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Lester Populaire - 10/2/2020 6:16:34 AM

The micro sandwich doesn't make sense for your application as far as i can tell.
The only application i can think of right now would be a surfboard where you want to use a very light core to keep weight down. This core is usually fine with the shear stress but over time it will dent underneath the feet as it can't cope with the compressive stress.
In this case the micro sandwich makes for a skin that has a higher bending stiffness and will spread the load over a bigger area of the core which results in lower compressive stress.

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f1rob - 10/2/2020 9:31:22 AM

how about starting with a foam core too,in its simplest form you can cut it along its lengh in both the vertical and horizontal plane
carbon wrap each off these smaller farmers before joining an then wrapping the whole assembly.
you now have a caton tube with 2 internal webs, if you leave the ends open you can remove the foam too lighten it even more.
you also have the option of cutting the foam into as many sections at the start and having a many webs as needed

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Rosta Spicl - 10/2/2020 12:03:39 PM

Mike, your questions are very interesting:
Double sandwich:  The sandwich panel theory says the rigidity (stifness) depends on final thickness. It doesn't matter how many sandwiches are bonded together, the final rigidity depends on their final thickness...but not so easy. In this case the final rigidity is affected by the skins own-rigidity, depends on modulus of elasticity (MPa). It generally means, you have 100mm thickness sandwich and 10x10mm bonded sandwiches together, the final thickness is the same (100mm), but it's final rigidity will be a little bit higher on 10x10mm than 100mm sandwich. The ratio is possible to analyticaly calculate, but you have to know exactely material parameters, especially skin and core material included and that's a big issue on fiber composites.
Cores: From the panel rigidity point of view is totaly doesn't matter what kind of honeycomb is used, if the core is symetrical. It means HEX = circle = foam. There is a difference between HEX and OX, depends on load directions.
Visit www.compotech.com, this company located in CZ developed their-own spreading technology, and FESTKA brand cycles producer (www.fetka.com) uses their tubes.

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konaMike - 10/2/2020 5:37:57 PM

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f1rob - 10/2/2020 9:31:22 AM

how about starting with a foam core too,in its simplest form you can cut it along its lengh in both the vertical and horizontal plane
carbon wrap each off these smaller farmers before joining an then wrapping the whole assembly.
you now have a caton tube with 2 internal webs, if you leave the ends open you can remove the foam too lighten it even more.
you also have the option of cutting the foam into as many sections at the start and having a many webs as needed

I hear you and my sense is that honeycomb or pure foam may not be the lightest approach beyond a certain panel thickness. This is the open question. Perhaps the webs you have suggested could also be panels to increase their buckling strength? I have actually tested a similar approach with a urethane foam core. I moulded the foam plug using two part urethane foam. It came out at 550 grams, so a little heavier than 2 lb/cu.ft. But 550 grams for the core is 70% of my total target weight...so way too much. I then drilled out a few 1.5 inch diameter cores using a whole saw. Each core weighed 2.61 grams which is slightly heavier than the equivalent diameter carbon tube  of 1.5 in dia x 2.5 in len x 0.007 in thk wall. This is when I realized that by replacing the foam with carbon tubes, it may be possible to reduce the core weight by 50% or more and still increase its shear strength. The problem of course becomes that the out face skins must be stiffer to span across the large diameter tubular cells, so why not convert the skins to thin panels made with nomex honeycomb?