Analysis of the cracking causes in an aluminium alloy bike frame

Two cracks were detected in the bike frame of an amateur cyclist, before the final failure of the component and after using it for around 35,000 km. The cracks were located in the joint between the bottom bracket, the chain stays, the seat tube and the down tube.

This paper analyses the causes of the cracking process and comprises, basically, chemical analysis for material identification, visual inspection of the frame, microstructural analysis, microhardness measurements, mechanical characterisation through Small Punch tests and SEM (plus EDX) analysis of the fracture surface.

The analysis concludes that both cracks were caused by corrosion and Stress Corrosion Cracking (SCC) processes that are justified by the differences in the solution potentials between the base material and equilibrium precipitates produced by an overageing treatment. The latter are more anodic and hence easily dissolved in a humid saline environment.