Failure Analysis of the Steerer Tube of a Bike

In the present work, the activity of analyzing the breaking of the steerer tube of a bicycle is presented. After a description of the steering assembly of the bicycle, the problem is presented. The problem consists in the fact that the fork which consists of a head a steerer tube and 2 stems that make up the fork has a clear break at the junction between the head of the fork and the tube. The section has a typical case of fatigue failure. In fact, about three quarters of the section are hammered, while the remaining part is ripped. Then the observation under the electron microscope is presented for better understanding what was the real state of the surface of the fracture section of the tube, and then it presents the observation activity to the scanning electron microscope in order to identify the composition of the metallic material in order to check whether there is a discontinuity in the composition of the material. Finally, a structural-type finite element analysis is carried out to verify which was the stress state at the level of the breaking section. It results that given the open conformation of the tube, the geometrical discontinuity due to the opening creates a peak of tension that goes to about 1000 MPa which is sufficient to trigger a crack for fatigue failure. An estimation fatigue life of the joint was carried out and was demonstrated that the life duration had a reduction from unlimited to about 2500 cycles due to the presence of the notch. This way it was shown that the conformation of the tube was the cause of triggering the fatigue break.