Yet another carbon monocoque thread

[Aaron@Kaist]

Just to recap, I have posted previously on this subject, and I wanted to see your guy's opinion on the direction of a building a carbon monocoque. Just to review several opinions that were given to me before:

HenningO:
recommended that we get a carbon sponsor, and that calculating the amount of carbon and core was well estimated by assuming that the end product was around 15~30 kg.

PatClarke:
recommended that a carbon chassis was a push for a first year team, and that a steel tube would be easier because there can be repairs and modifications unlike the carbon monocoque.

exFSAE:
stated that the monocoque should be well justified, and cost to benefit ratio should be considered. Also recommended that data analysis concerning the rigidity of the chassis is important to consider before pursuing the chassis.

RiNaz:
recommended that funding should be procured before attempting the chassis, and to use prepreg carbon rather than dry fiber because it is easy to deal with. He also recommended that we get some training at a local composites lab.

Screwdriver:
stated that doing stuff differently can backfire causing damage in our reputation and end product.

vreihen:
introduced the cut and fold method used by WWU's team.

Davo:
suggested that if we had the know how and resources that we should go ahead with it, but it would be putting a lot of pressure on our team. Also stated that steel is a lower risk venture.

to continue,

What we have done so far:

1) we attempted the steel space frame chassis, by practicing welding. We also designed a space frame chassis on solidworks.

http://fsae.kaist.ac.kr/car/chassis.jpg

than we proceeded to weld various structures to see how the welds would turn out.

Main problems were that we did not have a tube notching machine, a proper saw for the pipes and the fact that none of our students could really weld to a decent level.

so once again we are stuck with the steel space frame idea.

going back to the monocoque,

What we have done so far is,
we took the advice from this particular thread and tried to do what everyone told us to do.

i) we got carbon fiber sponsorship:

70 meters of generic twill weaved dry fiber,
width of 150cm)

and 100 meters of prepreg carbon which we plan to use for our carbon chassis,

Thickness: .227 mm
Total Weight per square meter: 336 grams
Tensile Strength: 450 kgf/㎟
Tensile modulus: 24 x 10³kgf㎟
Fiber density: 1.77 g/㎤
Resin density: 1.2 g/㎤

we determined this amount by using HenningO's method of calculating from total weight of carbon chassis, so we requested around 30 kg of prepreg carbon.

ii) we recieved training from a local composities shop as RiNaz had suggested, but mostly with wet lay techniques, not much of prepreg stuff.

iii) we obtained access to a autoclave at the local composities shop with a 150 cm wide entrance.

iv) we bought ansys to analyze the carbon fiber, honeycomb sandwich structure.

v) we obtained sponsors for aluminum honeycomb core, machined to a one inch think core. Each honeycomb thingy have a width of 12mm. (blocks of 1250mm x 900mm x 25.4 mm)

vi) we bought several books:

The composite materials handbook by Technomic
Analysis and performance of fiber composites by Agarwal
Fibre composites and hybrid material by Hancox
Finite element modeling of composite materials and strucutre by FL matthews

So far our plan is to

1) model the car's suspension, find the mounting points
2) use those mounting points with steering and suspension design to find contraints in addition to the new rules to find the limits of the monocoque
3)design a general model of the monocoque based on the engine and suspension contraints.
4) analyze the design and optimize
5) manufacture.

If anyone would be so kind as to review this method we are considering and guide us? We are planning to start this project, learn how to do it and maybe use it for the year 2010.

For the manufacturing, we are considering a two peice construction that bonds the top and bottom parts. I saw some pictures of teams producing the whole thing at one go..is that even possible?!?! I was wondering how that would be done.

we were considering making both a female and male for both the bottom and top peices, after making the bottom and top plugs seperately. For the inserts and hard points we were considering using machined aluminum.

If anyone has any input or anything they want to say, it would be a great great great great great great help. I hope this thread will be able to help starting up teams in the future as well.

Aaron Park
Team Manager
KAIST University

[MarkoFSBR]

if the welding of tubes is your biggest problem why didn't you rent out a professional welder?

it would be much cheaper and faster( and not to mention way better job) than doing it yourself.

tube ends can be cut with an ordinary angle grinder if you are careful enough. it can be done on a mill , it can be done on a lathe...

how do you expect to make a monocoque if you cant even machine an ordinary tube end??

for the price of the material and labor involved in fabrication of monocoque you could have bought a good band saw ,and a tube bending machine , and save a lot of time and money.

[Chris_S]

You don't need a tube notcher to profile the ends of the tubes:
http://fsae.com/eve/forums/a/tpc/f/125607348/m/42910687241?r=56710097241

And if you cant weld, find a good welder, and ask them to teach you the basics. Its not difficult once you know the basics and after that its just practice.

