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konaMike
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Group: Forum Members
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Hi fellow members,
I am building a structural rectangular carbon box beam 2.5” thick x 6.0” wide and 60” long. The structural objective is to equally maximize the specific bending (in both the vertical and horizontal axis) and torsional stiffness of the beam using no more than 800 grams of cured material. In the application for the beam there are no significant point loads and load distribution can be considered to be uniform across the entire surface of the object . Other factors such as the ultimate strength, fatigue strength, surface finish, layup, sourcing material, etc can be ignored for the purposes of this inquiry. At this point in the process I am only concerned with identifying the form that will produce the stiffest and lightest beam. As my background is more in the area of materials engineering, I am seeking the advice of forum members with structural experience. Having said that, my sense is a closed edge sandwich panel with uni-carbon faceplates and a carbon honeycomb core may be the best approach to achieve this objective. Would anyone disagree or suggest another approach?
Also, if a sandwich panel construction is the best approach, owing to the 2.5” thickness of the panel, I am having doubts about whether the standard “thin face sheet” sandwich panel construction will produce the lifghtest and stiffest result as opposed to a “double sandwich” design in which the face sheets of the main panel are also sandwich (perhaps with nomex core) panels albeit very thin - say 0.25” thick. My theory is that since “face panels” will be 10x stiffer thin face skins and only marginally heavier, this approach will allow the cell size of the main 2.5” thick panel core to be much larger and therefore much lighter. For the 2.5” core I am considering using “end-grain” pre-cured laminate sheet so my second question is whether the cell shape should be a hexagon. My sense is that a set of circular cells (short tubes) bonded to adjacent cells along their tangents would be lighter, have better bucking strength and provide superior shear transfer?
Thanks in advance for your consideration and assistance.
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torsten Ker
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Group: Forum Members
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Hi Mike
That really depends on what axial, vertical and horizontal load, distributed over the length of centre point that beam in gone have to take
You are creating a box profile
Your bean has total area of 1800 square inches, 1.2sqare meter
Looking at your requirement of mx 0.8kg carbon fibre that makes no more than 900gsm material for the outside wall without internal bulkheads and pvc core installed.
Even that it is carbon fibre and assuming that both sides are anchored solid to prevent twisting I would estimate 150kg to 200 kg horizontal load on the centre before distortion and twisting will occur, gradual deforming at max load support can be ruled out, when carbon goes, it goes at a bang
Overall hard to tell without knowing the use of the final beam
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konaMike
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Group: Forum Members
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+xHi Mike That really depends on what axial, vertical and horizontal load, distributed over the length of centre point that beam in gone have to take You are creating a box profile Your bean has total area of 1800 square inches, 1.2sqare meter Looking at your requirement of mx 0.8kg carbon fibre that makes no more than 900gsm material for the outside wall without internal bulkheads and pvc core installed. Even that it is carbon fibre and assuming that both sides are anchored solid to prevent twisting I would estimate 150kg to 200 kg horizontal load on the centre before distortion and twisting will occur, gradual deforming at max load support can be ruled out, when carbon goes, it goes at a bang Overall hard to tell without knowing the use of the final beam Thanks for the feedback Torsten, do you have any additional thoughts on my theory of ' double sandwich' construction as a weight saving measure or the advantages of cylindrical cells rather than hexagons?
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torsten Ker
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Group: Forum Members
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'double sandwich' is mostly or 'sandwich' is experimental at best, supporting and enhancing rigidity of such parts and do distribute the overall load stress a part can take before distorting or disintegrating structurally on exceeded load The problem is carbon fibre, it is strong and ridged but does not flex well and cannot deal with shock forces as much as glass fibre or Kevlar or Aramid That means if it flexes, twists, bulked out on load, it will break without warning As for fatigue, I have seen load of carbon fibre bike frames separating the re-enforcement (breaking the epoxy bond) on high stress areas such as fork heads which have turned into balsa wood like substance Have a look at that, I found it interesting https://oscarliang.com/carbon-vs-fibreglass/
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konaMike
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Group: Forum Members
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+x'double sandwich' is mostly or 'sandwich' is experimental at best, supporting and enhancing rigidity of such parts and do distribute the overall load stress a part can take before distorting or disintegrating structurally on exceeded load The problem is carbon fibre, it is strong and ridged but does not flex well and cannot deal with shock forces as much as glass fibre or Kevlar or Aramid That means if it flexes, twists, bulked out on load, it will break without warning As for fatigue, I have seen load of carbon fibre bike frames separating the re-enforcement (breaking the epoxy bond) on high stress areas such as fork heads which have turned into balsa wood like substance Have a look at that, I found it interesting https://oscarliang.com/carbon-vs-fibreglass/ Thanks again for your input Torsten. At this point in my design process, I'm still in ' big picture' mode. I just want to identify the most promising experimental roads to go down. I've been closely involved in the bike industry for many years and even owned a triathlon shop for a few years in the early 2000s. Unfortunately the bike industry is very traditional to the extent that innovation is limited to small incremental improvements. It's time for me to get out of that box with my new project LOL.
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torsten Ker
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Group: Forum Members
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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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Lester Populaire
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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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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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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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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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