Fatigue fracture expertise of train engine crankshafts

A failure investigation has been conducted on three cases of failed diesel engine crankshafts used in train and made up of forged carbon steel. The chemical composition and the mechanical properties of the crankshafts material including tensile properties, micro-hardness and toughness were evaluated. The crankshafts examination shows that all failures occurred after a fatigue process. The failure zones comprise the fractured surfaces observation, show the presence of beach marks with semi-elliptical shape surrounding the fracture origins indicate its progressive growth character. The cracks initiation can occur as a result of mechanical and thermal fatigue loads, due to the high stress concentration on fillet radius and the unusual friction between journals and bearings, respectively. Nevertheless, the cracks propagation was only attributed to the mechanical fatigue produced under cyclic bending and torsion loadings.