Table of Contents

Materials and Structure




The photograph shows a fatigue crack in the frame tube of a bicycle. The crack started at a tube joint region and then propagated, under the fluctuating stresses from road shocks, into the tube. In this picture the crack has been made visible by filling it with chalk dust. An alternative method is to use a liquid fluorescent dye that will soak into the crack and can be seen (after the tube surface is wiped clean) when the bike is illuminated with ultra-violet light. The mechanism of crack propagation in a metal frame tube is the same as that for the spokes considered above. The crack will grow until a critical crack length is reached at which point the tube will fail by fast fracture.

For steel frame tubes there is a 'fatigue limit' - - a stress value below which the fatigue life of the component is essentially infinite. If the tubes are of aluminum alloy, the fatigue lifetime increases as the maximum applied fatigue stress is reduced. The frame tubes are also susceptible to stress concentration due to corrosion or local surface damage. The tube shape and wall thickness should be selected so as to reduce the maximum tensile stresses and thereby increase fatigue life and decrease tube elastic deformation under load.

From: Apps, "The Bicycle Book,"
Smithmark (1996)