Materials and Structure

Spoke material may be tested for its fatigue life properties using an apparatus of the type shown. The spoke material is held in a colet and rotated about its long axis. A force is applied to the free end of the spoke, causing it to bend by a controlled amount. The stress in any small element of the spoke is then periodic about zero as shown in (b), with the maximum amplitude, s_max, being furthest from the point of force application. One revolution of the spoke takes the stress amplitude through one cycle.

This test is destructive, with the spoke being rotated under constant load conditions until it fails. The test is repeated for other "identical" samples under the same or different load conditions, and the number of cycles to fatigue fracture, N, is recorded as a function of the applied stress, S. The data is then plotted on a semi-logarithmic plot of S v. Ln(N), a so called S-N plot.

An important design issue is that there is a wide spread in N for a given S, the failure being statistical in nature and dependent upon the surface condition of the sample.

From: McMahon and Graham,
"The Bicycle and the Walkman,"
Merion (1992)