Failure mode analysis of two diesel engine crankshafts

A failure analysis of two damaged crankshafts are presented: one obtained from a diesel engine of a mini backhoe, and another one from an automobile vehicle. The diesel motor suffered a serious mechanical damage after 3 years and 5000 hours in service: the connecting rod n° 3 broke and, in consequence, the crankcase and motor block suffered damage. The motor was repaired by a non-authorized workshop, but maintaining the same crankshaft without being properly inspected. After 1100 hours working the crankshaft failed on the 3rd crankpin. The second crankshaft failed after 105 000 km in service. In both crankshafts a crack grew from the crankpin-web fillet, and their symmetric semi-elliptical crack front profiles confirms the effect of a pure mode I (reversed bending). Fractographic analyses show the semi-elliptical beach marks and results indicate that fatigue fracture was the dominant failure mechanism of these two crankshafts.