About carbon fiber, part I.

So you're interested in this carbon fiber stuff.
We'll be divulging lots of hard to find info here! When we talk about carbon fiber here, we're using it as a woven reinforcing component in fiber reinforced plastics. We dont' restrict ourselves to just carbon either. We use glass, aramid or even polyester, paper or cotton as a reinforcing material, depending on the desired properties. The end result is still a composite matrix of fibers and plastic. The freedom of design offered by creating the part and the material at the same time allows for some very innovative products. A short list of some good and bad things about composites:

+composites+
strong per-weight
highly moldable to complex forms
has a high visual appeal factor
ideal for small runs of high quality complex parts
shop startup costs less than machining equipment
tooling is often made with composites

-composites-
weak when purely planar
potentially expensive raw materials
production is very labor intensive
production doesn't scale well
potentially toxic byproducts of production
skilled workers needed
not a lot of information available to start

We'll explore these indepth and more in part II, but now for a few common questions

so what exactly is it?
Carbon fiber, or graphite, is a classification for any fibers which are at least 92% (by weight) carbon in composition. Most of the time these are continuous fibers bundled and most often woven into a textile form factor.

so why is it so strong?
The fibers' high strength is achieved on a per-fiber basis with each fiber being made up typically one plane of carbon in a crystaline structure along the fiber's length. These carbon planes have a high modulus of elasticity in their plane, but when stacked, slide easily on each other due to very weak out of plane bonds. This is why in other applications graphite can be used as a lubricant or bearing surface. Naturally, long fibers are especially strong, as shorter, less pure or planar fibers break easily. These long fibers get spun, bundled and woven Into the fabrics we commonly call carbon fiber. In the end, carbon fibers themselves are strong in tensile and compressive loading, but nothing else. However when woven and put into a matrix of plastic it's main strength lies in tension, as when woven, a fibers linear path of compression is compromised. If used incorrectly, carbon fiber is a very weak substance, and most composite matrixes cannot stand up well to heat, even though the raw carbon fibers themselves can. Most of the strength of a composite structure comes from proper design and application of the composites.

how do you use composites properly?
Well, when engineering something for composite production you have to keep the fibers working for you, not just taking up space. There are many considerations unique to composites that have to be addressed throughout all stages of production.

a few major considerations:
The fabric's fiber orientation on the part for strength, and how to minimize waste in angular cuttings on your roll width.

Other strength issues like layup thickness, resultant cure times and possibly heat build up.

The tow orientation, wrinkles in the fabric, smooth transitions for cores. to Aesthetic qualities like print through or a cosmetic layer and joint orientation. Logistic issues like mold splits and laying up in the completed mold, as well as mold sag, complete wet out and application of vacuum or pressure in said mold. When bonding in fasteners and pre-existing hardware into the layup, everything gets much more complicated, and you have to take it all into account in mold and part design.

Engineering, experience and often times common sense are used in creating a part that comes out of the mold easily and is strong. They also let you know when not to use composites. As a general rule, the best application of a composite matrix is in a thin sculpted skin. When applied in such a fashion that there are fibers under tension when distributing the load, allowing for a very lightweight yet strong structure. strength over flat spans can be achieved in a double skinned structure with a core material or bonding or otherwise attaching other materials like steel or titanium.

bonding?
Yes, this is actually quite simple. Bonding fasteners or existing parts into a composite matrix will be much stronger than bolting them on afterwards, has better load distribution and you can create lighter, stronger, less numerous completed parts with hardware bonding. bonding in studs, threaded bosses or a nipple on a plate in or on the layup is a favorite of mine

won't a part just split if it cracks?
No, actually, like a drilled hole, woven fibers stop a crack in it's tracks. When a crack is introduced into a matrix of woven fibers, eventually the crack will be perpendicular to a fiber or bundle of fibers. The crack would have to travel around the fibers, perpendicular to the direction of the crack's stress and progress, to bypass the impeding fibers. Thus the crack stops until enough force is put to it that the fibers in the way would be broken.

Why use it if it takes so much engineering?
Because inherently composites have some very good properties. And we'll take an indepth look at the good and bad in detail in part II.