In real engineering
situations materials experience fluctuating loads during use. The magnitude
of the stress fluctuation may be within the elastic range of the material,
however, it is found that material failure can occur through the process
known as fatigue.
In the bicycle, two types of location are prone to
fatigue failure: the spokes that are always in tension and experience a fluctuating
stress for each revolution of the wheels, and the joints between frame
members. The photographs show a spoke that failed by fatigue. The nipple
section (a) shows that the crack started in the threaded region of the spoke.
(b) shows the fracture surface, the black arrow showing where the crack started
and the top, matt grey section being the final fast fracture region.