Composite calculus

Discussion in 'Boat Design' started by JimmyJames, Aug 28, 2009.

  1. JimmyJames
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    JimmyJames Junior Member


    I am a recently graduates mechanical engineer and I'm on a new project of boatbuilding. My job is to calculate the tickness and the composition of sandwich laminates. I had some course on that at school but I don't really remember anyting on that. I have all the mechanical proprieties of the resin, clothes and core material and I just want some advice on how to calculate the sandwich proprieties. I would like to make my calculus by hand first and maybe later i will use a FEM software. Could someone helps me get some informations on composites calculus method?

  2. jehardiman
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    jehardiman Senior Member

    If you are interested in analysis of the layup only, get a copy of Introduction to Composite Materials by Tasi & Hahn. If you need more, pick up Engineered Materials Handbook Vol 1, by ASM.
    Last edited: Aug 28, 2009
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  3. daiquiri
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    daiquiri Engineering and Design

    A good book is a prime choice. If you cannot afford it or don't have access to it, you can search the internet for "Classical Laminate Theory". There are sites around with some really neat explanations and lessons.

    Or, you can get a copy of Scantling Rules for ships by some classification society.
    For example, Bureau Veritas' "Rules for the Classification and the Certification of Yachts" (which can be found on their internet site, and it is free) explain (or prescribe) the laminate calculation and verification process in the Part B, Chapter 12, Sections 1 through 5.
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  4. Ad Hoc
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    Ad Hoc Naval Architect

    Once you have followed the above advice, be very careful using FE with composites. Since it is not isotropic, the properties that require an input into the software assumes an "ideal". In reality this is never the case.

    If you can, or know someone who can for your layup from the above advice, then make a sample/coupon. Mechanically test it to obtain the usual parameters which you can use to validate a simple bend test in the FE.

    Once that is done, never read direct the through thickness results.

    This all assumes you have an understanding of FE and how FE works, regards of the materials used..this is a mine-field initself!
  5. jehardiman
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    jehardiman Senior Member

    Very true, as poor lay up practice can reduce ultimate strength by 80% or more. Additionally, it is important to always keep in mind flexture as related to layer shear and therefor FE end fixture, as the lay up that works best might not be the stongest and neccessary tabbing may cause failure in unexpected ways.

    There are many pitfalls in actual composite fabrication, so there is a fair amount of trial and error in some of the more 3d shapes like hull and bulkhead joints.

  6. ABoatGuy
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    ABoatGuy Member


    All good advice. Get to know the yards your working with and test as much as you can. You will soon get a pretty good indication of their capabilities vs. computational solutions. And if you have the chance work with someone that has been doing the job for a while. As noted above composites, by their very nature, are not isotropic (and that can be a great design tool), so you need to be conservative until you get some test results to back up your numbers.
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