Metallurgy for Cyclists V: Carbon Fiber Boasts Tremendous Potential
by Scot Nicol
If you've followed parts I through IV of this series on bicycle metallurgy, you've learned a lot about those characteristics of aluminum, titanium and steel which you need to consider when designing bicycle frames. This instalment takes a step outside the realm of metallurgy to look at the use of composites in bicycle frame applications.
It's common to use the terms 'carbon fiber' and 'composite' interchangeably, but in fact not all composites are carbon-based. Concrete, for example, is a composite material. The dictionary definition of a 'composite' material is a combination of materials displaying enhanced properties not provided by the constituent materials alone. In scientific usage, the term 'composite' refers particularly to materials made by dispersing particles, short fibers or long fibers in a matrix.
Those fibers won't necessarily be carbon. In the Duralcan metal matrix composite used to build the Specialized M2, the fibers are aluminum oxide whiskers dispersed in a 6061 aluminum matrix. Advanced composites -- the types used to build 'carbon' bicycles -- have fibers embedded in a matrix, typically epoxy. The fibers might be true carbon, Kevlar (a.k.a. aramid), boron, ceramic, silicon carbide, quartz, polyethylene... and probably a bunch of others that I'm not aware of. To qualify as an advanced composite, it is generally reckoned that the embedded fibers should be continuous, comprise more than half the material by volume, and offer mechanical properties superior to those of fiberglass.
A Simple Lexicon
Here are the terms you need to know. A fiber is a single strand of reinforcing material. A bundle of parallel continuous fibers is bound together with a glue, or matrix. A single layer of this matrix is called a ply, and multiple plies are laid up to form a laminate. If you've forgotten about the other terms used in this series -- like tensile strength and elongation -- re-read the first installment to reacquaint yourself with them, because they'll be essential to our discussion.
The Numbers Look Good
When you look at the numbers boasted by carbon composites, your initial response might be that it's crazy to build a bike out of anything else. But you astute students of the School of Bicycle Geekdom already know that numbers aren't the whole story -- you need to check out the fine print. And get this: with carbon fiber, you have to throw most of what you already know out the window.
It's true that the potential for composite frame materials is tremendous. Unfortunately, the results of some composite bicycle-frame projects have been less than satisfactory. There are reasons for the high failure rate of composite frames, but the fault does not lie with the material. I know you may find this hard to believe, but sometimes even rocket scientists make mistakes...
The most common folly of carbon bike designers is to underestimate the complexity of the task ahead of them. Carbon-fiber structures are less fault-tolerant than metal structures, making good design and execution even more important than usual.
And sometimes the fault really does lie with the material. Epoxy is brittle stuff, and brittle is not your friend on a bike. In the worst cases, an impact on a composite frame -- a rock on the downtube, say -- will open up a network of microcracks throughout the matrix. Everyday stresses work on these cracks, until eventually, long after the impact, the frame fails. And the failure modes of composite structures are plentiful and dramatic: exploding laminates, fibers pulling free from the matrix, first-ply-failure, matrix cracking, and delamination. Sheesh!
All in the Lay-Up
Of course, a smart designer knows how to work with their material to minimise the risk of such catastrophes. Those wacky composite guys get more variables to play with than their metal counterparts.
First off, they choose their material, just like the metal guys do.
Then they choose a geometric configuration -- angles, shapes, thicknesses, that kind of thing. Sure you can make a frame with tubes and lugs like Trek or Giant, but you can also lay out a wholly new shape of your own design, like Kestrel or Look do. Those new shapes need whole new sets of equations to model them.
But the designer's responsibilities don't finish there. They also determine how the composite matrix is laid up -- that is, how the different plies are built up to form the laminate. Structures made from identical composites and with identical geometric configurations but with different lay-ups can yield completely different results -- not just in stiffness, but also in fracture stresses and failure modes.
This is what makes the material so cool. You can dictate the exact characteristics your tube or structure is going to have. Stiff in torsion, soft in bending. Soft in both, stiff in both. Your choice. This ability relies on the phenomenon of anisotropy, and you just can't do it with metal.
That said, let's look at how the properties play out in comparison with those of aluminum, titanium and steel.
Tensile vs Compressive
As you know, tensile strength is measured by pulling on a sample until it breaks. Imagine we're pulling on a bundle of carbon fibers. It performs well. Extremely well. Weight for weight, it's way ahead of steel.
