Calculating Stress on a Panel, and Material Selection

Discussion in 'Boat Design' started by Zac Penn, Dec 10, 2015.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Flat panel is a dopey solution anyway, something with plenty of shape to it will be stiffer and not need a sandwich.
     
  2. fredrosse
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    fredrosse USACE Steam

    Corner Joint Loads

    Some quantified information as to how the panel joint loading is worked up. Please note that this information is just for illustrative purposes, and details of setting the design criteria, and development of actual design should be worked up with a person who is qualified to do the design. A legal requirement where potential personnel safety is concerned.
     

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  3. TANSL
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    TANSL Senior Member

    fredrosse,I do not know if you've had in mind that a tank should be calculated for a loading height of at least the height of the vents above the tank top, plus some other consideration, if large tanks.
     
  4. Zac Penn
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    Zac Penn Junior Member

    Okay I am back...
    I don't have time or money right now to deal with an engineer so I am going to just move ahead and see what happens in my parking lot. My fiberglass sales person is a very easy going guy and is as interested in this project as I am so we went ahead and made a quick little test panel to see how it reacts to certain stresses. I just molded and hand laid it up yesterday morning. Here are a few pictures showing how it came out...If nothing else I can now make some toboggans!
    20151217_140902.jpg

    20151217_140927.jpg

    This panel was with two 3/4" thick balsa cores beveled at 45 degrees. We did one layer of 1808, 3/4" balsa, mat, 3/4" balsa, 1808, 1808

    This at least gives you guys a better idea on how I am envisioning with this project. I only molded the flanged corners on one side due to time.

    I am already becoming spoiled about fiberglass layup (freaking messy, sticky, and smelly) and I am want to vacuum infuse the next test panel. I can gather up the supplies pretty easily. This will not only yield me a lighter panel but a stronger one and probably take less time to produce each panel. Time is money so it may end up being close in price over the long run.

    Due to the balsa being a porous core material as it soaked up resin it off gassed the trapped air under the top layers of 1808. We didn't notice this until about an hour after laminating and I was told it wouldn't have been a problem if it was either vacuum bagged or infused. So that is another reason to go that route. If I gather the supplies to vacuum bag then i might as well infuse right?

    I think for the next panel I am going to go with 1 1/2" Carbon-Core plastic honeycomb material.
     
  5. Zac Penn
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    Zac Penn Junior Member

    After plenty of searching I ended up finding this paper that has what i believe to be good formulas for figuring out the deflection on sandwich panels....
    https://engineering.purdue.edu/AAE450s/structures/sandwichstructure.pdf

    I am pretty confident that I have all of the variables figured out except for the correct Moment of Inertia formula for a sandwich panel. Unfortunately that nice Hexcell paper did not mention a formula for finding the moment of inertia for a simply supported sandwich beam. Or at least I couldn't find it. Thanks for any help.
     
  6. TANSL
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    TANSL Senior Member

    The calculation of the moment of inertia I can do it for you, if you give me the composition of your sandwich panel, the number of component layers and their thicknesses.
    I must warn, however, that the calculations in the attached document to your post, refer to the beam theory but what you probably have to calculate is a panel with the 4 recessed edges. That is, you should use the theory of plates, not the beams theory.
     
  7. Zac Penn
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    Zac Penn Junior Member

    Thank you for offering that. If you could please post the formula I would appreciate that. It will help in future core thickness estimations.

    The panel I am trying to find deflection for will be...
    Two layers of 1808 resin infused at 0.028" thick per layer
    CarbonCore Plastic Honeycomb core at 1.5" thick
    Two layers of 1808 resin infused at 0.028" thick per layer
    Total panel thickness should end up at 1.612" thick
     
  8. TANSL
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    TANSL Senior Member

    I regret not being able to explain how the inertia of a rectangular section composed of several layers is calculated. My difficulty with English do not allow me to give a clear and reasoned explanation.
    The moment of inertia must relate to a certain axis which, for each layer, will be a line passing through the centroid of the section. The moment of inertia relative to said axis is equal to (B * H ^ 3) / 12, where:
    B = section width (measured parallel to the axis)
    H = height of section (perpendicular to the axis)
    This is not enough. If anyone can explain, I beg do it for me.
    Simplifying for your example, if you consider a strip of 1 "wide and 1.612" thick:
    I = 1 * 1.612 ^ 3/12 = 0.349 inch ^ 3
     
  9. rxcomposite
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    rxcomposite Senior Member

    Can you give us a link? I have never come across a carbon core plastic honeycomb.
     
  10. Zac Penn
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    Zac Penn Junior Member

  11. Ad Hoc
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    Ad Hoc Naval Architect

    If you understand structural analysis, as you claim to it does not require a full command of English (which never stops you form commenting on thread and attacking posters), it is very straight forward.

    The structural strength comes from the structural stiffness, the EI.

    Thus, you establish the Young's modulus (E) of all the materials being used. Then pro-rata the size as an equivalent section for each one of the differing materials from a base line, usually the material with the lowest E. Then it is a simple second moment of inertia calculation using the new 'equivalent' size, just as one would for any other structural section made up of several sections.
     
  12. TANSL
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    TANSL Senior Member

    Thank you, Ad Hoc, the truth is I did not expect it was you who kindly help me explain it. It has been, almost, a lecture, with good knowledge and use of language. Very friendly.
    Now, what does all this explanation of stress and strengh with how to calculate the moment of inertia of a section of a beam?. That, if I have not misunderstood, is what the OP asked.
    I'm afraid I do not know how to explain my self (probably due to lack of knowledge) but you do not know understand what others say (probably too much pride.). Perhaps it is, as you were explaining in another post, an error with your iphone.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Well, this is very clear.

    Either it is your English or your technical ability....please state which. If the former, why do you post if you are unable to explain simple structural concepts in the language of the poster?

    No....it is not enough.
    That is only the "I" of each individual "layer", of the built-up section. One must adjust for the actual "I" of the whole section about an axis (Using I=ah^2 for each) and thus use the parallel axis theorem to obtain the the inertia about its own neutral axis about a datum before using the parallel axis theorem to correct for the whole section. .

    Why do you post?..what is your motivation..other than trying to be the first to post...yet as the thread and questions evolve, you claim your English is insufficient (fine, I admire anyone with dual or more language ability), yet as it becomes more technical you hide behind this excuse as it is clear your ability to answer the actual question in hand is lacking. Maths/science is a universal language.

    Thus why do you post??....it does not seem to benefit the poster at all..only your ego and when that is not satisfied, you resort to attacking posters for pointing out your flaws. Very professional!
     
    Last edited: Jan 13, 2016
  14. TANSL
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    TANSL Senior Member

    Ad Hoc, answer your endearments will not add anything positive to this thread.
     
    Last edited: Jan 16, 2016

  15. rxcomposite
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    rxcomposite Senior Member

    Zac,

    I did some calculations with the following assumptions;

    250 lb/ft2 (12 kN/m2) pressure at bottom. Safety factor is 3.
    4' X 8' (1.22 x 2.44m) panel size. ends firmly fixed

    Deflection is 0.23" (5.9 mm) with 40mm "Carbon Core" and 3.5 mm and 4.6 mm inner and outer skin.

    Carbon Core has a very low shear strength equivalent only to 60 kg/m3 of corecell A300. It ranks very low on Specific Shear Strength, only slightly better than polystyrene or rigid PU foam. In any case, it is better than Nomex honeycomb but those are typical values for honeycomb cores.

    If you have some other dimensions/pressure I will recalc and post the result.

    What is the weight of your glass 1800? Is it a woven roving?
     
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