[screwdriver]

Theres's just one flaw, I can see in your process. The molds. Did you plan to build them yourself or did you find a company with a big mill?

Another question is what kind of prepreg you got yourself. Woven or uni-directional (u-d). We use a mix of woven and u-d at various angles in our layup.

We built carbon negative molds out of another very thick woven tooling prepreg.
The positive molds for the negative molds are made of a special model-building foam which comes in big blocks, which are glued together on a plank to produce enough raw material for the mill to produce the whole part.
Once the mill is done, the molds are sanded and cleaned, spray-painted, polished and treated with mold sealant and mold release chemicals.

[Aaron@Kaist]

thanks for the input....

to screwdriver: we are using plain twill woven prepreg. just a question tho, what size do you cut your foam into before machining it? Do you use a CNC machine? we plan on using regular carbon fiber wetlayed onto our positive for the negative molds. do you do it build the entire thing in one cure? or are you bonding several pieces together?? Sorry to ask so much, just curious to see what methods you are using.



"if the welding of tubes is your biggest problem why didn't you rent out a professional welder?


it would be much cheaper and faster( and not to mention way better job) than doing it yourself.

tube ends can be cut with an ordinary angle grinder if you are careful enough. it can be done on a mill , it can be done on a lathe...

how do you expect to make a monocoque if you cant even machine an ordinary tube end??

for the price of the material and labor involved in fabrication of monocoque you could have bought a good band saw ,and a tube bending machine , and save a lot of time and money."

to MarkoFSBR:

first of all, we could outsource the tube frame...but whats the fun in that? Plus it would be very hard to find a welder who could accurately put the frame together to our specifications in Korea. We can machine it if it had to be done, but considering the effort required to train, buy equipment etc for a steel frame, we'd rather do the monocoque. Like I said before we are going to comp with a steel frame, just doing the carbon one if we can finish this year, if not, taking it in 2010.
plus you obviously did not read my post because you are telling me that it would be cheaper to do a steel frame for us.

Our cost for carbon: 0
Our cost for Tooling and autoclave: 0
Our cost for honeycomb: 0

Cost of steal: 250 bucks for 40 meters
Cost of equipment you listed: 1xxx dollars
...

maybe we can save some time, but considering money and what we can get out of this, I would say the carbon is a better deal. Please don't get me wrong I'm not trying to argue with you guys, just read and move on with your life if you have nothing helpful to say.

thanks so much

Aaron Park

[Brian Evans]

Couple of thoughts - if FSAE is, at it's heart, an educational experience in learning how to be an engineer - then the CF route is absolutely the way to go. what a wonderful opportunity to gain experience with the material that is quickly becoming the most important structural material in industry.

If tubeframes are your baby - I do tubeframe work semi-professionally, and while I have the notchers and the mill - I most often fit tubes with a hacksaw and files. Once you learn how, it's fast and easy, it's quiet (for a race car guy I hate noise) and then, obviously, you need to learn how to weld...

Brian

[screwdriver]

The foam blocks are quite large, about 1,5m long.
http://fhm-racing.de/!news.php?file=1205005278.htm
There's a picture of it. We glue up to 5 of them together for one of the bigger molds. They're beeing machined on a very big mill which is usually busy manufacturing body panels for prototype cars out of massive blocks of material.

[Aaron@Kaist]

dear screwdriver,

do you have a two multi-piece construction for your chassis or do you lay it all up in one shot? I'm curious as to how lap joints are accurately set after all that sanding and smoothing out. can anyone elaborate on this issue? Thanks so much, really appreciated. Truly.

[RiNaZ]

Aaron, in a way, you're right, what's the fun in outsourcing your welding jobs. Also, generally, a lot of first year team will be sent back to their pit area when going thru tech inspection to do some re-modification on their car. This is quite common, especially if you're an international team. So, like it or not, somebody in the team has to learn how to weld. Hey, if my girls baja team at my school can learn how to weld, you guys can too.

my internet connection is very slow, so i dont have the chance to convert all your numbers, but i think you needed at least 250-300 square yards for 2-core-2 layers. So if your design requires more than 2 layers, then you probably need more fabric.

I strongly discourage you from doing carbon tub for your first year. I just cant see how you can modify or fix the carbon tub in case something happens at, during or before the competition, at least in a quick way. But like i said before, if you cant find the motivation to do a spaceframe, then you might as well do the carbon tub, even if that means you're not gonna finish in time for the competition.

