hi everyone, last week I infused a flat panel with following reinforcement:
1/4 divinycell foam and on each side one layer of 3k 9oz 4x4 carbon and 2 layers of 9oz woven glass.
infused each panel separately and then glued them to the core with thickened resin under vacuum. it is 67" at the bottom and 18" width … IT is just as rigid as I wanted it and weighs 7lb. however, now i wanted to know beside using thicker core, how can i achieve a lighter part?
also, can I use brass tubes to protect the core where I am using bolts?
The glass in the layup does not really take any load in tension. However, the increased thickness of the laminate might help resist buckling loads in compression.
You could replace your 2 layers of glass with 1 layer of carbon, and probably be slightly stiffer (although increasing the core thickness is more of a help).
Also infusing the part in 1 go will help reducing weight. Hard to get it 2-sided smooth like glass, though. (I do not know if that is a design criterium)
Instead of wovens you could also opt for multiaxials, which do not look that nice, but have a better Vf, so are lighter. UD carbon is also an option.
If you produce two plates and glue them together use a nomex honeycomb instead of foam.
And only use carbon fibre, it will be a big difference if you do these two things.
If you only need ony shiny side infuse it in one shoot with a light foam as core, safes time, money and is just a little bit heavier than infusing two plates and glue them together with a honeycomb.
thank you guys, well the reason i decided to go with additional 2 layers of glass was after i made some test samples and as you can see in the pic the panel with 2 additional layers of glass (left one) was stiffer than the one with only one layer of carbon on each side (right one).
the second reason was to add little thickness so when i secure the panel with bolts and washers the core wouldn’t get crushed(right pic). honeycomb is out of question, but which one would be better?
1- going to 3/8" core thickness and get rid of glass and use one layer carbon on each side and maybe doubling or tripling where i am going to drill and put bolts (buying a thicker foam is cheaper than adding extra layers of carbon)
2- using same core (1/4") and use 2 layers of carbon on each side (the advanteg is that i still have some 1/4" foam still left )
3- or both together? 
would going to higher weight carbon or type make any difference?
by the way thank you herman with helping me earlier with using freekote and glass mold, it is much easier and better finish than waxing and my shoulder thanks you as well and thank you dominik for free mti sample, it worked like a charm and I am actually making a little video of making the whole piece using MTI which I will upload soon)
Why not three layers of carbon instead of glas? And why no honeycomb?
My thoughts:
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9 ounce carbon is heavier than 9 ounce glass. Changing out an equal weight of glass for carbon will increase its weight. Since the density of carbon is 1.8 g/cc and glass is 2.55 g/cc a carbon fabric of equal weight is thicker. This will take more resin to saturate. If you want to move to an all carbon FRP sandwich then you need to use less than you were with glass. If you are getting a Vf of 50% then you need to use 11 - 12% less carbon than glass by weight to arrive at the same weight.
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Moving to an all carbon layup would increase the stiffness and buckle resistance of skins
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Moving to UD materials will will increase fiber volume fraction and reduce the amount of resin in the part. The downside with an increase in Vf for a specific volume of fiber is that the skin thickness decreases. This can make it more prone to buckling. The bonus is that UD materials have better buckle resistance than wovens of equal thickness. The interface between the skin and core becomes very critical in preventing the skin from buckling away from the core. The compression strength of the core in critical in preventing the skin from buckling into the core. The challenge with doing multiple layers of of UD materials is material availability. If you need fibers going in 4 directions than you need 4 layers of material…which may end up being heavier due to material availability.
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When you start pushing the specific strength then the fiber orientation and amount of fiber in each direction becomes very critical. Since you are working with a thin panel, having fibers in the 4 primary direction is often required for stability. I sometime use light glass on +/-45 to maintain the torsional stability of the part and then use heavier UD on 0* and medium UD on 90*. This creates a structure that has the required orientations for stability but maximizes the amount of fiber in the correct directions for strength.
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You might consider using some strategic hat stiffeners on one side of the panel if possible. This may allow you use less fiber but still maintain the required stiffness.
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Increasing the thickness of the panel is the easiest way to maximize it’s stiffness. Inserts can be used in the bolting location to prevent the fastener from crushing the core.
Often when pushing for a higher specific strength (strength to weight ratio) then some experimentation and failure testing is required.
Why not three layers of carbon instead of glass? COST
And why no honeycomb? No experience with honeycomb
If cost should not increase you will have to stay at glas and foam. Using carbon and honeycombs is more expensive.
You can try to infuse in one shoot, but than you will have only one shiny side.
The most weight is in your glas and core.
Wyo explained it very well.
Since we don’t know this parts application ( looks like an auto under tray including perhaps a splitter)…I can only guess some options to add strength while making it lighter.
Can this part be made with some shape added ? I mean as in adding bulges or spines along its length or even opposing?
