What Are Composites?

In the following article written by Scot Nicol specifically for VeloNews, we feel that Scot was successful in explaining the basics of advanced composite technology. With permission from VeloNews, the article is reprinted here. Please feel free to read through and enjoy it. If you have any further questions on "composites", you can contact either VeloNews or Zipp directly.



by Scot Nicol

If you've followed parts one through four of this series on bicycle metallurgy, you've learned a lot about the physical characteristics that are important to consider when designing aluminum, titanium or steel bicycle frames. This installment takes a step outside the realm of metallurgy, and looks at the use of carbon-fiber composites in bicycle frame applications.

The Wonderful World of Composites

It's common to use the terms carbon fiber and composite interchangeably, even though all composites are not carbon fiber. For example, both plywood and concrete are composite materials. The term composite refers to combinations of materials that result in enhanced properties not provided by the materials alone (concrete is a composite of cement, sand, gravel and water; Cheeze Whiz is air, artificial flavors and artificial colors).

In scientific terms, composites are generally acknowledged as those materials in which either particles, short fibers or long fibers are dispersed in a matrix. In the case of the Duralcan metal matrix composite that is found in the Specialized M2, aluminum oxide whiskers are dispersed in a 6061 aluminum matrix; while advanced composites — the types used to build bicycles — have continuous fibers embedded into a matrix (typically epoxy). To qualify as an advanced composite, it is generally thought that the fibers are continuous, greater than 50-percent fibers by volume, and the fiber has mechanical properties superior to fiberglass. Fibers can be carbon, Kevlar (a.k.a. aramid), boron, ceramic, silicon carbide, quartz, polyethylene … and probably others that I'm not aware of.

A Simple Lexicon

Here's a simplified explanation of how terms will be used. A fiber is a single strand of reinforcing material. A bundle of parallel continuous fibers are 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. The plies can be laid up in various angles to produce different characteristics of the laminate. If you've forgotten about the other terms used in this series — like tensile strength and elongation — re-read the first installment of this series ("Gadgets" March 7) to reacquaint yourself with those terms, because they'll be essential to our discussion.

The Numbers Look Good

If you look at the numbers that carbon fiber can boast, your initial thought 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 are not the only thing to look at — you need to check out the fine print. And get this: With carbon fiber, you need to throw most of what you've learned out the window.

Yes, 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 that composite frames have endured, but the fault is not that of the material. I know you may find this hard to believe, but sometimes even rocket scientists make mistakes. The situation is similar to what happened with titanium in the 1970s. Teledyne made some frames that failed, not because the material was bad, but because the design was bad, or the execution of the design was bad. Similar things have happened with composites, and the image of the material is not as good as it should be.

The common folly is for the designer to underestimate the complexity of the bicycle frame. Since carbon-fiber structures are not very fault tolerant (unlike metal structures), the design and execution plays an even more important role. And sometimes the fault is not in the design or execution of the structure — the fault may be a big rock coming in contact with the downtube. While the tube might not fail from such a large impact, the repercussions are usually hidden on the inside of the laminate, or within the laminate. Microcracks can then spread through the matrix, decreasing the ability of the fiber to transfer load. Metal tends to do a bit better in these situations — but you can make metal frames that break without warning, too.

It's All In The Lay-Up

What I'm getting at is the fact that composite materials are very complex … more complex than metals. In addition to the material itself having greater complexity, the structures are not as straightforward as metal structures. As you have learned in this series, the designer of a metal structure has two variables: material choice and geometric configuration (like tube sizes, shapes and thicknesses). Those wacky composite guys not only get those same two variables, they also get to determine how the composite matrix is laid up. Bear in mind that two structures of identical geometric configuration, weight and composite material, but with different lay-up, could yield a completely different result. Not only is it possible for the obvious — like stiffness — to vary, but fracture stresses and failure modes could also vary tremendously. And the failure modes of composite structures are plentiful: exploding laminate, fibers pulling free from a matrix, first-ply-failure, matrix cracking, and delamination. And I thought designing a metal bike was tough…. Another curve ball thrown into the mix is the geometric shape of the frame. Sure you can make a frame with tubes and lugs like Trek or Giant, but you can also lay them in a shape of your own design, like Kestrel or Look. With lugs and tubes, the designer at least has metal frames with which to compare; but with a new shape, a whole new set of equations needs to be developed.