FRP bonded connection calculations

Discussion in 'Boat Design' started by boats_designer_fr, Jan 31, 2019.

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  1. rxcomposite
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    rxcomposite Senior Member

    Agree with that. The forum is not only about the poster asking self serving questions. There are others who are following and would like to know the answer or how to solve a similar problem.
     
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  2. gonzo
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    gonzo Senior Member

    If the core fails, the UTS of the plate and laminate is irrelevant though.
     
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  3. fallguy
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    fallguy Senior Member

    Yes, of course, but if the design is done right; it would alleviate concern about the core. i.e. of great enough area. And backing is a rather obvious solution, but not always possible.

    I am actually doing something similar in my build where we did not imsert a hd core into the hull and made an engineering change. So, we will be glassing an HD core over the hull. The difference for my project is the forces are not the same direction. They are up and down ftmp.

    My concern is interlaminar shear of the substrate, because I know it is weakerish(weakerish!). I am tempted to remove the weaker glas even and just lay in new..

    The feather in our cap, if you will, is we can bond in two places as the work is on a corner, but there will be walking on the middle of the part where my interlaminar shear concerns are.... I am going to be asking you to do those tests for me very soon. Please also look for a private message as I am hoping for your help on another part of the project.
     
    Last edited: Feb 5, 2019
  4. Ad Hoc
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    Ad Hoc Naval Architect

    And in the attached joint (and applied load) by the OP...through what mechanism would the core fail?????
     
    Last edited: Feb 5, 2019
  5. Angélique
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    Angélique aka Angel (only by name)

    If not checked for strength, maybe like this...

    fail 1.jpg

    Or this...

    fail 2.jpg
     
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  6. fallguy
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    fallguy Senior Member

    If you glue the base to the hull/bh; and you do nothing else but apply load, either the core or the laminate would probably fail. This is basically a coupon test, right? Of course, this was never the plan in the first place...we expect to laminate the plate with glass of sufficient strength and area to stop it from being a simple coupon test.

    I think Gonzo is estimating the core or glass could still fail with the glass overlay.

    At some point, to be fair, there is a weakness in the system, no?

    Kind regards.
     
  7. Alik
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    Alik Senior Member

    The best solution is this case would be steel T-fitting, backing plate, and/or solid laminate in attachment area. It is difficult to predict how core will behave. Cores are very inconsistent in properties; also many new cores are not well proven.
     
  8. TANSL
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    TANSL Senior Member

    Someone, besides me, thinks that the union proposed by the OP is not correct? Is it the first thing that should be "designed"?
     
  9. fallguy
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    fallguy Senior Member

    May I?

    While not ideal, we do not know the circumstance. A better answer would be, not ideal, but if you insist, here is how you calculate for your less than ideal system.

    So, yes, even a novice like me says not ideal, but it may be sufficient.

    So, very frustrating to see people with more knowledge than me just say not best, even if that is valid.

    Kind regards Ignacio.
     
  10. TANSL
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    TANSL Senior Member

    @fallguy your kind and conciliatory tone is very much appreciated.
    The issue that I raised from the beginning is that this union is not valid and, therefore, any calculation procedure that is explained for the "valid" union, it is not possible to apply to this case. You may not be an expert but I am sure you have common sense and, since you do not need to discredit me at all, you may admit that I have some part, perhaps not all, of reason. In summary, if someone knows how unions of that type are calculated, please tell us. I do not know it and I'm not going to pretend to be an expert by telling generalities about how the correct joints are calculated. Speaking of generallities is useless. What would you say if I answered: the union should be calculated as best as possible ?. Would you believe that I am an expert since I did not say anything wrong or false ?. Well, that is what is being said in this thread.
     
  11. Angélique
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    Angélique aka Angel (only by name)

    Things like these down below give me peace of mind...

    peace of mind.jpg
     
  12. fallguy
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    fallguy Senior Member

    Actually, had you stated you did not know how to answer as drawn and that it is not the optimal solution; I doubt AH would have gotten so riled up. Some of his frustrations were very valid because you offered so little and essentially said, 'not that way'.

    For example, had I responded to the OP, you need a backing plate; it would also be worthy of rebuke. But is not a backing plate a better solution? Arguably, it is...

    But the OP did not ask that.

    And in boat building; we may strive for ideals, but they are not always attainable.

    Had the OP asked what is the best way to do this connection; your response would have been better accepted.

    Here in the US, we call the answer you gave cheap..because it doesn't help much. Is it true there are better ways to connect?' Sure. A truth does not an answer provide.

    Kind regards. Please don't be offended.
     
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  13. TANSL
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    TANSL Senior Member

    And why should I feel offended, because you have expressed your opinion, in an educated way, about my posts? No, I'm not like that. You have taken an opinion about me based on my answers that, may be, poorly expressed on my part have led you to make certain comments and it is very much appreciated that you express them in the very weighted terms in which you have done so.
    Contrary to what you think, I believe that I can not (nor should I try) to give an adequate solution to that union and I will try to explain myself. A structure is formed by a set of pieces that fits like a puzzle. Each piece is interacting on several other pieces of its environment and, if you do not know anything more about the whole, or at least the surroundings of the area with problems, you can not give the appropriate solution. The only thing that, in my opinion, can be said is what I said: that solution is not correct. The materials used for the core (if that is a sandwich laminate) are not designed to withstand efforts in the direction marked by the OP, they are not worth for that. From this point of view, the proposed union is not correct. The union of RPF and steel is very problematic. Only the different behavior with changes in temperature can lead to problems that are not solved with "normal" solutions. From this point of view, the proposed union is not correct. What is the correct solution? : I do not know. And therefore I can not say how to calculate it.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.
    But that would imply that the OP would either:

    1) select materials and an arrangement that woudl fail
    or
    2) Not.

    As already noted by:
    The post from Gonzo was just his typical one liner throw away comment that does not answers the OP's question and adds nothing to the debate/question at hand and is merely argumentative - as has been pointed about by many others on many other threads. Ergo - what is the reason for the pointless obvious comment?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Not quite.

    A coupon test is where you make a piece of of your proposed layup, all made as you would in the shipyard. Cut a piece out of the sample made and then you test it. To determine its mechanical properties,, its 'E' in the 3 axis of a 'volume', in a nut shell. We don't do this for steel ally or any other metals, because these are isotropic materials. That means their mechanical properties do not change with the direction of axis - same in all 3 axis. So whether you apply a load vertically, horizontally transversely, the the behaviour of the metal remains the same.

    But with composites, this is not true. They are generally orthotropic properties in 2 of the 3 axis being the same or sometimes anisotropic where it is different in all 3 axis. Coupled with that, the method of manufacture, the QA part, also plays a role in the final properties.

    Thus you make you laminate, as you woudl do later, but a small test sample for testing. You test it, and hey, you now know the properties of your material layup.

    So when applying a load, you can calculate - will it pass or will it fail.
     
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