Hello everyone,
We are building a formula student car and this is our first time. I have no prior experience in carbon fiber fabrication. I have decided to do vacuum infusion. How to decide the number of layers needed? I've already calculated the required thickness for the strength I need but there are few things which make me confused,
- If 3K 200GSM 2/2 twill fabrics are used, how to know the thickness of 3 layers with resin? 
- If a core material is used could I reduce the number of carbon fiber layers? 
- Any suggestions on the fabric type, number of layers and core material combinations to get 'good' strength so that it won't fail when hit on a cone, rock or something hard (I mean a rule of thumb from your experience)?
Thanks!!

As far as I know, formula student cars have to be calculated for safety, and this is nothing to quickly take up in a forum. You will need someone who really knows this stuff.
200g gives you about 0,2mm thickness. What i can tell you, is that 3 layers is not nearly enough for a crashbox.

Sorry, but the last two questions of your first post seem to me, as if you don't have that much experience in mechanical calculations. To answer them:
-it depends. When the part has to endure bending forces, a core material can reduce the necessary amount of fibre reinforcement significantly. But when the part faces tension or pressure, the core doesn't reduce the needed layers at all. Except stability is the main issue. This has nothing to do with composites, it would be the same principle for metal.
-this is basically 100% dependant on your design, and varys greatly over the whole structure. Again, this is a universal principle and has nothing to do with composites.

One of the beauties when doeing infusion is that the thickness of the final laminate is close to the thickness of the dry cloth. So the answer is: both. You can simply measure your cloth with a caliper.

Check in detail the regulations for nose impact attenuator for the specific competition you are entering.  Most of the western FSAE competitions have either a specified impact attenuator design for a specific tested foam material and/or have a chosen supplier who can offer an off the shelf unit pre-made.

If you choose to go away from that path, you will spend days  testing and calculating for your new crashbox as you have to provide proof of structural equivalency for it to be considered acceptable.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Hi Warren,
We are using the standard impact attenuator. And our chassis is spaceframe.  What I want is an advice on body panel fabrication.
Thanx...

Thanx a lot Oekmont. That thickness detail is very important! By the way what I m talking about is not crash box but body panels to cover the spaceframe chassis. The stresses would be maximum at the mounts right? I did rough calculations using the possible load components on a panel from panel weight and aerodynamic forces for mounting points. And then decided the thickness. And next did a FEA simulation using including gravity and pressure forces taken from CFD. I'm sure I got the thickness right. 
Thnx again for information!

I am not an expert of these regulations, but it may be the case, that you body have to fulfill safety regulations besides the frontal crash box. You should check that first. Just for self carrying panels 1 layer of 200-240g carbon on each side of 1,5-3mm soric (or soric lrc) with no core, but additional layers carbon at the fastening points would be a good point to start iterating.

A composites analyses with only thickness as parameter isn't a proper analyses. This would include fibre orientations, as well as the distance of each layer to the neutral plane, for which you would already need the thickness of each layer. And the use of a sandwich (and the understanding how it works) should be taken into consideration before the first iteration too.
The substitution of an isothropic material with an orthopic one (or many orthopic layers) may change the load paths significantly. An isothropic fem isn't suited for composites.