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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:
- 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.
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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.
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