3-2.5 Ti Comparison with Other Materials

MMC (Metal Matrix Composites)

There are many available types of MMCs, but only one, a particulate-type from Specialized/Duralcan, is presently being used in bicycle frames.

Particulate-type MMCs are the least-expensive form in current production. The Duralcan MMC is an aluminum oxide particulate matrix in an aluminum medium. Other MMCs under development for bicycle use are also particulate types. One employs silicon carbide, the other boron carbide, both in an aluminum base.

MMCs vary in the base metal from aluminum to titanium to copper, and in matrix additives as noted above. The formats of the additives also vary, from particulates, whiskers and wires to continuous and discontinuous fibers. Each factor plays a large part in the strength and other mechanical properties of MMCs. One thing common so far to all particulate and whisker MMCs is a loss of ductility and fracture toughness, which has had a negative effect on potential fatigue life.

Duralcan's 6061-T6 15% particulate MMC has the following advantages over pure 6061-T6 aluminum:

Tensile modulus is increased 30% to 12.7 ksi. The higher modulus helps offset the material's low fatigue life, since a stiffer frame has a lower stress cycle.

Yield is increased by 15%, from 40 to 46 ksi.

Disadvantages of the Duralcan MMC include:

1. Elongation hovers at a meager 5.4%, potentially decreasing fatigue life. (Theoretically, if the frame were designed for no flexure whatsoever, elongation would not affect fatigue life, since the joints would not move. In practice, however, this seems unlikely.) Elongation drops another 50% or more for other MMCs. The lower the number, the less ductile the material. 6061-T6 aluminum has 14-17% elongation after welding and heat treatment. High-quality bicycle steel is 10% before welding, 20-25% after welding. Titanium's elongation is 10-19% before and 15-30% after.
   
   
2. Stress vs. Number of cycles (S-N) fatigue curves remain almost identical to off-the-shelf 6061-T6: 17 ksi at 107 cycles for Duralcan MMC vs. 16 ksi for 6061-T6. Therefore, the fatigue strength-to-weight ratio is almost identical to standard 6061-T6. Note that this fatigue strength is hypothetical because, like monolithic aluminum, MMCs do not have true fatigue endurance. Instead, they must be designed with a much more conservative safety factor.

Fatigue strength is the most important consideration in frame design, regardless of which frame material is under consideration. Most frames fail through fatigue, not from one-time overloading, as in a crash. Ultimate strength is of secondary importance, because a high UTS alone does not and cannot make a durable frame.

The most obvious theoretical benefit of any MMC is the potential to create a stiffer material, as in an engine block where rigidity can reduce noise and vibration. This, however, is not necessarily desirable in a bicycle frame. Ride quality is an important consideration that must be incorporated, even if the fatigue issues are satisfactorily resolved.

Welding is also a complication. Most MMCs lose strength after welding, and some of that strength remains unrecovered after heat treatment. In the area closest to the weld (as well as in the weld itself), the particulates become dispersed, which can cause anomalies and strength problems. Heat treatment cannot restore these particulates to their pre-welded state because the metal does not liquefy during heat treatment.

Finally, it should be noted that a bonded MMC frame can never match the weight of a welded MMC frame. Thus, it is doubly unfortunate that many MMCs have serious mechanical degradation after welding.