Investigation of failure in a crankpin of a motorcycle engine

This study performs a failure analysis of a failed crankpin connecting the crank shaft with the piston rod in a motorcycle engine. The failed crankpin under examination is embedded in a 16NiCrMo12 alloy, a typical steel designed for carburizing treatment. The crankpin experiences a periodic compressive stress associated to the compression stroke featuring the cyclic movement of the piston rod imposed by the rotation of crank shaft. During the failure analysis, such mechanism has been associated to a pulsing fatigue phenomenon. The failure of the crankpin concerns the surface, which is affected by significant spalling phenomena causing the detachment of steel fragments, as the element is subjected to constant friction because of relative rotation. Since not all the produced crankpins undergo the observed damaging phenomena, the aim of the study is to identify the factors governing the failure phenomenon and to provide a working solution to the problem. The employed experimental techniques include microstructure analysis by means of optic microscopy and scanning electron microscopy, and hardness measurements. The combination of the obtained results has allowed us to identify the presence of large amounts of non-metallic inclusions underneath the surface as the cause of the damage