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Carbon-wrapped Titanium and Aluminum
Titanium or aluminum tubing wrapped with a bonded layer of carbon fiber composite has
been proposed as a method to achieve a synergistic improvement of material properties.
(In fact, carbon-wrapped aluminum tubing was produced by Easton for Raleigh for two
years, before the withdrawal of that frame from the market.) The main objectives of
this approach are:
- To improve the performance of a low-strength tube. Aluminum's low strength-to-stiffness
ratio, for example, can be boosted appreciably with a layer of high-modulus composite fiber.
- To protect the abrasion-sensitive carbon within a metal exoskeleton.
These approaches have a number of drawbacks:
- External carbon wraps do not solve the problem of abrasion damage to the composite.
- Internal carbon wraps do not necessarily protect the composite from impact failure
either. To create a frame of reasonable weight, the titanium or aluminum tube must be
very thin, and consequently not resistant to denting. Since titanium is very ductile,
it can spring back from minor impact with no appreciable damage. However, the internal
wrap will suffer local cracking, which can spread into a serious fault.
In addition, delamination of the composite from the tube surface is a serious long-term problem.
It has at least three sources:
- Delamination can occur from impact. Once the composite has cracked, it will continue to
fail along the fiber orientation. The fissure created by the initial fault becomes a point
for peeling or cohesion.
- Delamination can occur at the ends of the supporting tube due to applied bending and
torsion during use. Adhesives are weakest in peeling and cleaving.
- Delamination can occur from stress. When used in a wrap, the adhesive must perform
two duties, first as the bonding agent between the fibers, and second as the glue between
the composite and the tube. Ideally, two different adhesives and primers would be specified,
but this is not always possible.
Carbon-wrapped tube frames also suffer from a weight disadvantage, since these tubes cannot
be welded once the composite has been applied, and so must be bonded in a lugged frame.
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