I understand that in most production structural carbon fiber parts there is a team of engineers to calculate necessary thickness. Not all of us have that information available to us. I am wondering how to determine necessary part thickness/appropriate layup schedule? Is there come sort of calculation I can do as a person who does not have engineering training? My upcoming project is to make a freestyle stand up jet ski. These hulls weigh approx 60-80 lbs. from the companies currently producing them. These skis often get 1200+ cc motors and are literally backflipped, barrel rolled, and jumped 40+ ft in the air when surf riding. I need to be sure the hull is strong enough, but weight is a factor. I am interested to hear from the pros how I can figure out the appropriate layup schedule? Thanks
Here is a link to a calculator - http://www.saertex.com/materialrechner/rechnere/rechner.php
It came from this thread about laminate thickness - http://www.compositescentral.net/showthread.php?t=7288
That link only gives a calculator for determining the thickness, not the strength.
- Make friends with an engineer. Possibly a college student. Though typically results will be directly proportional to the engineer’s experience and not necessarily education.
- Reverse engineer an existing product.
- Do a That Looks About Right (TLAR) and test test test.
… or overbuild knowing the relative strengths of the existing materilas used and those of carbon/resin you are using… the poor uneducated mans method 
Reverse engineering a current product will be hard because they are quite closely guarded and as it is my potential competition I dont think they will appreciate me taking a digital caliper to their layup. Thats not really an option. I do need to have an idea though, especially as I need to start estimating material costs, so mainly I need to know how many layers I should be using and of what fabric. I plan to use 2x2 twill as a visual layer, but need something beefier behind that. Also I know core materials can add a lot of strength and thickness. I need to find a material with minimal print through and resin consumption though. Would love to have some recommendations on this.
I plan to test the hull extensively before offering them for sale, and also plan on laminating plenty of test panels. Mainly though I really need to begin the process of estimating material cost…
You know the weight of the original, so when looking closely you can also guestimate the layup used. Taking a thing apart and measure it is nothing to worry about. Some even cut samples, put them in the oven at 350 degrees C (some 750 F) to burn out the resin. Then you can carefully take the laminate apart, and weigh it.
Keep in mind that stiffness equals thickness. If you want the part to be just as stiff, make it just as thick. Funny thing is the material used does not matter that much in that respect. (things are different in tension though)
The production version of stand up jetskis are made of SMC (sheet molded composite). Is the thickness still comparable? Isnt part of the benefit of carbon that it takes less material to acheive the same strength?
The primary advantage to carbon is it’s tensile modulus. This allows you to make a structure that is stiffer for less weight than glass or kevlar. It also has a lower density (1.8 g/cc) than glass (2.55 g/cc) which allows you to build up more thickness for less weight. This increases the flexural modulus of the laminate which will help resist buckling.
The attained Vf due to the processing can significantly change the thickness of the laminate. 4 layers of 5.7 ounce 3K pan based carbon with a Vf of 40% will yield a laminate that is .042" thick and weight 45 ounces/yard^2. If you boost the Vf to 60% then the laminate will be .028" thick and weigh 33 ounces/yard^2. The tensile modulus of the laminates will be nearly the same but the buckling resistance of the thicker laminate will be noticeably higher. You could add another layer (5 total layers) at Vf of 60% and still be thinner (.035") than the 4 layer at 40% and weigh less, be stronger, stiffer and have near the same buckle resistance.
I don’t know how applicable it would be for Jetskis but there are design guides for designing hull and deck laminates for small boats such as ISO 12215 part 5. VectorLam from Vectorply is a free excel based program that will allow you to estimate the mechanical properties of various laminates and also has a feature to design hull laminates.
Thanks Infusible!