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Carbon Fiber
Carbon fiber is a blanket term for a wide variety of carbon-impregnated polyesters,
graphite fibers, and polymerized carbon fibers that are used within a matrix of adhesive
to create a clothlike structural material.
Within the family of fibers considered appropriate for bicycle frame use, the raw fibers'
stiffness-to-weight ratio is roughly 3.5 times higher than 3-2.5 titanium. The ultimate
tensile strength is roughly 70% higher.
However, these figures apply only to the raw fiber strand, before it is impregnated by
and retained within an epoxy resin matrix. The epoxy adhesive's structural properties
are significantly lower. Moreover, epoxy normally occupies 50% or more of the cross-sectional
area of a sheet of carbon fiber cloth. This ratio of resin to carbon must be maintained
to hold the fibers together; a lower epoxy content reduces the fiber weave's layer-to-layer
shear strength. A 50% volume of adhesive reduces the finished product's strength-to-weight
ratio by a factor of two.
In addition, carbon fiber is anisotropic, which means that it displays directional properties.
For example, a fiber with a modulus of 20,000 ksi when measured longitudinally will have,
at best, a transverse modulus of 4,000 ksi. Similarly, the ultimate tensile strength may
measure 220 ksi longitudinal, but will be, at best, 10 ksi transverse.
This anisotropic property can be exploited beneficially in some structures, such as leaf
springs. However, bicycle tubes must be able to carry stress loads in many planes at
once-in tension, compression, fully reversed bending and clockwise and counterclockwise
torsion. Thus, it is virtually impossible to utilize anisotropy to any significant
extent in a frame.
In addition to the modest structural properties displayed by the epoxy resin, carbon fiber
has extremely low ductility and poor abrasion resistance. Historically, low ductility in
those bicycle frames that do not use separate lugs has led to joint failure and stress
cracking. Abrasion is a particularly thorny problem since composites are notch-sensitive,
such that even minute inconsistencies in the material can develop into large cracks,
eventually leading to failure.
Abrasion problems can be reduced at the cost of added weight by a protective skin or veil
of fiberglass or, at higher cost and somewhat greater strength, Kevlar fiber, but the
abrasion resistance of these and similar polyester and aramid fibers is also low.
Abrasion and impact damage can be repaired with epoxy-based fillers and additional cloth.
However, since the integrity of the structure is dependent upon continuous fibers in
tension, the strength of the repaired area will be lower than the original material,
and the weight of the repair will be higher.
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