Divinycell vs Core Cell

Discussion in 'Materials' started by CatBuilder, Dec 23, 2010.

  1. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    PB Number 51 Feb/March 1998. You should read the issue because it explains why different core fail. SP Gurit-Corecell A foam- for Hulls and dynamic loading also has graphs and some illustrations but is incomplete. There is also similar article on Composite fabrication why core fails.

    Anyway, if you go by the direct weight convertion, the shear stress strength will be high so you are safe.
     
  2. Brian2009
    Joined: Jan 2011
    Posts: 22
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Sun Valley, Idaho

    Brian2009 Junior Member

    Sorry for my ignorance, what does "PB" stand for in the reference you quoted? I haven't been able to google it successfully. Thanks.
     
  3. Steve W
    Joined: Jul 2004
    Posts: 1,824
    Likes: 63, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    I assume he means Profesional boatbuilder magazine.
     
  4. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    That's correct. Thanks Steve.

    You can't google it. You can get a free subscription but you have to pay a minimal fee for old issues.
     
  5. fcfc
    Joined: Feb 2005
    Posts: 779
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    I am a bit surprised of your graph, because of http://www.diabgroup.com/europe/literature/e_pdf_files/man_pdf/H_man.pdf

    I know it is manufacturer figures, but it states H100 has a minimum 25% shear strain, and typical 40%. Which is better than what your graph show.
     
    1 person likes this.
  6. CatBuilder

    CatBuilder Previous Member

    You can get the issue in question for $3.50 as a pdf from the publisher.

     
  7. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    The comparative graph was published by PB magazine and quite agree with "Corecell A-Foam - For Hulls & Dynamic loading.mht" which is currently in the web.

    As for the material properties, it was taken from a DIAB CD which was given to me in a boat show some 3-4 years ago.

    However, looking at the site you linked and comparing it with what I have, it seems there is some dicrepancy. The numbers are generally off by 7 to 15%. Some higher, some lower. I wonder why? It is the same material specs for Dvinycell H.

    I am getting 20 % nominal and 12% minimum for H80 (shear strain) and 24% & 14% for H100 according to the tables I have v.s. 30-15 for H80 and 40-25 according to the link.
     
  8. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    fcfc,

    Ok I got it now. The PB issue might be dated but it still hold true.

    If I assume the link is the most updated one, the compressive, tensile, and shear strength of the diab is better by about 7 to 12% which places it higher in the graph closing in on balsa. The elongation is much better but it will not be in the same category as corecell.

    Corecell is "weaker" but more elastic.
     
  9. fcfc
    Joined: Feb 2005
    Posts: 779
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    Not exacly related to this thread, but there is also big difference in shear depending on how foam is contoured : http://www.acmanet.org/crj/volume2_issue1_papers/trofka.pdf


    Also, for core issue, from memory, seems to remeber that high modulus, on a technical paper no longer accessible, classified cores in 3 category.
    Balsa and honeycomb as low elongation core. cross linked pvc (all brands) as medium elongation and corecell airex r63 linear pvc as high elongation.
     
  10. Herman
    Joined: Oct 2004
    Posts: 1,618
    Likes: 89, Points: 48, Legacy Rep: 1240
    Location: The Netherlands

    Herman Senior Member

    The problem with manufacvturers numbers is that they are all bullshitting the story. In all promotional materials their own material is the best. I even know of a company that claims their product retains more than 90% of its strength over time. The fun thing is that the strength at T=0 starts at 110%.

    The only way is to have tests done in a controlled environment, all cores the same treatment (layup, resin, temperature, etc) and then test it. And do the same for panels with equal stiffness (99% of cored panels are designed for stiffness).

    Unfortunately I have no means of doing this, although I can arrange something (one of my customers has some testing equipment, although no hydromat). Perhaps for the summer...
     
  11. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    The comparative graph is only a guide for someone designing in composites. Manufacturers data sheet should always be used for computations to prove it will work. Material testing increases the level of confidence in the materials and computations.

    The core is only one part of the equation. There is still the matrix (resin) and the fiber and the process to be taken into account. There is also the environmental conditions to be satisfied such as heat and slamming. It is the combination of the materials and process that satisfy the equation.

    For example, if the structure is designed to absorve the whole of impact load, the designer will choose carbon fiber, end grain balsa with epoxy resin infusion. All in the "stiff but strong" category.

    If the structure is designed to flex to absorve most of the impact, then a core that elongates (corecell, airex) combined with Vinyl Ester resin (longest elongation) and uni directional fibers (thin, low resin content, flexes) works well.

    An in between solution could be infused epoxy. uni, and divinycell as long as the flexing and strength is there before catasthropic failure.
     
  12. Herman
    Joined: Oct 2004
    Posts: 1,618
    Likes: 89, Points: 48, Legacy Rep: 1240
    Location: The Netherlands

    Herman Senior Member

    The elongation of VE that I see over here is generally 2-4%, where epoxy is 3-7%.
     
  13. CatBuilder

    CatBuilder Previous Member

    Just for a data point, the Silvertip Epoxy I am using to laminate with has an 8% elongation. The adhesive/gluing/filleting epoxy I'm using has an 11% elongation.
     
  14. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    I have VE= 5-6%, Epoxy=4%.

    Again, those are just design guide. You have to choose the material property that satisfy the equation. The idea is not to deviate much from the original design criteria.

    As Herman has said, the manufacturer is BS the story. If the ads says that elongation is good, would you grab an Airex, then choose a carbon fiber for the skin because the ads say it is stronger?

    There is a wide range of material in the same group to choose from, especially epoxy.
    If it says 8% elongation, what is the ultimate strength? And what will be the strength at what temperature?

    Take the case of the core. If the calc shows that you need 1" to satisfy a given shear strength and finds Divinycell fits the bill, but then choses a corecell with a lesser shear strength, you need to increase thickness of the core.

    Or take the case of a boat built with traditional Mat/Roving and used all Unis instead. The Unis (at the same tensile strength) will flex more due to reduced thickness leading to flexing, then to cracking, then to osmosis. If you are a manufacturer, you will be in trouble due to warranty claims.

    Catbuilder is very cautious. Before he switch material(s), he ask the designer and everybody else in the forum.;)
     
    1 person likes this.

  15. fcfc
    Joined: Feb 2005
    Posts: 779
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    Just a thought about the initial post.

    For resin and fiberglass or even plywood, you can have a choice of resellers and somewhat standard prices.

    But for foam core, it is much much harder. As said in french, prices are made on customer's face (à la tête du client).
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.