 2nd biax layer pull.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    There is only one definition. It is like saying one is either pregnant or one is not. There is no halfway.

    You need to understand what brittle failure is first before you answer that one.

    Ductile failure requires a lot of energy. Brittle failure requires very little energy.
    In brittle failure the parts can usually be put back together rather like a jigsaw, as eventice of the brittle fracture surface. In ductile failure there is often significant elongation into the plastic zone that it is impossible to put back together.

    The issue with foams is that they are polymers. As such they all have a chain linked structure, either in straight lines, zig-zag doubled/single bonding etc. Applying a load tends to unravel these long links/chains thus typical polymers have a high degree of elegnation. If you look at a typical linear polyethylene structure is is considered a "soft" material. The degree of polymerisation affects the behaviour. Since the same polymer now with now more covalent bonds such as phenol formaldehyde has a high degree of polymerisation. Thus despite being a polymer - high elongation linked behavior - it has a rigid structure and as such exhibits a brittle like behaviour. This leads to what many called the rough rule of thumb in terms of rigidity of polymers. The rigidity and melting point increases with the complexity of the molecular structure.

    But to depressed the foam causes what looks like brittle failure, as little pressure, or energy, is used to cause it to fail as such. It cannot be put back together as it is in many small pieces.

    Thus there is no one statement that fits all when it comes to polymers/foams.
     
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  5. gonzo
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    gonzo Senior Member

    I think that if you treat delamination as crack propagation, then there is a start, growth and final total delamination. I do understand the difference between ductile and brittle failure.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    You can't redefine words and terms to suit your opinion/narrative. Engineering is based upon facts - not opinion.

    Ductile.
    If you apply a load to a material...it slowly begins to yield. It stretches and stretches until the yield point has been exceeded. It can then continue into the plastic region of the material. At this point the elongation is considerable, compared to its original length. In doing this process a lot of energy is used.

    Brittle.
    If you apply a load to a material...it has no deformation or elongation. There is no going through the yield to the plastic region. The deformation is negligible. In other words - it doesn't stretch.
    In doing this process very little energy is used.
     
  7. rxcomposite
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    rxcomposite Senior Member

    We are talking composites here as shown by the diagrams posted. Malleable materials such as lead, copper, gold can be hammered/stretched out of shape without breaking or cracking. You are introducing other terms to the topic which muddles up the issues.
     
  8. gonzo
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    gonzo Senior Member

    I am not re-defining words. I pointed out that I was looking as delamination between layers of fiberglass as a crack. If you disagree that it is a crack, I am open to discuss it. However, I did not twist any definitions to suit my narrative.
     
  9. rob denney
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    rob denney Senior Member

    The epoxy is loaded in peel. A very small area to resist a large load. Something has to break. The glass is stronger than the epoxy, and the load is spread over a much larger area at the foam/glass interface. The more brittle the epoxy, the easier it is to break it in peel loads. If you want to increase the strength, post cure it.

    This assumes there was no contamination on your cloth and the resin was mixed correctly.
     
  10. fallguy
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    fallguy Senior Member

    Post curing is planned; however, it has not beem done, yet. And was not done on the sample.

    What a great response.

    Thank you.

    If I send off samples; I will also post cure them.

    Thank you twice.

    By the way, there is no glass contamination and there is no mixing error.

    During the course of my build I did have two mixing errors. One time I moved the epoxy unmixed to a paint tray and forgot to mix in the tray. I never move to tray ever now without mixing. The other occurred when I had planned a 4/8 mix to 12 ounces, then ran to 5 and instead of running to 15; I added the 15 and went to 20. That was a fillet batch and required removing it all. The other one required removing two layers of biax. In both instances; it was the last project of the day and I was rushing to finish. On all the vac table work, I take extra steps to verify mixed quantities and we always run a two minute timer on mixing. It is a bit unneeded as the epoxy becomes clear when adequately mixed. The fact that I even know what caused my errant mixes ought to help define the level of precision. I have used 100 gallons of epoxy in this build.
     
    Last edited: May 15, 2018
  11. Tungsten
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    Tungsten Senior Member

    I get the same peel results Fall man,a thin biax skin separates, its only when i peel test a thicker skin like 4 layers 12oz thats when the whole skin comes off including a thin layer of foam.
     
  12. fallguy
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    fallguy Senior Member

    I just bumped the part on an edge and it peeled.

    Sort of scary for the newb.

    I figured I must have f'ed up badly, but how? I got a little snippy with the epoxy manufacturer, now I have to send their tech guy chocolates and flowers.

    Thanks for the post.

    I am getting a 9oz layer that peels what seems too easily.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Because a thin layer will pull at about 90 degree to the laminate. A thick one somewhat at a lower angle. Lab test (shear) is in plane, parallel to each other.
     
  14. gonzo
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    gonzo Senior Member

    That is not correct. There is a stress concentration factor (Kt) depending on the sharpness of the crack. Further, crack propagation, like fatigue, is a function of time. A fast pull test may show a higher value than a slow because it didn't give enough time for the crack to propagate.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    That's nonsense. Two layers pulling apart is delamination - that's it.

    Kt is a geometry related issue with regards to stress flow around a discontinuity. Two completely separate issues. You're mixing apples with oranges, and just guessing.

    Well..so many issues all unrelated int this it is hard to know where to start.....so it'll go down the usual rabbit hole of polemics and misdirections to save face. Pointless!
     
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