Having said that, there are many ways of constructing the tub. There are the right ways, and the wrong ways. And in the case of FSAE, it's usually using the wrong way, especially when money is very hard to come by.

Just to get on the same base with you on terminology/label ...
male = plug = positive
female = mold = negative
use either one, dont jumble them up together, you will confuse ppl. Usually, i can just talk to ppl using the term plug or mold.

Yes, the easiest way is probably to make two halves of top and bottom, and bond them together.

Do you have a CNC machine to cut foam? If you do, i would suggest that you glue all the foam together in stacks like the picture that screwdriver posted. And then have the machine mill it out for you. But most CNC only allow for certain heights on the foam. If that is the case, then you have to individually CNC it one by one, and then stack them together. This is probably what you might end up doing. Make sure that you have line-up holes in your foam too, so that it's easy for you to line it up when you glue them together (probably hard for you to imagine w/out diagrams, maybe i can draw some diagrams on paint when i get a better connection).

Another thing about pre-pregs, once you cure it, you cant attach the core to that pre-pregs. So some ppl use some kind of adhesive film to attach the core in the middle. So make sure you figure out if you want to lay everything in one shot, or lay up the first 2 layers and core, and then cure it, and then come back with 2 more layers and curing it.

Why not do it all in one shot? There is a chance of bridging or the fabric doesnt adhere to the surface of your mold. Usually, it's a good idea to vacuum your fabric after the first several plies, so you can conform your fabric to the mold surface before adding more plies. This is called de-baulk (this process usually consist of heat).

So make a lot of planning and thinking before you start making the tub.

i got so much to say, but since i know you're going to the competition with a spaceframe, i think i'll stop here. Plus, i think you should concentrate more on the spaceframe if this is the chassis you'll be bringing to the competition.

[screwdriver]

Aron our design is a bit special. It's got two parts, the front, basically the cockpit and the back which houses the engine and all it's components.
http://img257.imageshack.us/img257/6950/clipboard28cb0.th.jpg (http://img257.imageshack.us/img257/6950/clipboard28cb0.jpg)
This shot from the building phase illustrates it.

The two indivudual parts are laid as one piece. They consist of about 10 layers of basic layup which covers all of the part. We vacuumed the part (without heating it) four times during the basic layup, if I remember correctly. After the covering layer, after the middle layer and bevore and after the covering layer on the inside. Then the part was cured, the core glued in and the strengthening layers were laid. After that was in, it was again vacuumed. Then the same basic layup was put on again in reverse order with the same vacuuming as before.

[Aaron@Kaist]

Thanks so much Rinaz and Screwdriver for the input, I signed our team members up for welding lessons today, and got our team a tig welder which we will be picking up later this week.

"my internet connection is very slow, so i dont have the chance to convert all your numbers, but i think you needed at least 250-300 square yards for 2-core-2 layers. So if your design requires more than 2 layers, then you probably need more fabric."

just had a few questions on this section, by 2-core-2 layers, does this mean that there are two layers of carbon prepreg fabric, and than the honeycomb, and 2 more layers of fabric on top of that?, 250~300 square yards of carbon would equate to roughly 80.4 kg of prepreg for us, or, 177 pounds of prepreg, is that correct? I know calculations must be made on our side, but it concerns me if that is the amount your team is using, because we do not have that much. We found a huge CNC mill to assist us with the foam milling, and will be using foam that is used for making mock ups for prototype vehicles. I hope this is the one, they say it is fairly resistant to heat so I hope it will be able to endure the heat generated from the curing of the resin.

Dear Screwdriver,

must say that is an amazing looking car!!!
I now get a basic idea of how it is done. May I ask you what kind of amounts of carbon fiber are used in making such a chassis?
If we do make a monocoque style car, it will be from the main roll hoop on, and the rear subframe will be constructed of steel tubes. For the core, do you use aluminum core or nomex? We have yet to receive samples of the honeycomb core and were wondering if the core is easily bent into curving shapes???

One last question, sorry guys,
we are testing panels on ansys that are using 1 inch high honeycomb cores, but my suspension team is telling me it will make the car too big, what size of cores are you guys using??

Thanks so much once again, and next time we meet, please let me buy you a beer.

Sincerely,
Aaron

[screwdriver]

I don't know the exact amount we used this season, but it was a lot. A lot more than what you said you have got. I remember drawing markers onto one roll of u-d prepreg for how many meters were remaining and it was 89 at that time. And I know for sure we used at least three rolls of that u-d prepreg alone.
On the other hand our vacuum pack broke three times in the autoclave (after the basic layup). Twice on the front half and once on the back half. Which meant starting all over again each time.

