Glass/epoxy tensile strength ?

Discussion in 'Boat Design' started by Guillaume C., Jan 25, 2014.

  1. Guillaume C.
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    Guillaume C. Junior Member

    Hello, I have a hard time to find a proper value of tensile and compression strength of unidirectional fiberglass and epoxy resin.

    FYI, this data is to be used for making dinghy davits. The beams will be made in wood and unidirectional fiberglass laid in grooves on the upper and lower surface of the beams.

    I'm sure lots of you have those numbers laying around your desk! Thanks
     
  2. Guillaume C.
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    Guillaume C. Junior Member

    Soooo.... If I got the concept right, this is the place to argue for countless pages on stupid projects, but where no one share info to actually do something, right?

    Anyway, I'll do my scantling to what seem to be a fairly conservative 50ksi in tension with a safety factor of 5 considering all the weight on one davit. If anyone has better value, this would be the time to share as Ill do the glassing tomorrow

    Cheers,
    Guillaume
     
  3. groper
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    groper Senior Member

    1100Mpa in tension 900mpa in compression @ 55% fiber volume fraction - UD E-glass in epoxy matrix.

    Modulus of Elasticity = 43/42Gpa tension/compression.

    Source = Hexcel composites design guide.
     
  4. TANSL
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    TANSL Senior Member

    For small vessels has been written the ISO 12215 standard, which provides formulas to determine the mechanical properties of the laminates.
    It is not easy, nor prudent, given a value, because the properties are highly dependent on the composition of layers has been applied, or % by weight of the fiber.
    Not that we don´t want to share information with you but the question can not be answered with a number, if one want to be rigorous.
     
  5. groper
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    groper Senior Member

    Sure, but its hardly feasible to conduct coupon testing just do he can build some dingy davits... come on, how over regulated our thinking has become, thank god for litigation...
     
  6. TANSL
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    TANSL Senior Member

    groper, I don´t know if you know what I'm talking about. What I am saying is that there are formulas, very simple, giving completely appropriate values​​, and no recommendation that nobody knows where they came. Because the values ​​obtained in a project need not serve for another project built differently.
    Is it difficult to apply this formula: compression ultimate tensile strength = 150 * FI + 72 N/mm2 ?
    (FI = fiber content in% by weight)
     
  7. PAR
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    PAR Yacht Designer/Builder

    I'm inclined to agree in as much as the information Guillaume C. is after is commonly available, either in terms of accepted engineering standards or the physical attributes of the various materials involved (fabric types, resin formulations, etc.). This said, following ISO or other standards would be a bit of over kill on some things, but in the case of a davit, I think a prerequisite. Davits have to tolerate tremendous loading at times and it's at these high load times, you'll really want the structure to retain it's shape and duty service.

    The calculations for davits and their mounts, aren't especially complex and assuming a reasonable safety margin, not terribly difficult. I'm not sure of the value of unidi in wooden grooves for this application, but it might be possible to reduce some of the mass associated with a wooden davit, using this approach, if this is the goal of the engineering tactic (which isn't clear at this point). So, Guillaume, what are you trying to do, at least in regard to the unidi and wood interface? I do believe it's possible to incorporate what I'm picturing in my head (from you brief description), in this regard, but it would be more helpful if you provided some more detail and engineering goals.
     
  8. waikikin
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    waikikin Senior Member

    Cheers Guillaume,
    You started already;)
    All the best from Jeff.
     
  9. TANSL
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    TANSL Senior Member

    Maybe I was wrong to interpret what was intended is to know whether the deck, GRP, will was able to support the stresses transmitted by the davit. If I was wrong, sorry.
    On the other hand, if the aim is to make a GRP davit, I think it is not the most suitable material. But perhaps here too, I'm wrong.
    A davit is just a beam or frame under certain loads and is so difficult to calculate as any other beam.
     
  10. SamSam
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    SamSam Senior Member

    Using the search term "proper value of tensile and compression strength of unidirectional fiberglass and epoxy resin" this came up.
    You're figuring for straight down loads, have you figured for side loads as the boat gets slammed around and the dingy goes from side to side?
    http://www.matweb.com/search/datasheet.aspx?matguid=3f2253a553404b13893830617250b5d8&ckck=1
     
  11. Guillaume C.
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    Guillaume C. Junior Member

    Hello, thanks for the answers so far

    The goal is to make simple davits that will hold the 150 pounds inflatable and solar panel. The davits arms will be bolted trough the deck at their base but also rest on the stern rail so that the forces at the bases are not too great. Obviously the highest load point will be where the davits rest on the rail. The dinghy will be lifted 39" from the stern rail, and the davits are 3.5" thick at the rail.

    I am using fiberglass laid in groove mostly because it is easy to make a groove of varying depth, then fill it with the fiberglass, then fair everything to be flush with the top surface. Applying the fiberglass directly over the core would be a bit more tricky to get a nice aesthetic curve with the varying thickness...

    Side loads are not calculated as the dinghy will be lashed with diagonals when underway. (but there will be a carbon tube between the ends of the two davits to stiffen the assembly). Also wood is probably heavier than what is necessary for a core, but should handle the internals compression loads well, and is easier to get than high density foam!

    Anyway, I was planning to route out the groove tonight so I have to settle on a value. As you can guest I'm no engineer, so any help you can provide is very welcome
     

  12. Guillaume C.
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    Guillaume C. Junior Member

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    Ive settled for a low 50ksi tensile with a safety factor of 7. In reality I've routed the groove 1/8" deeper than required and fill them a bit more than supposed.

    Only problem so far, and this is the first time I laminate fiberglass so thick, is that I get some white spots in the layup. While playing with the fiberglass strands, they appears to be well wet but the resin is kind of creamy white. Should I be concerned with this?
     
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