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Lester Populaire
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+x+xhow 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? well at a certain point you just have a tubular structure and not a sandwich panel anymore. top skin a thin sandwich with a rohacell or nomex core, bottom skin as a single skin (on traction side) and on the sides just a +/-45°single skin to transfer shear loading and give torsional rigidity. but in that case i would switch directly to BIM as a production method. And just like that you are back to incremental improvements that you didn't like in the bike industry 
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konaMike
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+xMike, 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. Thanks Rosta, my sense is that there is an upper 'thickness' limit for a panel with thin face skins beyond which it is no longer the lightest approach for a given stiffness? For example, if my beam was 300 mm x 300mm, would it be lighter to use thin panels as the face skins with a set of internal bulkheads to carry the shear....or would a sandwich panel still be lighter?
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konaMike
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+xhow 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?
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konaMike
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+xThe 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. Hi Lester, yes, the surfboard use case represents the problem well. As you have observed, the density of the foam required to handle the point loads on the surface is unnecessarily heavy for handling the shear loads especially when you consider the board thickness. So to solve for both modes of loading, you could use a lower density core with stiffer face sheets for handling the surface point loads. My objective is to get try to get the main core weight below 300 grams which is well below that of a typical foam core using 2 lb cu. ft. density urethane or 3 lb cu. ft. density honeycomb. My theory is that by using much stiffer face sheets and the strongest possible core material, namely, end-grain carbon, I could lower the weight of the core (by increasing cell size) well below that of standard honeycomb or foam.
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Rosta Spicl
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Lester, you right of course. I would like to say only from panel theoretical rigidity point of view excluding all the other criteria. The rigidity is affected by the outer skins much more than the inner skins. The neutral axis closest skins has no effect..only increasing the weight. However the weight/material efficency/rigidity is totaly nonsense...I agree. Finaly , I think, we both agree, outer skins like a sandwiches has a positive effect for rigidity, but the weight is higher. There is a necessary to thinking about the ratio between more rigidity and incerased weight.
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Lester Populaire
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Group: Forum Members
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+xMike, 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. While what you are writing here is true, i feel like it is missleading as the bending stiffness of the assembled sandwiches would be much lower if you normalize it by it's weight. What you are describing is not an efficient use of material as you want to maximise the distance to the neutral axis (steiner's theorem). Same for torsional rigidity you want to maximise the cross-section. In reality this will have limits as at some point the skins will become too thin, the core will become too heavy or your design will become too bulky for other reasons.
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Rosta Spicl
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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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f1rob
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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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Lester Populaire
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Group: Forum Members
Posts: 311,
Visits: 13K
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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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torsten Ker
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"Unfortunately the bike industry is very traditional industry is very traditional "
Does apply to most, unless proven wrong or shown better we just carry on believing we are right :-)
I found EasyComp a very evolving and inovative company providing excelent materials I can trust. Been ordering stuff since 2012 only resorting to others if can't find it here
Let me know about the project, I love learning, mostly by trial and error, tough
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