Wednesday, March 3

How Composites Work

Strength Relative to Steel (Very High Strength Steel is Not Usually Used in Bikes)
The Trick is to Take Advantage of Carbon Fiber “Along the Grain” Strength
Lighter Than Aluminum and Stronger Than Steel
This was a boast often heard in aerospace in the 1970’s about carbon composites. You still occasionally hear it, including from bike makers. It’s true, if misleading and oversimplified. Carbon is an engineered material that can be oriented to provide strength and stiffness where it will do the most good. It also has the major advantage over fiberglass that it is very stiff and less dense. Don’t underrate the importance of that lower density. Bikes have a lot of locations where things are sized simply by “getting from point A to point B” rather than strength or stiffness.

Carbon Has Good Stiffness
Wood and Plywood on Hormones
If you take a thin piece of most straight grained woods, you will find the wood splits very easily along the grain, but won’t split against the grain. If you glue (laminate) many thin pieces (plies) together, orienting the grain so they don’t all run the same way, now you have a good analog to what’s going on with carbon. Carbon fibers are simply much stiffer and stronger than wood fibers. The epoxy is what holds them together. These fibers are made into plies for use by the bike manufacturer in two main forms. Unidirectional material, which has all the carbon running along the length of the raw material, and woven material, which has the carbon woven into what is essentially a high tech cloth. Usually, on a bike, if you SEE carbon in the finished product, you’ll see that woven form on the outside. If the manufacturer is smart, the ply choices are intelligently combined, laid into a mold, and then cooked under temperature and pressure. In principle, this is all simple enough that you could make this stuff in your garage, and I’ve seen stories of very reasonable homebuilt carbon bikes on the internet. Two pounds of raw material can make a completed bike frame. Compare that with the amount that would be needed to make even a small homebuilt aircraft wing. It’s one reason why carbon is not used very much in the automotive market. Bikes really ARE different!

Woven Carbon Raw Material Before Layup and Cure (cooking) - From Wikipedia

Low Density is Where Carbon Truly Excels
Loads – Please Don’t Interrupt
Since carbon fibers are really strong and stiff, loads run down them, in an exaggerated version of wood grain carrying the weight of a tree trunk. This works great, as long as there are no “knots” or other load interruptions. Examples of these load interruptions on a bike include brake and rack drillings, dropouts, tube intersections, threaded inserts, and bottom brackets. In the case of wheels, holes for spokes and the need to eventually attach the spokes to things compromise the capability. In the case of a typical cured carbon composite, the simple act of drilling a hole drops the strength in half. This is a real challenge, and one that manufacturers have taken many different approaches to accommodate. Read articles in Buycycling Magazine and you’ll be serenaded of stories about how this or that new solution represents a revolution. Luckily, as a happy and content carbon component owner, if these are done well, the only sort of load interruption you will have to worry about is that associated with a post-manufacture impact, unless you leave it out in the sun long enough for the UV to deteriorate the epoxy “binder.” Impact interruptions can take the form of either internally or externally broken carbon fibers, or delaminations and disbonds, in which the plies are no longer stuck together properly or to whatever they are glued to. This of course, is another chapter in this carbon saga.

Loads Repeated – Again and Again and Again
Carbon has problems wherever there are load interruptions. These problems will cause it to fail whether an excessive load is applied once or many times. While this is an oversimplification, it’s why a plan to replace a carbon component after a set time won’t work. Metal, on the other hand, also has problems with load interruptions. The difference is that metal shows these problems as fatigue cracks and failures. With carbon, you take the hit right away, but there’s no added penalty when the loads get repeated.

Effects of Load Interruptions on Material Strength
Bottom Line
The key to making carbon composites lighter than competing metallic materials is to take advantage of their strong direction preference and their low density. This explains the evolution of carbon in aircraft structure. The first applications were simply loaded parts where joints could be minimized. The aircraft equivalents of “straight frame tubes” so to speak. Complex structure with lots of load inputs and widely varying load directions, such as transmissions and gears, are mostly still made of aluminum or steel. It’s also why you don’t want to throw a tie-down chain over the frame of your carbon bike to keep it stable in the bed of your pickup, or start drilling “lightening holes” in it.

I know that Whareagle wants to hear about aero and composites. That will come, but this post is already long enough. For now, go ride a bike!

1 comment:

Ron said...

First time reader here.

Its interesting you state that that the hole will decrease strength by half but in what mode of loading? Was this stated in literature somewhere or you have this knowledge from experience?

Is it any different from the behavior of metals? For example, the stress concentration of a .125" transverse hole in a 1" OD .5" ID tube for bending is about 3.11, so factor of safety decreases approx. 33%.

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