Talking about vacuum packing, here's a tip:
Make sure that there's enough vacuum foil everywhere for it to lay itself into the shape properly. Otherwise it will rip like it did with us. Also covering screws and all sharp edges inside the pack with extra padding is a must.

[vreihen]

Originally posted by screwdriver:
http://img257.imageshack.us/img257/6950/clipboard28cb0.th.jpg (http://img257.imageshack.us/img257/6950/clipboard28cb0.jpg)
This shot from the building phase illustrates it.

Holy toe out, Batman!!!!! http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Aaron@Kaist]

thanks for the advice on the vacuum packing. We ourselves had a problem with the mold deforming once the vacuum pumps were set in place, I guess there is a lot more that can go wrong.

As for the amount of carbon fiber,

I calculated 336 grams per square meter, and 100 meters we have, which equates to 33.6 kilograms. Including the weight of the core which is supposed to be around 6 percent?? around there,should this not be enough?

Assuming we had the same density of carbon, and thickness, and you used 89x3 meters of u-d, this would mean that your carbon monocoque weights at least 90 kgs...

the only thing I can think of is if the resin leaving the carbon and making it lighter, or we are using different thickness of carbon fiber.

may I ask the total weight of your monocoque?

[screwdriver]

That explains a lot. Our prepreg was only 1m wide and doesnt have such a high density. The monocoque weighs 15kg without any add-ons.

Btw. I forgot to mention, we use a 10mm nomex-honeycomb core.

[Aaron@Kaist]

Thanks so much screwdriver, will take me a few hours to soak everything in, but you've been a great help in helping find direction...much appreciated

[Matt N]

Aaron,

We started with ~31kg of material of about the same weight you have (333gsm including resin) and after building our first monocoque and nose cone we still have about 35% of it left.

Matt
WWU FSAE

[Matt N]

Additionally, to give an answer to your question about core thickness, go to www.Hexcel.com (http://www.Hexcel.com) and find the sandwich panel design guide. If you spend a couple minutes with the hand calculations presented there, it will give you some ideas about how skin thickness and core thickness (and core strength and stiffness) change things.

Matt
WWU FSAE

[Aaron@Kaist]

thanks a bunch Matt, much appreciated. what was the thickness of material you were dealing with?

[Aaron@Kaist]

just one more question if possible to answer:

For general FEA of the monocoque, is it possible to use solidworks student edition?

My plan was to model the whole thing in solidworks, shell it outwards and inwards, and have those thin shells represent carbon by modifying the data, and the sandwiched middle section represent the honeycomb core. Is this a reasonable approach to this problem?

[Rellis]

you will want something more powerful than SW to do the fea

There are lots of different ways to do it. The hard part will be predicting core shear

[Aaron@Kaist]

could you elaborate on "if every thing is under control you are going to slow." ? I don't quite understand that one. Also, we recently obtained licensing for use of Altair's hyperworks software, do you have any advice as to how we should use this software? any teams out there using altair to design their monocoques?

[alumasteel]

could you elaborate on "if every thing is under control you are going to slow." ?

That's a quote of Mario Andretti. In other words, if you're completely in control then you are not pushing hard enough...

[Aaron@Kaist]

ok....So thanks to Rellis, we now know that solidwork's FEA is not powerful enough to analyze laminate structures, so we contacted Altair Engineering in Korea, and obtained a copy of their hyperworks program. Of course there are no classes and no books on the damn program so we turn to forum members once again to ask advice. To my understanding we can model and test the carbon monocoque structure all on Altair hyperworks. Is this correct? Or do we use shells from a CAD modeler like SW and import it into Altair's software to analyze it? Does anyone have experience using Altair's Hyperworks to analyze composite structures? Thanks so much once again.


fsae.kaist.ac.kr
Aaron Park
KAIST

[Mully]

Hi,
Hyperworks can be quite frustrating at times and I can understand any frustration you are experiencing, but in my experience it is well worth it.
You could try using 2D elements, so a shell sounds about right (my uni uses pro/e so excuse my solidworks ignorance).
In the 2D panel on hyperworks you will find your greatest friend for composite structures in the Hyperlaminate module. It is fairly intuitive so I will leave it to you to learn about it.
Also, there are some pretty good tutorials that are included with Hyperworks and I would strongly advise going through these.

[Aaron@Kaist]

thanks a bunch

[Aaron@Kaist]

by the way how did you guys get your 2009 car ready so quickly????

[MalcolmG]

Aaron, the car we have recently finished (well, not quite finished, but getting there) is our 2008 car. Because we build for the Australasian competition in November/December our build cycle basically follows a calendar year, so we started our design mid-January, so we had just under 9 months from beginning design work to having a running car. However, we built the car to be compliant with the 2009 chassis rules (or those that were proposed around this time last year) so that following the Australasian competition, we can take it further abroad, (Germany or UK being likely candidates) if the car is competitive (ie scoring over 800 points) and if finances allow.

