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COMPONENT FAILURE MUSEUM

The main sections of the museum are listed below;

Metal Fatigue
Manufacturing Faults
Bicycle Components
Corrosion
Polymers
Composite Materials

Tools of the trade, some ways to investigate problems;
Photoelasticity
Dye penetrant testing

Glossary
of materials engineering terms

 

Bicycle crank failure II

Bicycle crank

Yet another fatigue failure in a bicycle crank spider arm. This was a high quality component that had very high load cycles but was in excellent apparent condition until the final fracture that unseated the rider in heavy city traffic. Fortunately no long term injury occurred.

Select pictures below for larger images.

The fatigue crack initiated exactly at the location of the maximum tensile bending moment, close to the crank axle on the tensile side of the crank.

 

The crack progressed slowly through the crank arm (dark area) until the remaining fragment was incapable of supporting the bending moment generated by the force on the pedal and the crank arm fractured rapidly.

This long term fatigue crack in a high quality component took a considerable time to nucleate from a machining mark between the spider arms on this highly stressed surface.
However once initiated propagation was rapid and accelerating as shown in the increased spacing of the 'beach marks' on the surface caused by the advancing fatigue crack.

This unfortunate incident may have been prevented only by very close visual inspection or dye penetrant testing on the component during the relatively short crack propagation phase.

Return to Metal fatigue cases
Return to Bicycle components

29/1/02 ed:AG

 
 
© 2005 Materials Engineering - Page last modified 18-Dec-2007