Brass rivets were cracking during assembly, when the rivets were being set. Failure analysis of the rivets was performed as part of the process to determine the root cause of the cracking.
The rivets were manufactured by machining them from wire stock. The main requirements for the rivet material were:
- Easy to machine rivet from wire.
- Easy to form rivet during assembly.
Meeting these two requirements put mutually exclusive constraints on the brass composition. Lead is added to improve machining, however lead reduces brass cold forming properties. The alloy selected had 0.9 to 1.5 % Pb.
Rivet Failure Analysis
Stereo zoom microscope (up to 70x) examination revealed that rivets were cracking at the portion deformed during the rivet setting process, when two components were being joined.
Composition analysis of cracked and uncracked rivets was performed using atomic absorption spectrocopy. The results indicated that uncracked rivets had less than 1.1% Pb and the cracked rivets had more than 1.3% Pb.
The root cause of the cracking was poor design. A new alloy was selected that had lower Pb. This required slowing down the machining process, but eliminated the cracking.
The problem could have been prevented by conducting a failure modes and effects analysis (FMEA) when the product was being designed. An FMEA involves reviewing components, assemblies, and subsystems to identify failure modes, and their causes and effects. Had the design team performed a design FMEA, they may have identified the mutually exclusive behavior of brass with Pb. Information about how to perform an FMEAs is available in the publication “Potential Failure Mode & Effects Analysis”, which is available from AIAG at www.aiag.org.