Failure Analysis of Aircraft Crankshaft

Crankshaft is a mechanical component consisting of a series of cranks and crank pins and is responsible for converting the vertical linear motion of pistons to horizontal rotational motion. Crankshafts were designed for normal loads with margins of safety, however, any increase in force or additional stress, such as the localized high stress concentrations, a structural failure could be induced. In this study, failure analysis was performed on an aircraft crankshaft to determine the cause of failure. Microstructural and chemical composition analysis revealed a hardened AISI 4340 material. Progression marks indicative of fatigue failure and ratchet marks were observed on fracture surface. Striation marks of an approximate width of 0.2 micron were observed under Scanning Electron Microscope (SEM). The fatigue crack was found to initiate at the fillet section, a geometric discontinuity susceptible to high stress concentrations. Other contributing factors to the increased stress gradient leading to the onset of the crack are the circumferential machining grooves which was observed intersecting with the fracture plane, the possible pitting corrosion, and the heat and stress caused by the crank pin-bearing surface contact in the vicinity of the crack origin.