A tale of failing

My beloved bicycle elegently lean on a post in Melbourne Central

Some time ago, I was unfortunate to lose a 2.5 year old friend -- my purple mountain bike. The rear wheel axle was found broken.

As an enthused mechanical engineering student, I naturally had some though of the causes of the failure. After the dissection of the rear hub, and carefully examined the broken edge of the axle, I hypothesized that the axle was failed by fatigue due to cyclic bending load.


shiny or grainy broken surface of the axle suggested that the failure was due to fatigue.

Fatigue is a kind of failing mode which happens when cyclic tensile load is applied to the material. Unlike yielding, it can occur below the yielding stress of a material. Also, unlike human, fatigue of an engineering material cannot be recovered by resting.


Side view of the broken axle

Since the an axle was not subjected to torsion, it was unlikely that the axle was failed under torsion load. This hypothesis is also confirmed by the broken surface. If one examined carefully, it can be noticed that the broken surface is flat. Normally, in fatigue under torsion load, the broken surface would be in cone.

Since the bicycle is no longer performing its function due to broken rear axle (in other words, it fails to meet its objective), I cannot sell it. In fact, I would like to sell it, but no one will buy it (at least I had tried the Cash Converters).

So, lets give an honour to my beloved bicycle for its contribution during its life time.