Modelling of 2x2 twill carbon fibre fabric in ansys composite preppost 1

[omer31093]

Hi Guys! I am new here. I am currently doing my bachelor thesis on the Shell Eco marathon prototype car. I am thinking of making the chassis this time with carbon fibre+nomex sandwich pannels. So the carbon fibre we already have in stock is 3K 2x2 twill 200gsm carbon fabric. I am using Ansys composite preppost for modelling the carbon fibre, but i am confused on how to input the details in the software. In the materials library they have plain weaves, unidirectional and etc but no twill weave fabric data. Can i use a unidirectional fibre in 0 and 90 ply directions to simulate the twill weave? Or is there some other better method...Any help is really appreciated!!MANY THANKS guys!!I have attached a picture of a dummy analysis i did, so you can see the basic shape i am intending...

 

[RPstress]

Twill weave is very similar to plain weave. It should be a little bit stiffer and stronger but in practice it may not be, because a lot depends on the volume fraction achieved, although often plain and twill have almost the same volume fraction. See

http://www.hexcel.com/resources/dataSheets/prepreg-data-sheets/M49_eu.pdf

for an example of plain and twill weaves (the E and σ are very representative of the differences between them). Often, as there, the twill weave is produced with a higher areal weight, although in your case it probably isn't (200 gsm is pretty light for woven material). The areal weight mainly affects the thickness (which of course determines the sort of strength and stiffness you get for a given amount of carbon). For carbon/polymer with sensible volume fraction (about 55 or 60%) you can take the areal weight in gsm and divide by 1000 and that's the approximate thickness in millimeters.

The only time you'd need to model the difference between plain and twill is if you were investigating the meso- or micro-mechanics, where the weave featured in the model. As it is you should be modeling the material with its in-plane (and through-thickness, though this can largely be ignored with sandwich) properties, whatever they are, and those numbers should be almost identical to the plain weave numbers.

Are you modeling the flat elements as sandwich, or are you modeling the skins as separate elements from the core? Again, twill vs. plain won't make any difference. Just curious about your approach.

If you're using sandwich panels with right-angle joints it'll be quite interesting how you achieve that. It has been done with (e.g.) the Beech Starship's wing skins and ribs, but that was mainly for a shear connection (the wing wasn't wet to there was very little tendency for the wing skin to be pulled off the ribs). The fuel was in bladders in the center wing box and the inner wing.

That thingummy in the illustration looks like a bathtub tension fitting, not the sort of thing usually made out of sandwich panels, especially since it looks like it may have significant pull-off loading of the joints...

 

[omer31093]

I modelled the plies and core seperately, it can be easily done in ANSYS Acp. We currently have a complete sheet i.e. (48"x96") of 30mm thick Nomex and a little shorter of 9mm thick Nomex..density 48kg/m3. Yeah this kind of 2x2 twill weave is normally used for cosmetic purposes. But we have about 20 square meters or more. I was thinking of using for the base panel in the engine area the 30mm nomex with facesheets on the top of 0,45,90 and the same on the bottom and then the 30mm nomex would taper off near the driver area and become the 9mm nomex.I know the 30mm is an overkill but we bought it not knowing how strong nomex actually is in compression, i think 15mm would have been sufficient.

You are right the 90deg joints are a problem. For that there would be rib connections for the side panels (not shown in the diagram). Also there will be more CF reinforcements on the two sides of each side panel. Called a T-joint yeah..that should hopefully prevent the side supports from ripping off since the front wheels are connected to the side supports therefore they would be loaded in shear. The bulkhead as can be seen will only have the driver head resting against it(the force of which i modelled as 200N, is that too much??) Thanks for your reply! Please tell me what you think of the things i said above as well.. Many thanks, also i have attached a pic now of what we hope to acheive actually. This is the clean diesel team from Japan.

You can see two green circles..i was actually confused on how to design those inserts such that the side panels dont fail under shear. But then i came to know that normally inserts in such positions are only bonded to the two face skins which take the main shear loading. Please if you have any input or advice, I would be really grateful to hear from yoU! Please see the picture attached so you can get an idea of what i am planning.

 

[adfergusson]

Given the manufacturing, let alone stress analysis, complexities in your design, the difference in behavior between plain vs twill weave will be insignificant. The difference between the elastic props of the two reinforcement architectures will be negligible compared to toher uncertainties you'll have. As RPstress has alluded to, and unless something goes drastically wrong in your fundamental stress analysis, failure is not going to be due to global laminate failure and is going to be hardest to investigate (and thus likely to occur at) at the inserts/bolted joints and the t-joint... both areas where your FEA is not going to be much use so you'd need to go down an experimental testing and design guidelines route (also consider whether you realistically have the time/material/budget for the testing work that would be needed for such a concept).

I'd suggest that you go and speak with the technicians who'll be helping you about whether they think it will actually be possible to make the moulds, fictures, jigs, etc... you'd need to hit the tolerances that would be needed for your design concepts. Just because other Universities have manufactured certain designs does not necessarily mean you will be able to as they might have different equipment, staff, etc... Even if you might be able to make it, don't underestimate the potential benefits of simpler concept that you can can iterate a couple of times and improve upon in the same time it would take to do just one more exotic design. I say this as someone who has supervised composite projects for formula student projects at Imperial College.

 

[omer31093]

Actually We have no test lab and no such test equipment. But the loadings and stresses in Shell Eco Marathon, where top speeds are about 40km/h max. I beleive it is not as complex as the FSAE designs.. Normally static loads are about 500-700N at each wheel suspension point. Also for the side panel i was thinking to have the 30mm nomex core in the region where the wheels are attached. And in the attachment points, a region of the nomex is removed and a rectangular piece of balsa wood is embedded in the nomex, so that the nomex in the surrounding region does not have to take the shear load. Then as the balsa within the core structure is bonded to the two carbon face sheets on the two sides, the reaction forces on the wheel suspention points are just taken by the face sheets.. Does that make sense??