These bows or spines can be hollow and add tremendous rigidity avoiding any core at all. This might be a smart way to tool up and also av
oid secondary bonding. (aside from added weight from fastener reinforcements).
(EDIT:)- This of course requires that you make a mold from a plug and then you do lose the easy plate glass form.
What? 9oz/yd2 of carbon is heavier than 9oz/yd2 of glass? That is like saying a pound of feathers weighs more than a pound of ham. I hope it was just a typo.
I would say if you don’t care about thickness, go all carbon, use UD if needed, and thicker core slightly. Do not use brass as bolt inserts, use steel or aluminum. Brass will oxidize and crush. ALL metals must be primed or coated before using in carbon, else they will corrode quickly!!!
It wasn’t a typo. Maybe you didn’t read the whole statement to understand that I was referring to the end result after saturation.
For further clarification:
9 ounces of carbon saturated with resin is heavier than 9 ounces of glass that is equally saturated with resin (assuming equal Vf between the samples). 9 ounces of carbon has a theoretical volume of 141.75 cc while 9 ounces of glass has a theoretical volume of 100.06 cc. Since the 9 ounces of carbon has a higher volume it takes more resin to saturate it. An FRP made with a single layer of 9 ounce carbon is .0039" thicker than one made with 9 ounce glass (assuming a Vf of 50%). An FRP using a single layer of 9 ounce carbon and a Vf 50% would weigh 14.75 ounces per yard while an FRP using a single layer of 9 ounce glass would weigh 13.06 ounces per yard. I’m assuming that the density of the carbon is 1.8 g/cc, that glass is 2.55 g/cc, and the density of the resin 1.15 g/cc. When predicting the weight and thickness of layup you have to look at the volume of the fibers to predict the resin volume and weight in the part. Just looking at the weight of the components will lead you to wrong conclusions.
I’ve got oodles of test samples to confirm the math…
Agreed…I use laminated/engineered wood inserts along with stainless hook-footed inserts-(for wood use). Drill the proper sized hole and hammer in the clawed fastener. Then secondarily bond the wood. No rust/galvanic corrosion or oxidation to worry about.
There are a few types>>
great, thank you everyone. yes wyowindworks, it is splitter and undertray for s2000, however since i am not gonna mass produce, it doesn’t make much sense financially to make a mold. So I guess I am gonna go with 3/8" core instead of 1/4" and test with only one and then 2 layers of carbon on each side. and infuse in one shot since I don’t care for both sides being shiny. and as far as inserts, can I just spray some anti-rust primer on aluminum?
I always put fiberglass between the insert and the carbon to insulate it. You could also use cuts of pultruded carbon tubing to prevent the bolts from crushing the core. You can wrap them with fiberglass yarns to prevent them from splitting if it seems like it could be problem.
I think that a 1/4" core is very thin for use on a splitter. Im building a splitter myself and Im using 1/2" foam as my core (mine is for a 240sx) I am actually cutting out 1" holes in the core and using 1" disks of G10 for my hardpoints.
My personal opinion is that you should probably make your splitter thicker and stiffer. These things really do take load at speed.
If you think your carbon spliiter is heavy you should see how heavy my 3/4" plywood splitter from last year was!
thank you dustyk43, beside using 1/2" foam core what is your lay up? and I am guessing you put the G10 disk after infusion right?
and yeah i am sure plywood would be heavy. my neighbor just purchased a $350 “carbon fiber” splitter that weighs 20Lb and is barely 1/8". I believe it only has one layer of carbon on top for cosmetic reasons and the rest is fiberglass and a thin layer of balsa for core. and it mounts to the bumper and not the chassis.
You’re on the right path, mind what wyowindworks wrote in the thread because he is right, more importantly, make sure your hardware is isolated with glass from the carbon as he wrote…
We’ve made a few splitters and rear under-tray diffusers for a couple of race teams, the last set we went with Nida Core, it was pretty thin, I would have to check the lay-up schedule on it, from memory I think 1/8" but it was done 3-up with carbon in between. They worked well, the driver has gone farmer numerous times on the track and they are still together.
Keep us posted, so far it looks great and the surface finish is top drawer.
Other than for the core I’m using a 12K Carbon layer, about 20oz twill weave, plain weave kevlar, core, 20oz carbon. I actually didnt infuse my piece. I just did standard vacuum bagging for this piece. Im sure I’ll get a chance to test other layups out this year as I have a tendency to mow the lawn with my splitter.
Thank you, may i ask why the kevlar? and did you use the kevlar for top or bottom of the splitter?
I would hope the use of Kevlar would be on the lip edge and under side. Since this is predominately where it performs best. Friction.
I know my track splitter took some serious grinding on deep transitions both asphalt,concrete and dirt. The Kevlar was awesome at living through all of it and never actually fell apart…rather it was run over,lol. My bumper cover bolts ripped clean through…splitter was the tougher link in the equation.