Forensic Analysis - Bike Handle Failure
Sophomore Material Science Project
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The owner of the bike said that this aluminum part failed after 50,000 miles of riding. The purpose of this analysis is to determine a likely cause of failure and improve the design.
The failure hypothesis that was tested was fatigue failure.
The bike handle is made of Aluminum which does not have a fatigue limit and the bike had been used extensively, so this seemed like a reasonable hypothesis.
To test this hypothesis, the part was modeled in SolidWorks, the cyclic forces were estimated, finite element analysis was performed in SolidWorks, and the fatigue performance of the material was analyzed.
A zoomed in view of the CAD model. This photo highlights the sharp corners where the greatest stress is likely to be experienced. This region is also where the failure actually occured.
According to “Forces Applied to a Bicycle during Normal Cycling” by P. D. Soden and B. A. Adeyefa a bike handlebar can experience forces between .1 and 1.08x the rider's body weight during turns. For a 160 lb rider, an average force of 60 lbs on the handlebars per cycle was estimated.
Finite element analysis was performed by fixing the stem and applying a remote load based on the geometry of the bike handle bars.
The maximum stress according to FEA was about 1.2 x 10^8 N/m^2 or 120 MPa. The maximum stress occured at the predicted location where failure occured.
For this type of aluminum, at 120 MPa, failure should be expected at about 3 million cycles.
Given the geometry of the bike and the range of gear ratios used, 3 million cycles is a reasonable estimate for 50,000 miles of riding.
To improve the design, our team recommended increasing the fillet radius shown here.
Increasing the fillet radius decreased the maximum von Mises stress to 53 MPa which is estimated to increase the number of cycles to failure to 1 billion cycles for this material or about 6 million miles.
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