Forensic investigation of a failed connecting rod from a motorcycle engine

- It is most likely that the connecting rod failed on the end of exhaust stroke, due to the tensile force being at the highest at TDC.
- The sulphide scale-build-up on the connecting rod led to create a brittle phase that prone to cracking.
- Surface Cracks reached a critical size, propagating until the part rapidly fractured.
- Observation of the fracture surface reveals fatigue fracture morphology, as observed in similarly connecting rods.

In the present work, a failed connecting rod from a motorcycle engine was investigated for the root cause of and possible mechanisms leading to its premature failure. In addition to finding the root cause, the expectation from this study was to possibly improve the existing designs or practices to avoid similar failures in future. These results were validated using a finite element analysis (FEA) simulation. A Scanning Electron Microscope was used for investigating the mechanisms of fracture modes, optical microscopy for studying the microstructures and visual inspection were primarily utilised to determine the root cause of the failure. In conclusion, it was determined that the root cause for the premature failure of the connecting rod was the presence of scale build-up inclusions, which led to micro cracking during fatigue loading of the component.