[Rellis]

I would do some test layups to compare to your FEA results.

[Aaron@Kaist]

greetings!!!

our team is registered for west w00t....number 27, looking forward to meeting all you in person. Just got a question though, for the monocoque, with the outside skin laid down, core placed in, and inside skin being laid down, how do you make the two skins meet/bond???? I've been building it my head for several weeks now and I just cant figure out how it is done...anyone got a quick tip on how to do this?? Once again, how to make the outside skin of the monocoque meet the inside skin with the honeycomb in the middle. Thanks a bunch

[Rellis]

you dont put your honeycomb all of the way to the edge you need some overlap for you plies to meet back up.

If you are asking questions like this you may want to reconsider a composite tub. Might I suggest a seat first

[Rellis]

PS great number

that was my racing number of choice if I couldn't get that one I was 127

[Aaron@Kaist]

thanks rellis, i am learning more and more how hard composite tubs are to make, and to be honest am reconsidering. Only if I could see a tub just once I would be happy. And it would answer a lot of questions, but i guess i have to try before giving up. Can any of you suggest reading or learning material on this subject? I cant seem to find anything specific to composites in automotive applications

[Fred G]

What's wrong with bringing the outside skin over the core and then overlapping that skin with the inside skin?

We call these "close out". You have built a moulds to incorporate these have you not? If not, nothing a little bog can't conceal.

H<span class="ev_code_BLUE">_</span>HH
H<span class="ev_code_BLUE">|</span>#<span class="ev_code_BLUE">|</span>
H<span class="ev_code_BLUE">|</span>#|<span class="ev_code_RED">|</span>
H<span class="ev_code_BLUE">|</span>#<span class="ev_code_RED">|</span><span class="ev_code_RED">`</span>
H<span class="ev_code_BLUE">|</span>#<span class="ev_code_RED">|</span>

Where:
H - Mold
# - Core
<span class="ev_code_BLUE">|</span> - Outter Skin
<span class="ev_code_RED">|</span> - Inner Skin

Regards
Fred

[Rellis]

Originally posted by Fred G:
What's wrong with bringing the outside skin over the core and then overlapping that skin with the inside skin?

We call these "close out". You have built a moulds to incorporate these have you not? If not, nothing a little bog can't conceal.

H<span class="ev_code_BLUE">_</span>HH
H<span class="ev_code_BLUE">|</span>#<span class="ev_code_BLUE">|</span>
H<span class="ev_code_BLUE">|</span>#|<span class="ev_code_RED">|</span>
H<span class="ev_code_BLUE">|</span>#<span class="ev_code_RED">|</span><span class="ev_code_RED">`</span>
H<span class="ev_code_BLUE">|</span>#<span class="ev_code_RED">|</span>

Where:
H - Mold
# - Core
<span class="ev_code_BLUE">|</span> - Outter Skin
<span class="ev_code_RED">|</span> - Inner Skin

Regards
Fred Easier to get voids and wrinkles more complex molds and no core taper requires extremely precise core placement

Im not saying it cant be done but that would be my reason for not doing it

[Rob Klyver]

"I strongly discourage you from doing carbon tub for your first year. I just cant see how you can modify or fix the carbon tub in case something happens at, during or before the competition, at least in a quick way."

Repairs are easy enough but you should really consider small panel test construction first.

[RiNaZ]

rob, you should elaborate more on your comment. Im interested to know and learn how to make quick and easy repairs just as i would with welding steel tubes.

Remember, some ppl out there rather buy gallons of bondo but wont spend a dime on the bondo spreader.

[Zhefei Li]

RiNaZ,

There are loads of books on composite repair. Granted, I haven't had enough experience or read enough of them to say that it's quick and easy, but I have my hands on Advanced Composites, by Cindy Foreman, and it overviews way to repairing composites, mostly geared towards boats and small aircraft.

Welding repairs would require at least a welder, and composite repairs would require a vacuum pump. Both require skill and experience. I guess the cure time eliminates the possibility of a "quick" repair, though, if you're already at comp and something fails.

[RiNaZ]

Zhefei, yes, i know composite repair. My comment was for rob to share his experience based on his comment that repairs are easy enough.

My experience is telling me that no matter how good you are in composite, you're still at the mercy of the material and i just couldnt see how fixing composite can be faster than welding tubes.

And the context of this is in the FSAE world, not in some garage where you have all the equipment that you need to make repairs.