carbon fibre composite properties

For tubes try here

and other fun from Fabio Gigli or Google Santa Cruz carbon crush test or similar. Or perhaps, as mentioned by others, look at running prosthetics – hey, look in the sky at planes, at old carbon frame soft tails, at aero blades, carbon spokes, boats, leaf springs, at all sorts of shit.

In that youtube clip, they are applying a stress across the tube not along
it’s length with which the fibres align and hence it’s strength. They are
basically testing the material properties of the matrix i.e resin,
plastic,whatever the fibres lie in, which in this case bends. In a bike frame,
the stress largely lies along the length of the tubes, it maybe compressive e.g
the top tube, or in tension. Whichever, it will lie along the lay up of the
fibre in the matrix, the carbon fibre being (not the matrix) strong.

Ditto where they
bounce a weight on the fork. In use, the stresses resolve largely along the
length of the fork and the carbon fibres in the fork will lie along it’s
length. The only time you’d get the equivalent to that fork test in the video
would be if you rode your bike into a brick wall.

Basically, that video is marketing fluff.

A proper test of carbon fibre
composite is done by making a specimen with the fibres aligned along it’s
length. It will be round in cross-section and of a standard, known
cross-sectional area and length. A force is applied trying to pull
the specimen apart eg.it will be put under tension, the force
applied measured and the extension of the specimen measured and a
stress/strain graph can be plotted until the force is enough for the
material to fail. From that graph the engineer can measure it’s Young’s
modulus(E) and it’s UTS (ultimate tensile strength). E in practice is important
to the engineer because you can then calculate the strength of components of
various cross-sections.

A specimen of carbon fibre composite will fail with very little extension (low
strain). ie. it’s brittle. This is pretty boring to look at, which is why they
applied stresses across the carbon fibres in the video.

There are areas on a bike eg. around the bottom bracket, where the stresses are
all over the place. These stresses can be modelled by the designer/engineer
using CAE/FEA. Because the carbon fibre can’t in practice be aligned with
these stresses, they just beef up the area with more material to stop it
flexing or breaking.

Carbon fibre composite is a “wonder material” to the general public, to the
engineer it’s just another material with various pros and cons.

For example, I recently saw a review for a car where they had used carbon
fibre composite for the dashboard (it was a sports car IIRC). They had used it
purely for it’s “bling” factor. They could have used some moulded polymer like
cheaper cars FWIW but it doesn’t look as “cool”.

If you look at the dashboard of an old Bentley sports car for example, you
will see it’s made of aluminium.

In it’s day aluminium was expensive and hence “cool”.

Eros in Picaddily Circus is cast from aluminium. When it was put up, aluminium
was more expensive than gold. The purpose of making it out of the Al was to
say to the rest of the world “look how rich and powerful we (Britain) are”.

Materials are not always used for their engineering properties.