Introduction
Product Liability Expert was
asked to perform a failure analysis on a failed bicycle seat post. This seat
post failed unexpectedly and the rider was injured. The failure analysis expert
witness was sent the relevant parts and asked to determine the cause of failure.
This work was essentially a bolt failure analysis.
Conclusion
The seat post failure was
caused by a manufacturing defect. There was a deviation from the design. An
unspecified bolt was substituted for the specified bolt. The callouts for
chrome plated M8 X 1.25 bolts specify that only the final 13.0mm to 17.0mm of
this bolt are to be threaded. The failed bolt had the entire shank threaded.
The root of the threads of this faulty bolt acted as a stress riser and caused
the bolt to fail due to metal fatigue.
Procedure
Failure Analysis: The collapse of the seat post occurred because the chrome
plated M8 X 1.25 bolt that held the seat clamp failed. A root cause failure
analysis was performed on this bolt.
Evaluate the Design: The geometry of the failed bolt was compared to the one
called out by the design.
Results
Figure #1 shows the pieces of
the failed seat post that were submitted for evaluation
Figure #1: Photograph of the pieces
of the failed bicycle seat post that were submitted for the failure analysis.
Figure #2: Close up
photograph of the failed chrome plated M8 X 1.25 bolt. This bolt held the
clamping mechanism to hold the bicycle seat on the post.
Figure #3: Optical photomicrograph
taken of the fracture surface and it includes the origin. This bolt failure is
a fatigue failure. The fatigue crack initiated at the root of the rolled
threads. The thread root acted as a stress magnifier (Mag 12X)
Figure #4: Drawing of the required
chrome plated M8 X 1.25 bolt. It is important to note that the design specifies
that only the final 13.0mm to 17.0mm of this bolt are to be threaded. This
unthreaded portion of the bolt is required to create an “unthreaded shear area”.