@adFerguson - Since you are so highly experienced with composites, i would like to ask you, do you think a properly strong T-joint and industrial adhesive, to fix the side panels to the base will be sufficient? I mean for now i only have FEA to simulate this design..and i am not really sure how to simulate the adhesive joint in Ansys ACP though..I mean it will depend on the exact material properties of the adhesive yeah?

And thank you so much for taking your valuable time to read and reply!

 

[adfergusson]

@adFerguson - Since you are so highly experienced with composites, i would like to ask you, do you think a properly strong T-joint and industrial adhesive, to fix the side panels to the base will be sufficient? I mean for now i only have FEA to simulate this design..and i am not really sure how to simulate the adhesive joint in Ansys ACP though..I mean it will depend on the exact material properties of the adhesive yeah?

Composites and/or adhesives are never, ever that simple I'm afraid. For some reason many people think that adhesives and adhesive joints are a straight forward thing to understand...they are certainly not. To give you an idea of the minimum amount of work needed to start modelling bonding between plies of identical, flat, composite material aligned in the same direction (much, much simpler than what you're looking at), you can read a post of mine here:

http://forums.autodesk.com/t5/simul...sis/cohesive-material-properties/td-p/4416345

This takes me back to my point about starting with something simple that you can quickly put together, test and improve upon. Has your Uni got an aluminium (or steel) space frame car to start with? If not, trying to start from scratch with composites strikes me as a bad idea as you'd be able to develop a much more reliable design that is still light by sticking to aluminium alloys for the main load bearing duties and using composites for aero/body work that is non-critical. Composites may be 'sexy' but it might be worth you looking at high end custom mountain bike sites and comparing the difference in weight between otherwise identical bikes but with aluminium and CFRP frames. When all the other bits and pieces are taken into account you'll see that the percentage difference in weight is pretty small but the difference in price certainly isn't (you can still get gains through aero efficiency through composite bodywork). You might find the overall project would better off if you decided to reduce cost and risk by ditching the composite chassis, going with an aluminum space frame and then using the money saved to buy (for example) some really light CFRP disc wheels for your car (weight saving on the wheels is much more valuable than weight saving on the chassis).

 

[omer31093]

Hi Thanks for your reply! We already competed once with an aluminum chassis, but it was a little heavy.(mainly 100x40x2mm rectangular hollow sections). Though it could be made lighter no doubt. The reason i wanted to go for the composite chassis is because we already have all the materials required such as core material and facing carbon. Only adhesives and inserts required Last year we did exactly that, i.e a aluminium ladder frame coupled with a carbon fiber shell (no loads on it). I was wondering why waste the nomex and carbon we already have?? One of our guys is currently doing his thesis on carbon fibre wheels..and another member is doing his thesis on the shell (design, modelling,CFD analysis). Well i read your post on Autodesk..that is on a way higher level. Neither do we have the resources or time to do such stuff Moreover, are a very new university..you know..we have only 3 professors in the engineering department..and none of them are into composites sigh..anyway i was thinking for the main objectives of my thesis to analyse the aluminium chassis from previous year, improve it in terms of stiffness and weight reduction. And then also model and analyse (on a low level) the composite chassis. Then i compare them both in terms of cost, manufacturability, stiffness, reliability and weight etc.. Makes sense?? What do you think Sir?

Anyway i was thinking of setting up a simple experiment whereby a construct two small sandwich panels..and bond them at right angles with the right adhesive(still to be researched). One to represent the base and the other the stringer. Then an insert will be embedded in the stringer and then weights will be hung from the stringer (like consecutive 10kg weights) untill failurem, while the base is constrained by a jig.. Do you think this will make a good representative test do decide the failure mode at the wheel suspension point Sir? many thanks and hoping to hear from you soon.

I really appreciate your input, and yes since you have pointed it out to me, first i will try to improve and optimize the current aluminium chassis.

 

[omer31093]

And also we have no technicians in our uni..No machines..All we do is on our own with our own hands. So i need to decide if it is manufacturable and then manufacture it. I live in Oman..where we are still way back in terms of technology And yes accuracy and tolerances will be a problem in this design.

 

[adfergusson]

Do you think this will make a good representative test do decide the failure mode at the wheel suspension point

To put it simply: No.

I also think you'd encounter other problems of equal, if not greater, difficulty along this line of work.

For a start, you will not be able to accurately locate suspension hardpoints in a composite chassis with the equipment and personnel available to you.

Misaligned suspension will significantly reduce the energy efficiency of your car and can massively increase the loads that the suspension hard points will need to sustain and transfer to the chassis... and that is just for a start...

Anyway, I would suggest you consider the following:

Make the suspension mounting points out of steel (not aluminium). Create a jig that accurately locates and firmly holds/clamps these suspension hardpoints on both sides of the car with respect to each other. Then layup and bond your composite components to the hardpoints while they are attached to this jig. As the resin cures the composite material will warp and change geometry but, because your suspension hard-points will be held in the correct location and orientation by the jig, your suspension hard points will remain accurately located with respect to each other (You might need/want some sort of lateral bracing member between the suspension hardpoints on both sides of the car to counteract any springback that that the composite may exhibit when cured).

Making a sufficiently accurate steel jig without CNC and CMM equipment will require a bit of thought but can be done without much expertise and fancy machinery. Also, steel is very easy to bond to when compared to aluminium alloys so you can reduce the amount of steel you need by backing it with composite and composite sandwich laminates.