But what about the compressive strength of carbon? Not so good. In fact, the compressive behavior of carbon fibers resembles that of a bowl of spaghetti. To get compressive strength, you to bond the fibers together in a matrix, and thereby transfer the load between the fibers and the plies. Since the matrix and the fiber combine to make up the composite, we'll look at them together to give comparative results.
Density and Modulus
Remember how we said lay-up affects carbon composites? At the risk of being accused of comparing apples and oranges, I'm going to give you some figures for a generic carbon fiber lay-up. Bear in mind that I'm only making a comparison for the sake of continuity in the series.
The density of a carbon fiber laminate is in the neighborhood of 0.056 pounds per square foot, which is about 60 percent of the weight of aluminum, our previous winner in the lightweight stakes. Good, in other words. How about modulus? Well, measured in isolation, the modulus of a even a not-very-high-zoot carbon fiber is about 30 to 33 MSI, or about 10 percent higher than that of steel, previously the stiffest of the three materials we've looked at. So you can see we've got some stiff, light stuff here. But remember, we have to put that carbon fiber into a matrix before we can make bikes out of it. That's where things start to get complicated.
A well-made laminate has 62 -- 65% fibers by volume. The Rule of Mixtures says that the modulus of the composite is proportional to the percentage of fiber in the mix -- the matrix is just there to transfer load between the fibers. So a 65% carbon matrix will have about two-thirds of the modulus of the embedded fibers, say, 18 to 21 MSI. Still sounds good: density one third of titanium, and modulus about 25% higher.
But this figure only holds good in the zero-degree direction -- that is, in the case of forces applied parallel to the fiber in the ply. Try rotating the ply and measuring the modulus perpendicular to the lay-up. Now it reads a pathetic 1.5 MSI or so, the modulus of the epoxy. Yuk! What's worse, the modulus drops off precipitously between zero and 30°. Anything other than a parallel load, and the modulus is way down.
This matters because bicycle tubes (or structures) are subjected to torsional loads as well as longitudinal ones. No-one wants a bike that breaks when it turns a corner! The answer is to add layers of plies at different angles (often 45°) to the initial zero-degree layer. The result is a laminate with an overall modulus of approximately 10 to 14 MSI, still not too shabby. (Again, these are generic numbers for the sake of a simplistic comparison.) This is the basis of the anisotropic design-your-own material capabilities of carbon composites, mentioned above.
The Weak Link
Now for the bad news: elongation. Elongation is the bike designer's safety net, but with carbon it's low, low, low. Depending on lay-up, it's possible to get some elongation out of the stuff -- for example, by a scissoring of layers in the 45° plies -- but in general you're dealing with a material which doesn't have an overabundance of ductility. Composite designs are not meant to bend permanently. And when they fail, they fail all at once, so carbon designers have to build in a big safety net, just as aluminum designers do.
Most manufacturers are very secretive about their lay-ups, so getting good info isn't always easy. The Trek technical manual yields numbers for the specific modulus of that company's lay-up, quoting the modulus divided by the density. Backing these numbers out yields an 8 MSI modulus for the Trek OCLV lay-up.
The strength of the advanced composites can be very high. The zero-degree strength for even a standard carbon unitape (the building block of the laminate) is 300 KSI or better. Looking at the big picture, the strength of the laminate still ends up way above 100 KSI, even at very low density. Trek's specific strength numbers yield actual ultimate values of about 115 KSI. Compare the 8 MSI modulus and Trek's claimed 115 KSI strength with the figures for aluminum. The carbon has about 20% less stiffness, but double the strength and 40% less weight. Very impressive numbers.
A Brilliant Future
What's the future of advanced composites? Their reputation is definitely on the rise. These days, most of the hideously ugly carbon projects have gone away, and there are several very successful carbon production lines happening. The two biggest players are Trek with its OCLV bikes, and Giant, which markets its bikes under several different brand names as well as its own. My guess, after polling a few people in the industry, is that there are probably two to three times as many carbon-fiber bikes sold in the world today as titanium ones. Surprising, perhaps, when you consider all the hype that titanium has received. But when you look at how inexpensive a frame from Trek or Giant is, this starts to make sense.
The future of composite bikes will likely parallel my prediction for aluminum rigs -- that advances in the material itself will be a lot less significant than the advances in our knowledge of how to design and build with it.
Steve Levin, the engineering manager of Schwinn, gave Scot Nicol considerable input for this article. Thanks, Steve.
Next time: the whopping subject of exotic materials.