Hello, I have been reading everything I can find about composites but I haven't found any type of equation/calculator for determining composite thickness requirements. I know different resins, etc. have a bearing on the strength but, there's got to be a "safe" relation between how much composite equals the strength of an object made of steel/aluminum. I am trying not to waste too much material and loose the weight saving of making parts out of composites. I'm also not looking to make parts that risk my life on (bike frames, control arms), just automotive body panels, intake manifold or a pickup tonneau cover.  I think a formula wouldn't be too hard that would make parts safely 15-20% strong than OEM and still have a substantial weight savings. For example: How thick would a fender need to be if it was made out of e-glass or carbon fiber vs. OEM sheet metal?  I know a wheel well would have to thicker to survive a rock from the tire tread at speed, but how thick? Would a layer or two of kevlar add to 2mm of e-glass be good or over-kill?  Everywhere I see PSI tensile strength,etc. but does it mean that I would need half the thickness of an aluminum bracket if the fiberglass is twice the metal's strength rating?

The fundamental problem is you are comparing different materials with different properties.

It really depends on the properties unique to your application.  Everyone's needs and requirements are different, hence why you will struggle to find manufacturers who can give you precise advice as they simply wont have data that is ready to use in a practical sense by the layman or DIY composite enthusiast.    Many of these materials will often be used by companies manufacturing CAD designed products and hence the materials mechanical data is orientated towards those kind of applications where the raw mechanical figures are used in complex mathematical simulation to determine if the material is suitable and set the main parameters.  For the DIY user, often that is not really helpful unless you are prepared to do those calculations yourself.

It is hard to compare steel or aluminium directly to carbon in that sense as in some very important ways they behave very differently.  A prime example is dealing with fixings in metal compared to composite.  Most composites do not take a thread and do not like point loadings by fasteners hence a different approach is needed to spread the load.  This can make it hard to compare to steel which behaves entirely differently. 

Another example is how they behave in failure which will be essential to understand when taking into account safe loading etc.

For non-structural parts such as many body panels, you can follow "rules of thumbs" or copy other tried and tested lay ups you have seen before.  An example is the layups seen in our Video Tutorials which are in most cases are examples of parts our customers may like to make.  So you can use figures such as that for a basis to begin with. But even then, things can be more complex, eg our Fiat 500 bonnet in the video is plenty strong enough for our intended application, but a bonnet on a very large saloon or estate car may be a little bit more flexible with that lay up and hence you would need to adjust accordingly. 

Same with panels intended use.  For example we have found in our and our customers experience that a layer of 200gsm carbon either side of 3mm Lantor Soric can make a panel stiff enough for race car use. But it may well be too flexible for a regular road car or too fragile for a rally cross or rally car.  Also you may find you could go thinner on heavily curved panels or need it to be a thicker layup to stop excess flex on substantially flat panels.

So yes there are some rough rules you can follow as suggested above and in other posts on this forum and elsewhere online, but be prepared to take a good look at your specific application and adjust accordingly.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Warren,